Freshwater ecosystems are home to an incredible diversity of species. Rivers, lakes, swamps, peatlands and all the other freshwater wetlands harbour more than 10% of the world's species. Each week, we will profile one of the most extraordinary.
#31 Cuban gar
Also known as Manjuarí, the Cuban gar, Atractosteus tristoechus, is the rarest of all the seven known gar species. The gar family is quite unique - their eggs are toxic (to mammals, birds, and most arthropods, but not fishes!), and their lineage dates back to the dinosaurs’ age like the Sturgeons. Endemic to Cuba and the nearby Isle of Youth, the Manjuarí can grow up to two meters long. It lives in shallow streams, swamps, crystal-clear springs, and cascading waterfalls. This species is the most understudied gar and the most threatened. Assessed as Critically Endangered by the IUCN Red List of Threatened Species, Cuban gars’ declining populations face threats associated with introducing invasive species, habitat loss and alteration, and overfishing of prey items. A recent assessment shows that Cuban gars share their home with other 79 species of freshwater fishes, of which 54 are endemic to the two largest islands, Cuba and Hispaniola. It also revealed that 45% are threatened with extinction. The arrival of invasive species like the African Catfish, Clarias gariepinus, resulted in the steep population declines of the Manjuarí. As apex predators of some freshwater ecosystems such as the Zapata Swamp - the largest wetland in the Caribbean - Cuban gars support the presence and endurance of other native fish populations. Hence, the Manjuarí is a crucial species for conservation efforts, as it is a key species for maintaining their ecosystems’ health.
#30 Large red damsel
The Large red damsel, Pyrrhosoma nymphula, is one of the most common damselflies in Europe’s wetlands. But be aware, they are very similar to another species: the Small red damselfly, which is less red. Large red damsel males are bright red with a black thorax, but females can be almost entirely black. Most likely, you will see the active and robust Large red damsel flying over or near slow-ﬂowing and standing waters. However, in eastern and northern Europe, where it is less common, it mostly reproduces at running waters. They also have smaller numbers in north Morocco, northern Tunisia, and the mountains of Southwest Asia. They like breeding mainly in a wide variety of wetlands, from acidic bog pools to brackish ditches, especially sheltered waters with abundant aquatic plants. Their aquatic nymphs (or naiads) have elongated abdomens that terminate in three leaf-shaped tracheal gills. These fascinating gills can absorb dissolved oxygen from water and function as ﬁns during the undulating swim of the larvae. Damselfly nymphs tend to stay for one year underwater. When it’s time to emerge, they crawl onto the shore or cling to an emergent aquatic plant and shed into adults. Shed skins are a common sight on waterside vegetation, especially for Large red damselflies, as most adults emerge from the pond at the same time over a short period in the spring.
#29 Cape pondweed
Cape pondweed, Aponogeton distachyos, is an aquatic flowering plant endemic to Western Cape in South Africa. This species produces floating leaves and flowers from stems growing at the bottom of water bodies, where it’s rooted. Typically blooming in small ponds from fall to spring, its flowers are known to be vanilla-scented and often visited by pollinators like bees. Humans also appreciate their flowers – it’s a delicacy in South Africa where they are commercially grown for their edible roots, stems, and flowers. This species is present in traditional dishes, where the flowers and leaves work as replacements for cabbage in stews and young shoots as an alternative to spinach or asparagus. It is also used for traditional medicines, the stems as a soothing treatment to reduce redness from burns and wounds, and together with flower petals for acne treatment. Due to its attractive flowers, it is also an ornamental plant. Sadly, such a feature has also led to the introduction of this species in other parts of the world. Invasions of non-native species are a threat to the economy and can cause systemic ecological damage. In freshwater ecosystems, invasions are occurring at a fast pace, and their impacts are enhanced by increased nutrient levels, loss of top predators, and altered flow regimes.
#28 Maripano piranha
Pristobrycon careospinus is a rare piranha locally called Maripano by the Piaroa ethnic group in Venezuela. The rare photo - actually the first live pictures of this species! - was taken by the Photographer Ivan Mokolji, who explains how he managed such a difficult task on this great article. Found in the Atabapo river in the Orinoco basin, this species is part of the Serrasalmidae fish family - piranhas and pacus. Serralmids are keystone ecological taxa - some species are top riverine predators and/or primary seed dispersers in the flooded forest. Piranhas are notoriously famous for their ferocity, mainly due to feeding on corpses and nipping bathers to defend their nests. While piranhas are indeed generally carnivorous, some species like the Maripano are facultative frugivores: they can eat both other animals (e.g., insects and fish) as well as fruits, seeds, and leaves. Despite their keystone status and commercial significance throughout South America's Amazon, the ecology of piranhas and pacus are poorly understood. Pristobrycon careospinus is known from only the original description with a single specimen captured in 1974. Hence, records such as Mokolji's are essential for freshwater biodiversity assessments and conservation.
The gharial, Gavialis gangeticus, is a specialist fish-eating river crocodilian generally about 3 meters long, but some may reach more than 6 meters. With seasonal migratory behaviours and social hierarchies, gharials come together for mating and nesting during the dry season in the highly seasonal, monsoonal rivers in which they live. On the sandbars and river banks, females lay an average of 40 eggs in a hole nest, and for 1 to 2 months, they will look out for them typically together with one single large male. The females open the nests at the time of hatching, and hatchlings from multiple nests aggregate in “crèches,” numbering from hundreds to a thousand or more. Once the monsoon waters start to rise, adults migrate while the hatchlings disperse widely into the aquatic shorelines. Gharials were once very common and abundant in the main rivers and tributaries of the Indus, Gangetic, and Brahmaputra-Meghna Basins. Sadly, they are currently only found in India, Bangladesh, and Nepal and are regionally extinct in Pakistan, Myanmar, and Bhutan. According to IUCN, gharials are Critically Endangered mainly due to dams and barrages which disrupt river flows , mortality in fishing nets, and historical and unregulated hunting for their skin. Nowadays, threats also include sand mining and boulder removal. WWF-India conservation efforts include a gharial reintroduction programme in India in collaboration with the Uttar Pradesh Forest Department. WWF-Nepal started a Gharial Conservation Action Plan for Nepal in 2018.
#26 Freshwater pufferfish
Pufferfishes are also in freshwaters – meet the Freshwater pufferfish, Tetraodon mbu. Freshwater pufferfishes inhabit large rivers and lakes, feeding mainly on molluscs and insects. This species is widely distributed in the Congo river basin, besides living in the delta and lower Malagarasi River in Tanzania. In this restricted range, it is listed as Vulnerable by the IUCN Red List of Threatened Species due to increased agriculture and land-use change. Overall, their population trend is unknown, and conservation efforts would benefit from research and monitoring of the Congo basin. Pufferfishes present many fascinating features, one of the best known being their defensive behaviour against predation in which they inflate their bodies, looking like a colourful floating balloon. By swallowing water, they can reach a volume almost four times their average size. They are also among the most poisonous fish in the world – they have potent neurotoxins, which can cause fatal human intoxication. Some freshwater puffer species present a high level of toxins on their skin, but further studies on the Tetraodon mbu are necessary to give us a clearer picture.
#25 Amazonian Manatee
The Amazonian manatee, Trichechus inunguis, is the only manatee to occur exclusively in freshwaters. It is also the smallest and most slender member of the manatee family, distinguished by smooth rubbery skin and large flippers lacking nails. Endemic to the Amazon Basin, its range unevenly covers over an estimated 7 million km2 – from the headwaters in Colombia, throughout Ecuador, Peru until the Amazon’s mouth in Brazil. They prefer murky and calm waters away from human settlements. As herbivores, they feed on a large quantity (8-15% of their body weight daily) of aquatic and semi-aquatic vegetation during the rainy season. All food is stored up as fat, supplying them during the dry season - when the water levels start to drop, individuals migrate from the flooded areas to deeper water bodies with less available food. On the other hand, they are less vulnerable to hunting and predators. Females are very attentive mothers, teaching the young to swim, select plants to graze, and surface to breathe. Due to this process, she will only produce offspring once every four years. Listed as Vulnerable on the IUCN Red List of Threatened Species and decreasing numbers, this species is threatened mainly by historical and current hunting, especially for its meat. Entanglement in fishing nets and habitat alterations (e.g., pollution and dams) are also threats. Data is limited due to their secretive behaviour, which hinders monitoring and conservation strategies. Conservation efforts by WWF in the region include monitoring projects via drones and satellites. In Peru, WWF has recently worked with Prodelphinus to support the government in building the National Action Plan for the Conservation of River Dolphins and Amazonian Manatee approved in 2018.
#24 Dumbéa River pipefish
Freshwaters also have beautiful colourful pipefishes - meet the Dumbéa river pipefish, Microphis cruentus, endemic to New Caledonia. Closely related to seahorses, freshwater pipefishes mainly inhabit tropical and temperate seas. But there are a few species in the Indo-Australian Archipelago restricted to freshwater, such as the Dumbéa river pipefish. Known from the lower reaches of the Dumbéa River, this species likely feeds on small benthic and planktonic animals like insect larvae and crustaceans. Similar to other pipefishes, Dumbéa females deposit their eggs underneath the male's trunk or tail. And the male is the one that does the work of incubating them for several weeks in either partly or wholly concealed pouch. Not much is known about this species, being listed as Data Deficient by the IUCN Red List of Threatened Species. Recently, Shoal - an organisation dedicated to conserving freshwater species – launched the Search for the Lost Fishes campaign. They considered adding this species to the Top 10 Most Wanted species, as it hadn't been seen for a decade - or so they thought. Glady, thanks to the naturalist Damien Brouste, who recorded the Dumbéa on the citizen science and naturalist platform iNaturalist, Shoal discovered that this species wasn't a lost fish.
#23 Titicaca water frog
Meet one of the world’s largest exclusively aquatic frogs: the Titicaca Water Frog, Telmatobius coleus. Endemic to Lake Titicaca in Peru and Bolivia, this species spends its entire life underwater. Found at depths of up to 100m, Scientists believe Titicaca water frogs can breathe through their loose-fitting skin, which performs all gas exchange. Listed as Critically Endangered on the IUCN Red List of Threatened Species, this species population has declined dramatically recently due to harvesting for human consumption, degradation of its habitat (e.g., pollution from mining and agriculture), and water extraction. Its larvae are also thought to be predated by introduced non-native trout species. Humans harvest the Titicaca water frog for food and traditional medicine, being used as an aphrodisiac and treatment for memory loss and asthma, among others. In 2016, 10,000 Titicaca water frogs were found dead along the Coata River, one of the Lake’s tributaries, probably due to the river’s pollution. Although the Lake is under habitat management and protection, further action and better enforcement are needed. Recently, two dozen women – artisans who used to poach these frogs – started knitting frog toys and finger puppets to help protect the frog and the environment with the support of the Denver Zoo.
#22 Moss animal
Cristatella mucedo is a small freshwater invertebrate which is part of Bryozoa phylum, sometimes known as moss animals. C. mucedo has a wide range and is distributed across America, England, Norway, Poland, New Zealand, and likely many other places. As they are not as well studied as other species and we still have lots to learn about these bryozoans. This species likes stiller waters such as lakes, ponds, and slow flowing streams and rivers and is generally found up to 2m in depth. It grows on submerged substrates as colonies of genetically identical individuals called zooids. Each zooid is about 2 mm long and has a horseshoe crown with 80-100 ciliated tentacles, which it uses to capture and filter organic particles. Zooids form clear, soft, gelatinous colonies resembling a caterpillar. Most bryozoan colonies are strictly sessile, but C. mucedo is somewhat mobile – it moves a few cm/day by sliding, and scientists presume that a thin fluid secreted by its cells helps them glide. But it may be that muscles in the colony bottom wall also play a role. They are primarily asexual and reproduce via small encapsulated structures called statoblasts, which potentially form new colonies. Water currents, wind, and animals like fish and aquatic birds disperse the statoblasts. This tiny invertebrate generally does not like polluted waters, and its presence indicates good water quality.
#21 Giant South American river turtle
Podocnemis expansa, the Giant South American river turtle, is one of the biggest freshwater turtles of South America, reaching over 80 cm in shell length and 65kg in mass. This impressive species used to be abundant in most rivers of the Amazon and Orinoco basins, occurring across eight countries. They would migrate by the thousands up rivers to nest collectively on high sandbars, including hatchlings that tag along with the adults to feeding areas sometimes many miles away. The hatchlings and females are able to communicate with each other through underwater vocalizations. Although the freshwater giant is currently listed as Lower Risk/ Conservation Dependent by the IUCN Red List of Threatened Species, the IUCN Tortoise and Freshwater Turtle Specialist Group (TFTSG) recently recommended the categorization change to Critically Endangered. Giant South American river turtles are historically a valuable source of food and income, especially in riverine communities. Large-scale commercialization only started after Europeans arrived in the region, leading to the overexploitation of eggs and meat for consumption and oil extraction. These threats have decimated populations of this river turtle across its range. The construction of mega-dams is also an issue, hindering its recovery. Long-term conservation efforts by the Brazilian Government and other institutions led to the stabilization of some populations through the protection of areas (reserves) and the release of huge numbers of hatchlings from protected nests. However, to increase their number to viable populations, best practices should include protecting the entire river basin, besides improving monitoring and enforcement.
#20 Lister's river snail
The charming Viviparus contectus is part of a family of river snails – the Viviparidae. The family name is due to their ovoviviparity: they bear live young - their offspring hatch from the eggs inside the female's body. As a gilled snail, Lister's river snail can breathe underwater - it allows water to enter the shell and go through its gills to obtain dissolved oxygen. Due to this ability, they are also able to filter water like mussels. This trait is important in polluted waters, where increasing temperatures combined with rising nutrient inputs tend to intensify eutrophication symptoms. V. contectus feeds on organic particles and microbes and lives in large slow-flowing rivers, large drainage ditches, fenland dykes, ditches on grazing marshes, and large ponds and lakes. This species is widespread in many countries in the Palearctic region, but there are considerable gaps in the taxonomy. They can host different groups of parasites, including those of medical importance. This species has been used as a traditional medicine in Korea and China for liver diseases and alcohol poisoning. The latter country also uses it as a food supplement since it contains high amounts of protein, essential amino acids, taurine, calcium, iron, and zinc. This freshwater snail is also part of the aquarium trade, but the main known threats are pollution and habitat loss. They are listed as Least Concern by the IUCN Red List of Threatened Species and with a decreasing population trend.
#19 White-throated Dipper
The White-throated dipper, Cinclus cinclus, is one of the few riverine songbirds worldwide. Found widely across Eurasia, this territorial bird likes fast-flowing, clear-water rocky streams, and rivers with abundant freshwater invertebrate prey. It also uses glacial lakes and shallow watercourses - their freshwater habitat has to have riffles and exposed rocks for breeding. They set their large globular nests made of moss and dry leaves on rock crevice or cliff ledge, sometimes even behind waterfalls, but almost invariably over running water. Mostly found in uplands, this species is resident but does some post-breeding movement to more lowland rivers or sometimes to the coast. It's fascinating to see them in the wild – they are often bobbing when standing, as they are dancing to some unheard music. They generally plunge into the water to feed and they can also submerge completely! They can swim against the current by walking on the river bottom, using their wings to stabilize themselves as water flows over them. Listed as Least Concern by the IUCN Red List of Threatened Species, they are not globally threatened, but their populations are decreasing mainly due to pollution. White-throated dippers are widely recognized as environmental indicators - their status and abundance strongly reflect the water quality and rivers' habitat structure.
#18 fishing cat
Not all cats dislike water – the Fishing cat, Prionailurus viverrinus, is strongly associated with wetlands and, as its name implies, it often enters the water to fish. Their exceptional dietary choice among felids suggests even some morphological adaptations such as semi-retractile claws that can grip slippery aquatic prey and a double-coated fur, which prevents the body from getting wet. But this nocturnal and elusive cat also preys on frogs, crustaceans, snakes, birds, and scavenges on carcasses of larger animals. About twice the size of a typical house cat, this species preferred habitat is wetlands, which destruction and degradation have led to its populations' decline. Listed as Vulnerable by the IUCN Red List of Threatened Species due to threats such habitat loss (from urban encroachment) and other threats such as wetland drainage for agriculture, pollution, and logging, which has lead to the eradication of Fishing cats from much of their previous range across South and Southeast Asia. Since 1970, the world has lost approximately 87% of its wetlands, and those remaining are disappearing three times faster than forests. A healthy population of fishing cats, and their prey, can only be sustained in a healthy environment. Learn more with this great read.
#17 Freshwater pearl mussel
The Freshwater pearl mussel, Margaritifera margaritifera, can often live for more than a century, even two! In Sweden, scientists recently discovered the oldest known specimen, which is 280 years old. They have a complex life cycle, which depends on salmonid fish exclusively. Males release sperm into the water, and the female inhales it to fertilize its eggs. Within its gills, the female pearl mussel broods the eggs developing into microscopic larvae called glochidia. When the temperature starts to rise during summer, it releases the glochidia as a stress response. The timing is perfect - it coincides with the presence of salmonids, so the 4 to 60 million glochidia can find a fish to be their host. Just as the fish takes in water to pass over its gills to breathe, the glochidia snap into the fish’s gills. As a defence mechanism, the fish creates a cyst around the glochidia. They stay until the following spring or summer when they are juveniles, dropping off the fish into the river’s gravel, where they become adults. Found in North America, Europe, and Siberia, this species is listed as Endangered by the IUCN Red List of Threatened Species. Their populations are declining due to pollution, river regulation, and habitat degradation including dams, which also affect salmonid populations. Their conservation benefits the entire riverine ecosystem - they are excellent indicators of ecosystem health and filter up to 50 L of water a day, improving water quality.
#16 Afia Birago's puddle Frog
The Afia Birago's puddle Frog, Phrynobatrachus afiabirago, is named after the mother of Ghana's first formally trained herpetologist, Dr. Caleb Ofori-Boateng. Dr. Caleb discovered the endemic puddle frog in the Atewa Forest's swampy habitats in Ghana – a unique area of mountain forest-like vegetation in West Africa. This puddle frog likes large forest ponds and swamps, presumably breeding in the water. Its population inhabits only two known sites, and it's likely to be decreasing due to the ongoing decline of the extent and quality of their habitats. In 2016, a wave of bushfires destroyed most of the forest habitat close to one of the frog's ponds. Listed as Critically Endangered by the IUCN Red List of Threatened Species in 2019, Afia Birago's puddle frogs face threats such as ongoing illegal logging, artisanal mining, and farming. Scientists estimate that 80% of the already small population will disappear in the next ten years if bauxite mining plans in the Atewa Forest Reserve move forward. Dr. Caleb and many environmental Civil Society Organizations argue that mining in that region would be catastrophic to biodiversity and human health as Atewa is the water source for over five million people. Finding the endemic frog secured Atewa Forest Reserve's designation as an Alliance for Zero Extinction site, restricting mining as an activity in this habitat which is already a Key Biodiversity Area. Also, together with Synchronicity Earth, Dr. Caleb started an amphibian monitoring programme in the region.
#15 Russia sturgeon
The Russian sturgeon, Acipenser gueldenstaedtii, was once widely distributed in the Danube river. Now, it is listed as Critically Endangered by the IUCN Red List of Threatened Species, and it is the most endangered sturgeon species of the four that still exist in the Danube river. This species likes to swim in deep parts of large rivers, spawning on stone or gravel bottom in areas of strong currents. They would migrate a considerable distance upstream before spawning, going as far as Bratislava. Most of their spawning sites have been lost due to dam constructions, except in the lower Danube river. But this species also faces other huge threats in one of their last safe reproductive harbors: illegal poaching and trade. A new WWF market survey found that one-third of sturgeon caviar and meat products were sold illegally in the lower Danube region. Out of 27 samples from poached sturgeons, 22% were from Russian sturgeon – a very high amount considering the extreme scarcity of this species in the wild. Stopping the illegal trade is one of the key pillars of WWF’s global Sturgeon initiative. In 2019, WWF Bulgaria released thousands of farmed young Russian sturgeons into the Danube. But that is not enough to ensure their protection. WWF’s conservation efforts also include reducing pressure on their remaining populations by addressing poaching and ensuring their freshwater habitats are healthy.
#14 Splashing tetra
The Splashing tetra, Copella arnoldi, is an extraordinary fish species – it lays its eggs on land! At the water's surface, females and males lineup side by side and jump together out of the water to spawn. Females lay their eggs on hanging leaves, and males fertilize them immediately. The male is then tasked with keeping the eggs moist by splashing them with its tail for around three days until they hatch and fry fall into the water. The unique breeding behaviour and parental care can be seen in the Orinoco and Amazon rivers in South America - this species native range. They have high commercial importance as ornamental fish in the region. According to the Brazilian Red List of Threatened Species, the Splashing tetra is listed as Least Concerned. However, the last assessment was made in 2014, and their freshwater habitat is under pressure mainly from hydropower dams.
#13 Green hydra
The Green hydra, Hydra viridissima, is a freshwater species of cnidaria – a “relative” of the jellyfish. Green hydras are no more than 3 cm long and green due to the presence of symbiotic Chlorella, algae known to provide a significant nutrition source to cnidarians. Found at depths from shallow waters to 60m, Green hydras are widely distributed throughout the world’s ponds, rivers, and slow-flowing parts of streams and rivers. These tiny hydras are solitary polyps without the medusae phase, and their bodies only have two layers of cells. Still, they have regenerating capabilities - when subjected to pressures in the environment, they may lose body parts like their tentacles and later build them back up. Some scientists believe that they might even live forever under the right conditions – in the wild, they face disease, predators, and water contamination. Research shows that they don’t show any signs of deteriorating with age, unlike most multicellular species. They are sensitive environmental indicators and play an essential role in freshwater food webs, especially as predators when abundant. Green hydras use their tentacles to catch prey, which has stinging cells called cnidocytes that immobilizes it. Curiously, hydras only have a mouth when it’s time to feed – their mouth is not a permanent opening, vanishing after feeding. Between the ring of tentacles, a small mound called hypostome has a group of cells morphologically distinct. To open and close its mouth, these cells change shape, becoming longer and narrower to widen the gap and returning to their original form to close it – a similar process to when the pupils in your eyes dilate and constrict.
#12 Singapore freshwater crab
Singapore freshwater crabs, Johora singaporensis, are smaller than the size of a thumb, growing 2 to 3 cm. Endemic to Singapore’s freshwater habitats, this species hides under rocks along river edges and inside clumps of leaves and detritus. They feed on plant detritus and oligochaete worms present in the soft stream's mud. Listed as Critically Endangered by the IUCN Red List of Threatened Species, this species is sadly among the world's most threatened species. They prefer relatively clean and fast-flowing streams in the highlands, which are at higher risk due to habitat loss and habitat acidification. Conservation efforts include measures to increase this species' wild populations, led by the Freshwater Crab Working Group in Singapore. NParks (National Parks Board) of Singapore has successfully hatched more than 150 crablets in captivity and has begun to release these captive-born individuals into the wild. In 2018, in partnership with Singapore’s National Parks Board (NParks), WWF highlighted 10 threatened species in Singapore currently under active conservation, including the tiny Singapore freshwater crab.
#11 Baikal seal
Baikal seals, Pusa sibirica, are one of the few species of seal restricted to freshwater. Located near the Mongolian border in southern Siberia, Russia, Lake Baikal is the oldest (25 million years) and deepest (1,700 m) lake in the world, containing 20% of the world's total unfrozen freshwater reserves. These freshwater seals are mostly confined to the lake, although they travel short distances into rivers flowing into and out of the lake. They are amongst the smallest of the seals (ranging between 1.1-1.4 m and weights of 50 to 130 kg), and may have the greatest longevity. Approximately 10% of the population is over 20 years old - with records of males reaching 52 and females 56 - and they continue to produce pups until they are 43-45 years old. During winter, they are mostly solitary and spend much time in the deep water under the ice, making and maintaining breathing holes to allow them to feed. Females with pups or that are pregnant are more likely to be on the ice, building dens to protect their offspring from predators. Baikal seals generally dive for a couple of minutes, but they can dive for longer - the longest recorded dives were over 40 minutes during winter. They are threatened by contamination from organic pollutants coming from the growing industrial activity in the Baikal watershed.
#10 Freshwater sponge
Spongilla lacustris is one of the 248 known freshwater sponge species occurring globally. S. lacustris is the most common and widespread freshwater sponge species and the most abundant in Central Europe. It lives in rivers and preferably lakes, often growing attached under rocks or logs. Its colour is mostly white, but it can look greenish due to symbiotic algae. Freshwater sponges are subject to harsh environmental conditions, having developed a dormancy mechanism. When exposed to excessive cold or harsh situations, they form highly resistant gemmules that remain viable after the other part of the sponge dies. Once conditions improve, the gemmules germinate into a new sponge. These ancient invertebrates are sessile (fixed in one place) and filter water to feed on small aquatic organisms like protozoa, bacteria, and other free-floating life. A ﬁnger-sized specimen of S. lacustris can ﬁlter more than 125 l of water in a day. They are prey to ducks, crayfish, and various macro-invertebrates. Sponges play a key role in recycling organic matter and contribute to the primary productivity of freshwater ecosystems. Their presence in an ecosystem can be an indicator of good water quality.
The Piraiba, Brachyplatystoma filamentosum, lives mostly on the soft-bottom floor of the Amazon & Orinoco rivers. The Piraiba is one of the 11,000 species of migratory fish, with records of maximum distance travelled as far as 7000 km. Juveniles float downstream where they grow rapidly. Once they reach adulthood, they migrate upstream to spawn. Due to this migratory behaviour, artificial barriers like dams can obstruct the “pathway” and reduce their ability to complete their life cycle (Learn more about migratory fish with the Living Planet Index For Migratory Freshwater Fish). The Piraiba is an essential environmental, social, and economic resource in the South American basins, and it is threatened by over-harvesting. Hence, it is imperative to conserve this species, its fishery, and its rivers - learn more about this and other beautiful important fishes with the World's Forgotten Fishes report. And check out this video to see the Piraiba swimming.
Native to the Amazon, the Arapaima, Arapaima gigas, is one of the largest freshwater fish globally - it can grow up to 3m, but nowadays it’s rare to find a specimen more than 2m long. Its Brazilian name, 'Pirarucu,' comes from two indigenous words – pira means fish and urucum means red, referring to the reddish marks towards the tail-end. During the mating season, males' red colours become more intense. After pairing, males clean the river bed with their jaws by pulling out any weeds or obstacles in a selected area, where they dig a puddle for the female to lay its eggs and fertilize them. The female stays nearby during the incubation period, while the male circles a bit further to scare off any predators. Arapaimas have gills and a modified respiratory swim bladder which works like a lung – they need to return to the surface every 20 min to breathe air, making them easy to catch. They are economically and culturally important to many traditional communities. Climate change effects, habitat degradation, and increased demand on fish stocks put the Arapaima and livelihoods at risk. Local communities are leading the management and conservation of their resources with the support of the government and multiple organizations, including WWF. Check this video to see what local fishers say about the Arapaima and WWF-Brazil's work to support them. Learn more about this and other valuable freshwater fishes with The World's Forgotten Fishes report.
#7 Asian water monitor
Meet the largest aquatic lizard and the world’s second largest-lizard - the Asian water monitor, Varanus salvator, which can reach more than 3 m in length and 25kg. No lizards are strictly aquatic, but this semi-aquatic and opportunistic species lives in various water systems; mangroves, swamps, and wetlands at altitudes below 1,000m are its most important habitats. Frequently seen on river banks and swamps, this amphibious species can dive underwater to forage and/or to escape predators. They are remarkably widespread throughout southern and Southeast Asia – researchers believe their extensive distribution and adaptability could be due to their ability to swim and cross large stretches of water and their generalist diet. Asian water monitors will eat about any animal they think they can consume - fish, juvenile crocodiles, tortoise - there are even records of them scavenging human corpses! Regularly harvested and traded as a pet and for their meat, skin, and medicinal value, this species is the most intensely traded lizard species worldwide. Between 2010-2018, the two major source countries, Malaysia and Indonesia, exported more than 2.3 million skins of wild-caught specimens, mainly for the fashion industry. It is listed as Least Concern by the IUCN Red List of Threatened Species, but their population trend is unknown (last assessed in 2009).
#6 LOWLAND TAPIR
The Lowland tapir, Tapirus terrestris, has a wide geographic range - from northcentral Colombia and east of the Andes, throughout most of tropical South America. Mostly found in the tropical lowland forests and wetlands of South America, they are often seen close to water which they use as a refuge against predators as well as a place to forage or cool off. This species is known for being a strong swimmer - able to cross rivers and even dive to feed on aquatic plants. Lowland tapirs can reach the size of a small pony, growing up to 1.8m and 225kg. They are primarily solitary, using their large teeth to grind up plants, fruits, and seeds at night and resting or hiding during the day. This species is listed as Vulnerable on the IUCN Red List of Threatened Species. Main threats to this species are habitat loss due to deforestation, hunting for meat, and competition with livestock. Due to their long gestation period and generational time, tapirs are more ecologically prone to impact from prolonged hunting activity, making population recovery more difficult.
#5 Dalmatian Pelican
The Dalmatian pelican, Pelecanus crispus, is one of the biggest birds in the world and the largest of the pelican family, ranging from 1,60 to 1,80m. This huge pelican feeds mostly on fish and is widely distributed – it occurs mainly in wetlands, where it breeds in fragmented ranges from eastern Europe to east-central Asia. It nests amongst aquatic vegetation on floating or stationary islands isolated from the mainland, avoiding mammalian predators. Dalmatian pelicans are dispersive in Europe and migratory in Asia. When migrating, large lakes and wetlands form important stop-over sites. Their population suffered massive declines in the past, mainly due to wetland drainage and fishers' persecution. Disturbance from tourists, wetland alteration and destruction, water pollution are among other continuing threats. However, Thanks to conservation measures, populations increased in Europe, being downlisted from Vulnerable to Near Threatened by the IUCN Red List of Threatened Species. Nevertheless, as this species currently presents a decreasing population trend and the ongoing threats in much of their breeding and migratory range, conservation efforts must continue. The Dalmatian pelican is one of WWF’s Asian Flyways Initiative priority species. One of the aims of the Initiative is to improve ecological connectivity in the flight path of this and other migratory bird species.
#4 Diving bell spider
The Diving bell spider, Argyroneta aquatica, is the only spider to live underwater for most of its life. This Eurasian species inhabits eutrophic lakes, ponds, marshes, swamps, and slow-moving streams. To breathe underwater, their posterior body part called opisthosome is covered by fine hairs that trap air, being covered by a silvery envelope when they dive. They use plants from the dense aquatic vegetation to anchor their underwater silk webs. With their tiny body hairs, they collect air from the surface and release it in the web, creating an air-filled bubble - the ‘diving bell’, where the air is held by the surface tension between the silk fibres. The diving-bell works as an air deposit and as a physical gill - it absorbs dissolved oxygen from the water to meet the spiders’ metabolic oxygen requirements. They also use the bell to feed - after capturing the prey (generally aquatic invertebrates and small fish) by injecting venom into its body with their chelicerae (fangs), they carry it to the bell to consume it. Females use the bell to lay their eggs – they place an egg-filled cocoon in the upper region and lay on it for protection until hatching. The offspring only leave the bell once they have developed the fine hairs to build their first tiny diving bell.
#3 Giant floating fern
The Giant floating fern, Salvinia molesta, is a free-floating perennial aquatic plant adapted to stagnant or slightly flowing freshwater systems. Sadly, even though this species is native to south-eastern Brazil, humans have spread it in almost all tropical-subtropical regions, becoming problematic in many areas (southern USA, Australia, South-East Asia, the Paciﬁc and south, central and eastern Africa). But floating species like those belonging to the Salvinia genus possess unique features that fascinate scientists and boost innovations. Their leaves present a "superhydrophobic" (i.e., extremely water repellent) behaviour - when immersed, the leaves wrap themselves in an air jacket and remain fully dry. Salvinia's surface also has an affinity for oil, which is, in a way, a drawback of super-hydrophobia. Such features have inspired studies in Biomimetics. Dr. Wilhelm Barthlott, Professor Emeritus at the University of Bonn, created a textile prototype based on Salvinia's properties to efficiently remove oil films from the water's surface without using chemicals or external energy supply (check out this video).
#2 Arafura File Snake
The Arafura File snake, Acrochordus arafurae, is one of the 153 known species of freshwater snakes globally. You can find this species in northern Australia and Papua New Guinea - swimming in rivers, lakes, lagoons, floodplains and their associated wetlands. The Arafura is adapted to freshwater life, being considered an entirely aquatic snake – they forage in water and cannot survive without aquatic prey. Arafura File snakes are nocturnal and non-venomous. They feed exclusively on fish, which they immobilize by constriction - as they are agile and their skin flexible (not bendy) they can easily capture and hold onto their fish prey. They have nostrils on top of their head so they can breathe whilst underwater. Listed as Least Concern by the IUCN Red List of Threatened Species, this species populations viability depends on their freshwater ecosystems health.
#1 Common hippopotamus
The Common hippopotamus or hippo, Hippopotamus amphibious, is a well-known mammal restricted to the rivers, lakes, and wetlands of sub-Saharan Africa. Hippos love water – as their name suggests, they have an amphibious lifestyle. They spend the day mostly in the water and in groups - herds range from the tens to a hundred – only emerging at night to feed on grass and fallen fruit. Due to their dry skin, Common hippos rely on water sources to remain moist and cool. With their eyes and ears on the top of their heads, they can watch for predators while lying low in the water. They are the only mammals known to produce an amphibious call – their loud vocalization can be transmitted simultaneously through land and water. Hippos fight for territory can be brutal, in which they use their huge ivory canines. Smaller conflicts, however, are generally settled by threat displays like the famous "yawn". Listed as Vulnerable by the IUCN Red List of Threatened Species, hippos’ wild populations have decreased 30% in the last decade, and some are regionally extinct. Losing hippos could have devastating consequences for African freshwater ecosystems, as they play a key role in transporting nutrients between the land they graze and the water in which they defecate. The main threats hippos face are habitat loss and degradation and illegal and unregulated hunting for meat and their ivory teeth. They are also threatened by drought, like in the Mara River basin, shown in WWF’s report and its consequences to biodiversity highlighted in this article with WWF’s William Ojwang as an interviewee. WF Hong Kong is also working on hippos conservation, as Hong Kong is the single-biggest buyer of hippos’ valuable teeth. WWF is shedding some light on this issue by raising awareness and advocacy (read this great blog from WWF’s Thomas Gomersall to know more).
The Taimen, Hucho taimen, is the largest salmonid in the world - reaching lengths up to two metres. But size is not their only remarkable feature - they are top predatory species known for hunting prey in packs, earning them the nickname “River Wolves”. They also have a peculiar diet, which mostly includes fish, but also the occasional rodent and bird. Unlike many salmonids, this migratory fish is potamodromous – it spends its whole life in freshwater, in large, cold, and swift-flowing rivers, traveling upstream to smaller tributaries to spawn. But it takes time for them to grow up – Taimens take up to seven years to reach sexual maturity, and they can live for more than 30 years. This species occurs in Europe and Asia, including parts of the Caspian and Arctic drainages in Eurasia and portions of the Pacific drainage in Mongolia, Russia, and China. However, this species’ numbers decreased from much of their range, being listed as Vulnerable on the IUCN Red List of Threatened Species. Due to threats such as pollution, habitat loss, damming, and climate change, this species suffered substantial (50%) population declines since 1985. Significant populations remain only in Russia and Mongolia. Mongolia harbours part of the core habitat of one of the last healthy taimen populations in the world. These populations are primarily threatened by fishing due to the high levels of unregulated, illegal practices throughout the country. The Mongolian government has taken significant action since 2012 with protective legislation. Together with other NGOs, including WWF, they are trying to find a balance to curb poaching and promote regulated fishing and its revenue. As part of conservation efforts, WWF-Mongolia works on scientific researches, education campaigns, and public awareness. Take a look at the Mongolian Fish guidebook to know more!
#50 parading shrimp
A long time ago, a group of indigenous people in north-eastern Thailand discovered a distinctive phenomenon called ‘parading’– the synchronous mass migration of freshwater shrimps on land. Found in rivers and streams of Asia - Viet Nam, Yunnan (China), Laos, Thailand, and Peninsular Malaysia - Macrobrachium dienbienphuense is one of the species known to parade annually. During the rainy season, these adventurous shrimps leave the water collectively at night and walk upstream on land before returning to the river. But why would they leave the water? Recent research conducted by Watcharapong Hongjamrassilp in Nam Yuen district (Ubon Ratchathani province, Thailand), tested the hypothesis that parading was associated with spawning and breeding – and it is not. Evidence shows that the shrimps might leave the water to escape strong currents. Most of the parading shrimp were juveniles – larger adults can face a stronger current, being less likely to parade – and not ready to reproduce. They walked from 5 to 20 meters - depending on river velocity and riverbank structure - and some even stayed out of water for 10 minutes or more. To take in oxygen while on land, shrimp stayed within the rivers splash zone (20–40 cm from the river’s edge), and their exoskeleton seemed to trap water around their gills (similar to a reversed dive helmet). This mass migration is such a fascinating scene that it has been promoted as an ecotourism event in Thailand since 1999, drawing more than 100.000 visitors annually. Even though they are listed as Least Concern on the IUCN Red List of Threatened Species, recent evidence suggests that they are decreasing in number and body size.
#49 water boatman
You can likely find the tiny water boatman, Micronecta scholtzi, in the bottom of ponds and streams, rowing with their paddle-shaped legs. M. scholtzi is a species of lesser water boatman found in Europe. Micronecta’ means small swimmer, but the only thing small about them is their size. Despite being only 2 millimetres long, this aquatic insect can produce the loudest sound of any animal relative to its body size. Bigger animals generally create the loudest sounds – mechanically speaking, a small sound source cannot generate a high-level sound output. In terms of sheer decibels, M. scholtzi loses to whales and elephants. Yet, this little creature can still produce a sound loud enough to be heard from a riverbank several meters away, propagating it across the water-air interface. Only males can sing – they generate an extremely loud courtship call to attract females, often in a synchronized chorus. And how do they sing? Scientists believe they rub their penis against ridges on their abdomen and that layers of air around their body amplify the sound. Water boatman specimens trap air reserves through microscopic hairs as they dive to help them breathe. Such a sound-producing genitalia mechanism is relatively rare within the animal kingdom.
The iconic jaguar, Panthera onca, is the largest cat in the Americas. From Mexico to Argentina, jaguars typically inhabit regions with dense forest cover and water bodies, living near lakes, rivers, and wetlands. They are more strongly associated with water than any other Panthera cats. This majestic species is a confident swimmer known to cross large rivers, and it preys on aquatic species like turtles and even caimans. Thanks to their robust canines and a large head, jaguars can take down prey three to four times their own weight and bite through a crocodile skull or turtle shell – having the most powerful bite than any other big cats. Therein lies its name – ‘jaguar’ comes from ‘yaguar’, an indigenous word that means 'he who kills with one leap'. Sadly, they have been nearly eliminated from half of their historic range, with regional extinctions in El Salvador and Uruguay. Their preference for freshwater habitats - even within drier areas, they are only found around the main watercourses – quickly brings them into human-wildlife conflict, as they share the same resource as the expanding high-intensity agriculture and mining. Illegal wildlife trade, and habitat loss and fragmentation are other threats leading to the decreasing population of jaguars, listed as Near Threatened by the IUCN Red List. Their stronghold is in Brazil – in the Amazon region and the world’s largest tropical wetland, the Pantanal. However, the recent fires in both biomes magnify the threats to these populations - especially in the Pantanal, where a park known to hold the biggest concentration of jaguars globally has had 85% of its territory burned.
#47 GREEN ANACONDA
To say that the Green anaconda, Eunectes murinus, is fascinating is an understatement. They belong to one of the most remarkable Neotropical freshwater snakes groups - the Anacondas - known for their constricting abilities and as one of the world’s longest snakes. Green anacondas stand out as the heaviest snake – it can weigh up to about 250kg! They also have a “heavy” diet, including Earth’s biggest rodent (Capybaras - see below #39) and the occasional crocodilian (Caimans). This species is widely distributed through the swamps, marshes, and slow-moving streams of South America. Green anacondas’ eyes and nasal openings are on the top of their head, which allows them to “hide” as they stay completely submerged in the water while waiting to ambush their prey. With a lethal squeeze that halts breathing and blood flow, prey is killed and then swallowed. Green anacondas also form “breeding balls” – slow-motion wrestling matches of a dozen suitors for one receptive female, where they simultaneously coil around her until one of them gets lucky. The impregnated female retains the eggs, and her offspring emerges from the eggs before leaving her body. She then gives birth to two to three dozen live young, which are already about two feet long and almost ready to swim.
#46 Elephant fish
Campylomormyrus curvirostris has a very long trunk-like snout and occurs exclusively in water bodies within the Congo River basin. This strange-looking fish is a species within the family of freshwater weakly-electric fishes, the Mormyrids. This - mostly - nocturnal family is known to use their electric sense to communicate, actively find objects and food, and discriminate between fish from other species and their own. A specialized electric organ in the tail base generates a mild pulse-like Electric Organ Discharge (EOD). The electric pulse helps them to create a picture of their surroundings, in a similar way bats and dolphins use echolocation. C. curvirostris belongs to a particular group within their Mormyrids family – the Campylomormyrus –, which scientists say to have EOD play a key role in pair formation and mate recognition. The species in this group show remarkable differences in terms of EOD shape and duration, which enables them to perceive differences in the EOD generated by their peers.
#45 Giant Otter
As the name conveys, the Giant otter, Pteronura brasiliensis, is the largest otter species in the world - reaching lengths up to 170cm-180 cm. They live in large and slow-moving rivers, streams, lakes, and swamps throughout South America’s lowland rainforests and wetlands. Giant otters are very social, living in large family groups that do everything together – sleeping, hunting, and playing. Generally, a monogamous pair that mate for life leads the group formed by their offspring. As they are diurnal, noisy, and hang out in groups, they can be easy to see - making them highly susceptible to persecution. Giant otters were nearly hunted to extinction for the pelt trade between the 1940s and mid-1970s and lost from much of their southern and easterly range, including Uruguay, Argentina, and several states of Brazil. Besides poaching, they currently face threats such as habitat loss, pollution, and diseases introduced by domesticated dogs. The Giant otter is listed as Endangered by the IUCN Red List, with a decreasing population trend and a discontinuous and fragmented range due to local extinctions. Important subpopulations can be found in the Pantanal region and in the Amazon, including in Peru, where the otter biologist, Jessica Groendikj, worked on the individual identification of the Giant otter. One of this species' distinctive features is the pale markings on its neck - as unique as your fingerprints - making them identifiable from birth.
#44 Vagrant darter
The Vagrant darter, Sympetrum vulgatum, is a dragonfly species found in all kinds of standing, largely unshaded waters. Larvae of almost all 5952 species of dragonflies and damselflies depend on freshwater habitats, spending most of their lives in this stage (generally one or two years) in the water. The nymphs live on the bottom of still and slow-ﬂowing waters. To fully grow into an adult, the larvae catches and eats live prey at every opportunity. Both larvae and adults are predators, playing a signiﬁcant role in the cycle of materials both in aquatic and terrestrial ecosystems. The Vagrant darter is a rare migratory species with a wide distribution – you can find this species eastwards to China and Japan, in Central and eastern Europe, and is likely common in large parts of its Asian range.
#43 Sockeye salmon
For the Sockeye salmon, Oncorhynchus nerka – size doesn’t count. Even though it is among the smaller of the seven Pacific salmon species, the bright-orange Sockeye salmon migrates thousands of kilometres during their lifetime from rivers to the ocean and back again. They hatch in gravel nests in rivers or lakes of Russia, Canada, and the United States and stay in freshwater for one to three years before migrating to the ocean. They remain in the sea for one to three years until maturity - when they are ready to go back to their natal freshwater habitat, where they spawn and die. Scientists think that salmon navigate back to their natal rivers by following the geomagnetic drift, along with visual, olfactory, and environmental cues from currents, salinity, water temperature, and freshwater inputs from rivers. As with other salmonids, the Sockeye needs clean, cool, and highly oxygenated water for their sensitive eggs to thrive. Therefore, a healthy population of salmon acts as a good indicator of river health. Recently, the Living Planet Index for Migratory Freshwater Fish highlighted the 76% collapse in freshwater migratory fish populations around the world since 1970 due to overexploitation, river dams blocking their migration routes, and habitat degradation. Sockeye salmon is listed as stable by the IUCN Red List of Threatened Species; however, some subpopulations are declining, and there are cases of regional extinction. WWF conservation efforts focus on protecting salmonids and their habitats through initiatives such as Free-Flowing Rivers and programmes in the Arctic, where we work with local communities.
The (Duck-billed) Platypus, Ornithorhynchus anatinus, is one of the most peculiar animals on the planet. This species uniqueness starts with the order of mammals to which it belongs - monotremes, mammals that lay shell-covered eggs but nurse their young. With its duck-like bill, webbed feet, a broad flat beaver-like tail, and an otter type of body and waterproof fur, this distinct mammal is adapted to and dependent on freshwater ecosystems. Hunting underwater, these bottom feeders have skin folds to cover their eyes and ears to prevent water from entering, and the nostrils close with a watertight seal. They find their invertebrate prey by detecting electrical signals through sensitive receptors on their bill, storing the catch in their cheek pouches until they reach the surface for consumption. They are also among the few venomous mammals – males have sharp stingers on the heels of their rear feet, using them most likely to settle territorial disputes. But don’t worry - although excruciating, the toxic blow is not fatal to humans, but known to kill mice, dogs, and other platypuses. Nowadays, you can only find the Platypus in the streams, lakes, and lagoons of eastern Australia and Tasmania – it was extinguished from much of its range due to hunting, habitat degradation, river fragmentation from dams, and entrapment or entanglement in ﬁshing gear.
#41 Sulawesi Cardinal shrimp
The colourful Sulawesi cardinal shrimp, Caridina dennerli, is only known from Lake Matano in Indonesia (part of the Malili Lakes system - Sulawesi). However, the vivid colours of this freshwater shrimp are fading from the lake - they have not been recorded since 2013 and are listed as Critically Endangered by the IUCN Red List of Threatened Species. Reinforcing this trend, surveys conducted in 2017 and 2018 recorded not a single specimen in the lake, not even from sites where they were abundant before. Hence, this species is now considered Possibly Extinct in the Wild, being among the 1 in 3 freshwater species threatened with extinction. The Sulawesi shrimp is currently under the threat of nickel mining and hydropower installations on the outlet of Danau Matano, which impacts outflow and natural water level fluctuations. The rapidly expanding human population, erosion around the lake, capture for the pet trade, and direct predation of invasive species aggravate the problem. IUCN’s conservation efforts on the Lake Malili system aim to raise awareness and improve conservation and sustainable use by assessing freshwater biodiversity status and distribution. Shoal - an organisation dedicated to the conservation of freshwater species - is working on a Conservation Action Plan in Wallace’s dreamponds to step up the conservation sufficiently to a scale that will allow the endemic species to survive and recover. You can also learn more about the impacts on freshwater species and habitats with WWF’s Living Planet report 2020 and what you can do to help.
#40 Oriental darter
The Oriental darter, Anhinga melanogaster, is a waterbird known for its ability to swim to catch fish and other underwater prey. Found in Southern Asia, this beautiful bird lives in shallow inland wetlands, including lakes, rivers, swamps, and reservoirs. With their webbed toes, darters swim with their bodies submerged, while their long and sinuous neck remains visible above water. This behaviour display gives darters a snake-like appearance – accounting for their colloquial name “snakebird”. When foraging, they move slowly or hang motionless in the water, spearing fish with their dagger-like bill. Adapted to diving in shallow waters, darters are renowned for their extremely low buoyancy. Compared to other closely-related water birds like cormorants, they have uncommonly wettable plumage, thin and less spongy skin, denser bones, and smaller air sacs. Like other water birds, they are important bioindicators for the ecological conditions and health of wetland ecosystems. The Oriental darter is listed as Near Threatened by the IUCN Red List of Threatened Species, being regionally extinct in Thailand. Their populations are decreasing due to pollution, drainage, hunting, and the collection of eggs and nestlings. As they warrant conservation attention, Oriental darters are harboured by WWF’s Orissa Semi-Evergreen Rain Forests ecoregion.
The capybara, Hydrochoerus hydrochaeris, is the largest rodent on Earth. This mammal depends on freshwater ecosystems, occurring only in a habitat close to water like marshes, estuaries, rivers, and streams. They are adapted to a semi-aquatic life, using water for mating, to escape from predators, and to eat aquatic plants. Due to their dry skin (their sweat glands are not well developed), they stay in the water or under shade – especially in the afternoon - to regulate their body temperature. Capybaras have toes partially webbed to paddle around and eyes, ears, and nostrils on the top of their head to keep an eye on their surroundings. They are fundamentally social animals - female capybaras raise the offspring in groups and cooperate in their nursing. Broadly distributed, you can find capybaras mostly in South America - Colombia, Venezuela, Ecuador, Peru, the Guianas, all of Brazil, eastern Bolivia, Paraguay, Uruguay, and north-western and eastern Argentina. Their primary threat is hunting for meat and leather, with some local populations extirpated. The most recent threat is the gargantuan fires in the Pantanal – the world’s largest wetland and one of the habitats with the highest density of capybara (learn more about WWF’s actions against the fires here).
#38 Nile Crocodile
The Nile crocodile, Crocodylus niloticus, is an aquatic predator that can reach up to 6 meters. Found in eastern, central, and southern Africa, this reptile giant inhabits a wide diversity of water bodies (rivers, lakes, swamps, and coastal estuaries), including the iconic Mara river basin. WWF’s recent report on Mara’s freshwater biodiversity identified the Nile crocodile as one of the four freshwater reptiles in the basin shared by Kenya and Tanzania. This species plays a key predatory role in the perennial river, which is the only water source for migrating wildlife in a drought year. They mostly feed on fish but can attack zebras, small hippos, porcupines, birds, and even other crocodiles. This impressive animal presents a caring nature as a parent – females ferociously guard and defend their nests until the eggs hatch, often helping hatching babies emerge with their teeth and carrying them inside their mouths to the water. This species also presents other sophisticated behaviour like complex communication and coordinated group hunting, being considered highly intelligent. If you ever thought that a crocodile has a dinosaur-like feature, you were not far off - crocodilians are more closely related to dinosaurs and birds than to most reptiles. Between the 1940s and 1960s, the Nile crocodiles populations were depleted throughout much of their range due to hunting. Thanks to the protection afforded by national legislation and international trade conventions (CITES), populations have recovered in many regions.
#37 blue-spotted Mudskipper
The blue-spotted mudskipper, Boleophthalmus pectinirostris, is one peculiar fish. It belongs to a group known for its incredible ability to live in and out of water - the amphibious “gobies” fish. Thanks to adaptations like their strong pectoral fins, they can move on land in a way called “crutching” – similar to a human walking with crutches. They can also jump and climb up streams and waterfalls through their partly or entirely fused pelvic fins that function as a strong sucker. To breathe out of the water, mudskippers use their skin and chambers in their modified gills to trap air. Found in Northwest Pacific, the blue-spotted mudskipper lives in freshwater, brackish, and demersal habitats, including mudflats in Mai Po Nature Reserve’s wetlands in Hong Kong. During high tides, they stay in a burrow in the mud. They “crutch” around and browse the mudflats at low tide, being so numerous that the mud can almost seem alive. Males commonly display dramatic territorial and mating behaviours, like facing each other head-on and raising their dorsal fins as threats or to attract females. Sadly, illegal fishing and habitat degradation threaten their populations (learn more with this great read and check them out in this video).
#36 pink Amazon river dolphin
“Once the sun goes down, the boto morphs into a handsome young man dressed in white to seduce young women of local villages, leaving them impregnated before the sun comes up to return to the river once again as dolphins.”
That’s not the only myth surrounding the pink Amazon river dolphin, Inia geoffrensis. Also known as “boto cor-de-rosa” or boto, you can find this mystical animal throughout the Amazon and Orinoco river basins in South America (strictly speaking, also in the Araguaia basin). Unlike other river dolphins, the boto has a flexible neck that moves its head left and right to maneuver through the flooded forests for food and shelter. With their long snout, they feed on fish and other aquatic organisms like turtles and crabs. Locals believe that botos possess magical powers. If you swim alone in the river, you could be whisked away by the boto to “Encante”, a magical underwater city. For many tribes, including WWF, the boto is sacred. Listed as an Endangered species by the IUCN Red List, the magical boto can’t escape threats such as dams, mercury poisoning due to gold mining, and being killed in fishing nets or to be used as fish bait. Through initiatives such as River Dolphin Rivers and South American River Dolphin (SARDI), WWF works with local communities, governments, and other NGOs to develop regional and innovative approaches to protecting this species and their habitats. Since they are indicators of environmental river health, the pink river dolphin was chosen as the Ambassador of the Amazon rivers! With such seductive powers, that was an easy call.
#35 Piraputanga fish
The Piraputanga fish, Brycon hilarii, is renowned for its leaping ability. Endemic to the Upper Paraguay river basin, they play an important (long-distance) role in seed dispersal in the Pantanal region as they perform long pre-spawning migrations along all river drainages. The Piraputanga uses its scissor-like teeth to feed on fruits that fall in the water. They can also leap up to one (1) meter above the water to snatch fruits (and insects) from the riparian vegetation! This remarkable behaviour is a common attraction for tourists, who bait fruit above the water to see the beautiful Piraputanga leap. The Piraputanga also captivates its audience with its colour – when the light reflects on the water, the scales of Piraputangas showcase a flaring contrast of their yellow body, orange-colored fins, and red tail. They are also an essential source of income for local communities through recreational diving and fish watching, as well as artisanal or sport fishing. Sadly, the destruction of riparian forests and construction of dams negatively impacts aquatic ecosystems, threatening seasonal migratory movements of frugivorous fishes. Overfishing or other anthropogenic disturbances (like pollution) might also lead to the local extinction of fish–plant interaction, which may negatively affect the riparian forests in this region and have far-reaching implications on ecosystem services livelihoods.
#34 raft spider
Meet the European spider of the year: the raft spider, Dolomedes fimbriatus, one of the largest native spiders in the region. Found mostly in moorland and wetlands, they inhabit water margins of ditches, ponds, and slow-flowing streams, showing remarkable adaptations to this habitat. This free-living hunter spider moves skilfully on the water surface, overpowering its prey without using a web. They sit on emergent water plants or the water surface in a hunting position, waiting for unaware (aquatic) insects, tadpoles, and small fish. Thanks to its thick coat of body hairs, the raft spider can lay its whole body on the water, relying on surface tension to remain above the water. But when in danger, they can dive underwater and come out dry – an air bubble forms around their body, which bursts after they go back to the water surface. Raft spider females constantly carry their spherical egg-sacs (with up to 1000 eggs!) in their mouthparts called chelicerae (pair of appendages in front of the mouth). When the eggs are close to hatching, the female hangs the eggs’ cocoon in vegetation near the water’s edge, guarded and enclosed by silk threads. Unfortunately, the Euro-spider of the year is becoming rarer and more vulnerable, due to the destruction of its habitat – building work on river banks and removing reed beds and water lilies - and climate change.
#33 Harvest mouse
The super cute Harvest mouse, Micromys minutus, loves a wetland! Especially in Europe, where they are quite common in wetlands. Harvest mice make their nests in the reeds or along rivers and canal edges, and feed on seeds, green vegetation and insects. This species of rodent is an extremely active climber and has an acute hearing - they react sharply in response to rustling sounds up to 7m away, freezing or dropping into cover. But they can’t always react fast enough – their numbers have been noted to decline in many parts of Europe, particularly local populations, due to loss and degradation of wetland habitats. In Sussex, England, the Ouse and Adur Rivers Trust created the project Helping Hands for Harvest Mice Citizen Science, where as part of a wider river restoration project, local communities have helped researchers monitor harvest mouse populations.
#32 Sacred lotus
To shine some light on these trying times, we bring you the Sacred lotus, Nelumbo nucifera. This perennial aquatic plant has been revered in many cultures for thousands of years, mostly as a symbol of purity. Its name derives from its significance in the religions of Buddhism and Hinduism: a plant that rises from a humble origin, rooting itself in the muck below and ascending high above the water untainted. The leaves can sit directly or above the water, and the distinguished lotus flower blooms gloriously above the leaves. The Sacred lotus’s uniqueness is in all parts of the plant - the roots, stems, leaves, and seeds have all been used as a food source and in traditional medicines, especially in Southeast Asia. Also known as the Asian lotus, this species has also intrigued the scientific community for presenting remarkable features such as leaf super-hydrophobicity (i.e., water repellence) - known as the “Lotus-Effect” -, and seed longevity. Their leaves have a special waxy coating that sheds water and dirt, suggesting that the Lotus-Effect may play an important role in protecting plants against pathogens and contaminations. Lotus fruits were found buried underground over 1300 years in China could still be germinated, being seed longevity another fantastic feature of this sacred aquatic plant.
#31 Great diving beetle
The Great diving beetle, Dytiscus marginalis, is an aquatic insect generally found in still waters with dense vegetation. These ferocious carnivores can grow up to 44mm feeding on arthropods, tadpoles, newt larvae, and juvenile ﬁshes. With dorsoventrally compressed and oval bodies perfectly adapted for swimming, diving beetles propel themselves through their legs’ synchronous strokes covered with swimming hairs. As diving beetles mostly breathe atmospheric air, they carry a bubble of air under their wings when swimming under the water surface – quite like scuba divers! Likewise, they also have to come to the surface occasionally to replenish the oxygen in the bubble. When resting underwater in well-oxygenated streams, this air bubble can function as a physical gill capable of absorbing dissolved oxygen from the surrounding water.
#30 Mekong Giant Catfish
Able to reach a length of up to 3m and over 300kg, the Mekong giant catfish, Pangasianodon gigas, is one of the world’s largest fish! Native and endemic to the Mekong River, this species can live to be 60 years old. These migratory fish swim around Vietnam, Cambodia, Laos, and Thailand – most likely found in the Lower Mekong river basin. Sadly, they are one of the most endangered fish in southeast Asia. Listed as Critically Endangered by the IUCN Red List of Threatened Species, this species faces overharvesting for local and commercial fisheries, habitat loss and pollution, intentional hybridization for aquaculture, and bag-net fishery of Cambodia. In the last 50 years, the total number of fish declined by 90%. This river giant is a highly-valued food resource, not because it is a delicacy but because it has a god-like status - some believe it brings good luck forever once eaten, bestowing the gifts of wisdom and long life. Intentional fishing is currently banned in most areas, but this species is still harvested incidentally as by-catch. As the Mekong giant catfish most probably migrates long distances, dams’ construction is also a threat. Dams likely prevent them from reaching their spawning site and may cause extinction - there is only one known spawning site in Thailand, and the blockage of this waterway by two dams might be their demise.
#29 American beaver
The cuddly-looking American beaver, Castor canadensis, is one of the two species of beavers in the world. The North American native has darker fur and a shorter snout than its Eurasian “cousin”, but both are stars when it comes to their constructions - beavers are second only to humans in their ability to change and manipulate their environment. Living in areas of persistent water like lakes, ponds, and streams, beavers build dams and dome-shaped homes with branches and mud to protect themselves and their offspring against predators. The dams block streams and turn land into large ponds, facilitating the floating of food and building material to their homes - often strategically placed in the middle with only underwater entrances. Beavers’ leathery tails storage energy (fat) and their characteristic large front tree-razing teeth never stop growing, which they control by constantly gnawing on wood. Due to their diet of leaves and bark, to some people, beavers’ butts smell like vanilla! To waterproof and scent mark themselves, they use a special grooming paw on their hind foot to distribute throughout their fur castoreum oil - a chemical compound often the combination of castor and anal gland secretions, plus urine (watch this video to learn more about them).
#28 Ywangan crocodile newt
New species are often discovered in the biodiversity hotspot region of the Greater Mekong - over 2,216 new species have been found since 1997 (that’s two species per week on average! Check this report for more). Recently, researchers discovered a new species of newt. With a coloration almost solid black, the Ywangan crocodile newt, Tylototriton ngarsuensis, differs from its bright orange cousins, besides having a shorter head, larger size, and breeding later in the year than other closely related species. Found in Ngar Su Village, in Myanmar, this amphibian is so common that the villagers have long known about it. One of the researchers, Lee Grismer, said that “even in populated areas, new species can be found and in doing so underscores the need for field work so we can more accurately catalogue the biodiversity of this planet.” Especially when these species face threats such as habitat destruction, harvest for the illegal pet trade, and medicinal trade. Besides Lee Grismer, this new species was discovered by his colleague from La Sierra University, Marta S. Grismer, Perry L. Wood Jr. (University of Kansas), Evan S.H. Quah (Universiti Sains Malaysia), Robert E. Espinoza (California State University) Matthew L. Murdoch (Villanova University, USA), and Aung Lin (Fauna and Flora International, Myanmar).
#27 Mary River turtle
This week we bring you the mesmerizing Mary river turtle, Elusor macrurus. This captivating species can be found in flowing, well-oxygenated sections of streams in a single river of Australia – its namesake, the Mary river. One of their unique features is having an extremely long tail, which can be as long as 70% of the shell length. But what makes this funny-looking freshwater turtle even more special is its ability to breathe through its bum! The Mary river turtles have a cloacal ventilator – gill-like structures on their cloacas, which allows them to breathe oxygen underwater up to three days. Unfortunately, their unique looks are the reason they are listed as Endangered by the IUCN Red List of Threatened Species. During the 1960s and 70s, thousands of Mary river turtle eggs were harvested for the pet trade, leading to their dramatic population declines. It was their looks, on the other hand, that helped to prevent the construction of a dam in 2009 in the Mary River, which would have put them at even higher risk. The struggle, however, is far from over as the Mary river turtle continues to be threatened by feral animals, cattle grazing and water quality.
#26 Fisherman bat
This week we present a flying mammal that can also fish – the Fisherman bat, Noctilio leporinus. Also known as the Greater Bulldog bat, this fishing bat inhabits tropical lowlands from Southern Mexico to Northern Argentina, foraging over streams, ponds, rivers, and lagoons. The Fishermen bat can swim and take ﬂight from the water, and feeds on aquatic insects, ﬁsh, frogs, and crustaceans. Many bats are associated with freshwater, but ﬁshing bats possess adaptations of their posterior limbs to capture prey. Fishing bats tend to have 1.8 to 3.9 times larger feet, and their digits – ''toes'' - are elongated, ending with hook-like claws. When fishing, these bats fly within 20 to 50 cm of the water surface, echolocating to find ripples that signal underwater prey. Once they notice the disturbance, they fly closer over the water (four to 10cm) with their back legs and feet held straight down, “scraping” the water to gaff fish with their forward-facing claws. With the fish in their claws, they rise in the air and transfer the prey to their mouth. The Fishermen bats are also safe “flyers” – when on a collision course with other fellow Fishermen, one bat "honks" at the other! They can do this by adjusting the frequency of its echolocation call.
#25 Electric eel
To celebrate World Rainforest day, we present this shocking species, Electrophorus voltai. Two new electric eel species were discovered last year in the Greater Amazonia region. One of the new species, Electrophorus voltai, can generate an electric shock up to 860 V, the strongest of any known animal. More than five times the voltage of a standard American wall socket, researchers say that this shock wouldn’t kill a healthy person, although it would definitely not be pleasant and result in a brief muscle contraction, then numbness. Electric eels generally use their electric discharge to detect other fish and paralyze prey. The researchers, however, also found that electric eels seem to coordinate their predatory activity; they surround the fish as a group, release electricity, and kill it. If you want to know more, check out the scientific paper here.
#24 Water scorpion
The Water scorpion, Nepa cinerea, might look like it, but it is not a true scorpion. With its leaf-shaped and flattened body, this intriguing species is actually a water bug – an insect adapted to freshwater ecosystems. Frequently found in the Palaearctic region – a large ecozone including Europe, Asia, northern Africa, and the north and central parts of the Arabian Peninsula – the Water scorpion swims via strong strokes, and crawls and clings to water plants. When at rest, they slyly hide upside-down amongst the vegetation, letting their breathing tube out of the water to catch atmospheric air. They track their prey furtively, approaching the fry of insect larvae with considerably slow, almost inconspicuous movements. As soon as the unaware prey is within some millimetres, the Water scorpion swiftly embraces its victim with its raptorial front legs.
The Axolotl, Ambystoma mexicanum, is a rare and intriguing species endemic to Mexico and one of the few fully aquatic salamanders in the world. They have a deep-rooted cultural significance in the region, but nowadays are found exclusively in Lake Xochimilco, near Mexico City. Their funny-looking (and charismatic) appearance is due to a unique characteristic known as neoteny – the retention of juvenile features, such as external gills, despite reaching sexual maturity. Presenting an astonishing and one-of-a-kind ability to regenerate body organs and lost limbs, Axolotls are on the spotlight of cellular regeneration research – they can even receive transplanted organs from other individuals and accept them without rejection. Sadly, this extraordinary species is listed as Critically Endangered on the IUCN Red List of Threatened Species mainly due to two factors: poor water quality and the introduction of non-native fish species, especially the Oreochromis niloticus (Tilapia) through competition and predation. Within conservation efforts, the work of the Sisters of Immaculate Health stands out – they are skilful Axolotl breeders! To save one of the world’s most endangered and most remarkable amphibians, the amazing nuns partnered with conservationists from Chester Zoo (find out more about their work here).
The flatworm, Crenobia alpina, is not your typical freshwater star species. With two primitive eyespots giving them a charismatic look, this species can grow up to 15mm in length (take a look at them moving here). This harmless flatworm is a predator found most frequently in cold and clear mountain streams. Their preference for cold waters comes with a price in a climate-changing world – a 40-year study shows that this species (as well as other invertebrates) has vanished from Llyn Brianne and much of Wales when the temperature of the streams rose. As the professor of ecology at Cardiff University’s school of biosciences, Steve Ormerod says, “the loss of freshwater organisms is a hidden tragedy, and it’s amazing how much is left to discover”.
#21 Painted terrapin
The striking Painted terrapin, Batagur borneoensis, is a freshwater turtle native to Brunei Darussalam, Indonesia, Malaysia, and Thailand. They present plain grey skin and shell for most of the time, letting their beautiful colours only pop during the mating season from January to June. The males are the ones to go through the transformation and “paint” themselves – their carapace lightens in colour to showcase striking black markings, and their heads turn to pure white with a bright red strip between their eyes. The female terrapins nest in beaches, but instead of the new-borns (a.k.a hatchlings) heading to the open ocean like their sea-going “cousins”, they make their way towards fresh and brackish waters. After reaching their mating age, pregnant Painted terrapins return to the same beach where they once hatched to lay their own eggs - the circle of life! This predictable behaviour, however, is one of the reasons this species is listed as Critically Endangered on the IUCN Red List of Threatened Species – it makes them vulnerable to habitat destruction and poachers who steal the eggs to eat or sell. Learn more about this incredible species and WWF conservation efforts with this great read.
Endemic to Lake Malawi in East Africa, the Chambo, Oreochromis karongae, likes to swim around in loose shoals. Like other fishes of the Oreochromis genus, they present a curious form of parental care called maternal mouthbrooding. During spawning, soon after the male has fertilized the eggs, the mother puts them in her mouth until they hatch – a fascinating strategy to protect their offspring. The Chambo is also very important for local subsistence and commerce, being the most valuable food fish in Malawi. For this reason, they are threatened by overfishing. In the 1990s, the populations of the Chambo collapsed, with a 70% decline within ten years – unsurprisingly ending up listed as Critically Endangered by the IUCN Red List of Threatened Species.
#19 Spoon-billed sandpiper
The Spoon-billed sandpiper, Calidris pygmaea, is a petite wading bird with a unique spatula-shaped bill. With their heads down in shallow and wet meadows, they move their bills side to side to look for food, also using it sometimes as a shovel. This migratory bird has a very specialised breeding habitat found in north-eastern Russia – lagoon spits where they can nest among crowberry plants, dwarf birch, and willow sedges with adjacent estuary or mudflats. During winter, they migrate down the Pacific coast to Southeast Asia – relying heavily on intertidal areas of the Yellow Sea along the way. Unfortunately, their stopover habitats, as well as their breeding and wintering grounds, are being degraded, resulting in the Spoon-billed sandpiper being one of the world’s most endangered birds. They are listed as Critically Endangered by the IUCN Red List of Threatened Species. As a response, the Spoon-billed sandpiper is one of the priority species of our Asian Flyways initiative, which aims to restore and protect an international network of wetlands used by this and other migratory birds populations.
#18 Freshwater jellyfish
Did you know that jellyfish also exist in freshwater systems? Native to the Yangtze River valley in China, the Freshwater jellyfish, Craspedacusta sowerbii, prefers to live in still or slow-moving water bodies such as ponds, reservoirs, quarries, and lakes. Similarly to other jellyfish, they have a complex life cycle that generally includes 5 moments: microscopic podocytes (dormant "resting bodies"), two different larvae phases, polyp phase, and the most recognizable (but least common in the environment) – the hydromedusa (right picture). As opportunistic predators, the Freshwater jellyfish feeds on small organisms that come within its reach, capturing their prey with the (in)famous stingers. But there’s no need to worry next time you take a dip in a lake - the stinging cells are so small that they usually don’t hurt vertebrates.
#17 Lesser Flamingo
This week we celebrate the exquisite and outstanding Lesser flamingo, Phoenicopterus minor. This species is “lesser” only in the sense that it is the smallest of the six species of the flamingo family. They have the biggest population of all flamingos, with two to three million individuals. They are also tolerant to extreme environments – their habitat is hostile for many animals and plants due to high temperatures, salinity, and alkalinity, but the Lesser flamingos are superbly adapted. Living around lagoons or lakes of Africa and India, they walk through shallow waters with their thin, pink legs as they feed - bending their necks downward to filter their food. It is thanks to their diet - rich in carotenoid pigments - that their feathers become remarkably pinkish. The bright pink colouration may function as a signal of parental capability, attracting mating partners – as parents take turns in brooding, one of them needs to look for food efficiently. The Lesser flamingo also shines with their spectacular courtship dance: groups of more than 1000 birds march together with a straight-up posture like soldiers, moving their heads and necks synchronously. These extraordinarily social birds also synchronize other activities such as brooding, to “crowd-protect” their offspring. A recent study even suggests that they form lifelong friendships – making it impossible not to fall for the flamboyant flamingo.
#16 The Waterwheel plant
The Waterwheel plant, Aldrovanda vesiculosa, might seem harmless with its tiny and delicate appearance while free-floating its “rootless-self” in peat-bog pools, lakes, and river deltas. Especially for an unsuspecting small aquatic invertebrate, who gets unwittingly too close to the Waterwheel and…SNAP! It is trapped in their prey-catching leaves. This remarkable and rare carnivorous aquatic plant has a snap-trap system that can be set off inadvertently by their game - the prey stimulates, mechanically, trigger hairs found in the central region of the trap leaves, prompting the two trap lobes to rapidly (as short as ten milliseconds!) move toward each other. Sadly, this unique species is listed as Endangered by the IUCN Red List of Threatened Species, known in only 50 isolated populations across the globe. The Waterwheel plant faces many threats, including eutrophication and degradation of its wetland habitat, driven by unsustainable agriculture and development.
#15 Irrawaddy river dolphin
To celebrate Dolphin Day (April 14th), we present to you the charming Irrawaddy river dolphin, Orcaella brevirostris. With a rounded head and no beak, the three subpopulations of this distinctive dolphin can be found swimming along three rivers in South and Southeast Asia - Ayeyarwady River (Myanmar), Mahakam (Indonesia) and Mekong (Cambodia). Recent estimates indicate that there are around 79 individuals left on the Ayeyarwady, indicating this is a species right on the edge. Nonetheless, less than 300 freshwater Irrawaddy dolphins still live, including in Cambodia and Indonesia. Although they are not directly exploited, these river dolphins suffer many threats, including entanglement in gill nets and illegal fishing activities such as electrofishing - which unsustainably removes their food source. Nevertheless, fishing activities can be done in harmony with river dolphins. In the Ayeyarwady river, the local fisherfolk use a traditional fishing method that portrays a unique relationship with the dolphins, called “Cooperative fishing”. The fishermen “call” the dolphins by tapping a stick repeatedly on the side of the boat. If the dolphins are up to fishing, they chase shoals of fish towards the waiting nets and signalize the location of the prey. Once the fishermen cast their nets, the dolphins snatch the escaping fish. As this extraordinary interaction can be seen only in a few places around the world, many initiatives – such as our River Dolphin Rivers - work on the conservation of both the Irrawaddy dolphin and the fishermen’s way of life.
#14 Surinam toad
The Surinam toad, Pipa pipa, is a fully aquatic amphibian known for its unusual appearance and fascinating – some might argue horrifying - parental care adaptations. You can find this species mostly in the slow-flowing waters of the Amazon Basin in South America, generally hiding under submerged leaf-litter - with their extremely flattened body and triangular-shaped head, they blend in for looking like a mottled brown leaf. This species also exhibits a unique courtship behaviour: after attracting a mate by making clicking sounds, the male toad mounts onto the back of the larger female and embraces her around the waist - if she is receptive, their mating act starts, and the pair displays some aquatic somersaults that can last for more than 12 hours. With the help of the male, the fertilized eggs are rolled up onto the back of the now upside-down female – the eggs sink into the puffy and swollen skin and are almost completely enveloped by it. The pregnant toad carries the developing offspring for three to five months until the “toadlets” are ready to punch their way out of their mother’s skin pockets, in a bizarre scene that can make many people shiver.
#13 Net-spinning caddis
The Net-spinning caddis, Hydropsyche siltalai, is a species of Caddisfly; insects that are aquatic as larvae. They spend up to a year in lakes and rivers around the world before emerging as adults - and graduated engineers. One might think that larvae can’t be up to much, but the Net-spinning caddis larvae are quite busy building their retreat - a fascinating dreamcatcher-like silk structure. The larvae produce silk threads and weave it into fine-meshed nets that filter food particles from the water column while sheltering them. Recently, scientists found that these silk structures might influence local hydraulics – they can reduce flow downstream by 95% and upstream by 17%, besides altering turbulence (for more information, check the scientific paper). Even though the impressive caddis larvae seem to be well prepared to face the world and reach adulthood, recent research showed that they are in sharp decline - vulnerable to changes in climate.
#12 Arctic char
World Water Day was on March 22, and to celebrate our most precious resource, we introduce you to the northernmost freshwater fish in the world: the beautiful Arctic char, Salvelinus alpinus. These circumpolar fish species inhabit cold and clean waters around the North Pole. Numerous local freshwater populations of Arctic char exist in all Arctic states, plus a smaller number of sea-run populations - where fish can grow up to 10kg in weight. The Arctic char suffers from increasing temperatures due to climate change, especially the more southern populations in the Alpine regions, which can be at risk of extinction - water temperature and quality are significant factors in the survival of their sensitive eggs. According to the first report on The State of the Arctic Freshwater Biodiversity (2019), climate warming increasingly threatens freshwater biodiversity in arctic lakes, rivers, and wetlands. The report also suggests that the Arctic char – as well as other cold-water species endemic to the Arctic – are likely to suffer regional losses, or even local extinctions as a result (find out more with this great article). Similarly, the 2019 report of the Intergovernmental Panel on Climate Change (IPCC) states that changes in the cryosphere will alter freshwater communities - range shrinkage and extinction of some species can cause regional biodiversity to decrease. WWF Finland has been involved in a conservation project to strengthen the population of Critically Endangered Lake Saimaa Arctic char. Also good news exists: exploitation of the northern Arctic char population in Finland has decreased at the same time with improved protection, leading to improved conservation status in the Red Book of Species for Finland – bending the curve in freshwater species does exist!
#11 Tasmanian Giant freshwater crayfish
In the rivers and streams of Tasmania lives the world’s largest freshwater invertebrate – the endemic Giant freshwater crayfish, Astacopsis gouldi. The IUCN Red List of Threatened Species list this species as Endangered, mainly as a result of years of past overfishing, illegal fishing, and habitat loss and degradation. Capable of reaching 6kg in weight, they have been harvested as a food source at a local and national level. The effects of fishing pressure are aggravated by their biological characteristics - this menacing-looking giant is actually very easy to catch, besides being slow-growing with low breeding potential. This species has received attention as a species of conservation concern - in the 90s, the Australian and Tasmanian governments added the Giant freshwater crayfish to their environmental legislation, and, recently, the critical habitat has been redefined.
#10 Victoria water lily
A long time ago, a young warrior called Naiá, from a Brazilian Amazonian tribe – “Tupi-guarani”– would dream about the day she would meet with Jaci, the moon. According to the legend, Jaci was a goddess who every night would kiss the most beautiful women of the tribe, and before hiding behind the mountains, she would pick her favourite to transform the woman into a star. Every night, Naiá would walk around the hills searching for Jaci. One day, after seeing the reflection of Jaci on a lake, she threw herself inside the lake and drowned. The moon compassionately decided to reward Naiá for her sacrifice, transforming her into a star of the waters – the Victoria water lily, whose white flower only opens at night and changes to pink when the sun rises.
The myth of the Victoria water lily above is a fragment of the memory of WWF's Paula Martinelli, who remembers hearing this story growing up in Brazil. The Victoria water lily, Victoria amazonica, is native to the Amazon biome and the symbol of the Amazon state of Brazil. The legend of the Victoria water lily has some truth to it – the white flowers only bloom at night, its colour changing to pink or ruby red after spending the evening exuding a strong odour to attract beetles to pollinate them. The flower closes its petals in the morning, hopefully trapping the beetles and releasing pollen on them. They open again in the following evening, releasing the beetles to transport their pollen to other flowers – they now have different colours and are scentless, being less attractive to the beetles. This fascinating adaptation avoids fertilization by its own pollen, increasing the genetic variability of the world’s largest water lily.
#9 Beluga Sturgeon
To celebrate World Wildlife Day (tomorrow on March 3), this week we present to you the majestic Beluga sturgeon! Even though there is still a debate on which is the largest freshwater fish in the world, the Beluga sturgeon, Huso huso, is undoubtedly the largest migrating freshwater fish – with a record of 1,571 kg and 7.2 m, this female was caught in 1827 in the Volga estuary. But this is history - nowadays, you can seldom observe a length above 2m. This species has been recorded swimming in the basins of the Caspian, Black, Azov, and Adriatic Seas. The migration of the Beluga sturgeon was predictable, and as a huge fish, they were an easy target, providing abundant food for many people. Its current native wild distribution is restricted to the Black Sea (Danube and Rioni) and the Caspian Sea (Ural). It does occur in the Azov Sea and Volga River, but as stocked fish. Due to historically severe overfishing and poaching in estuaries and rivers, the Beluga sturgeon is listed as Critically Endangered (CE) by the IUCN Red List of Threatened Species. This species is wanted for its meat and caviar - one of the most valuable products on the food market. Their caviar is considered top class caviar and is among the most expensive – you can buy 30g in London at the modest price of €420, which adds up to a price of €14.000 per kg. The beluga sturgeons’ eggs can only be legally obtained from aquaculture. Unfortunately, illegal wild caviar is still in demand on the black market by customers who consider "wild" as the "real" thing. IUCN also states that sturgeons are more critically endangered than any other group of species – 23 of all 27 known species are considered to be threatened with extinction. The mighty Beluga sturgeon can live for over 100 years, spending several years in the sea and migrating longer distances than any other sturgeon – more than 1000km. Because of their particular long life cycle, the recovery of its heavily depleted stocks will need several decades. But they can still be saved. WWF is working together with communities, civil society, companies, scientists, law enforcement authorities and governments across the world to rebuild their population. For example, our Sturgeon initiative is tackling the illegal caviar trade and enhancing the health of river systems. And, last year, WWF helped to save a Beluga in the Danube.
#8 Giant freshwater stingray
The Giant freshwater stingray, Urogymnus polylepis, is one of the largest and heaviest freshwater fishes in the world. With the ability to gain more than 600kg and 2m disc width, this freshwater giant mostly swims the large rivers of south and southeast Asia ( find out more about the beautiful Giant stingray and other giants of the Mekong river in this report). The IUCN Red List of Threatened Species lists the Giant freshwater stingray as Endangered, mainly due to fisheries and habitat degradation. The development of dams is also one of the drivers for their – and other freshwater megafauna species – population declines. Among WWF’s conservation efforts is our Resilient Asian Deltas (RAD) Initiative, whose ambition is to secure healthy and functioning river systems from source to delta through unprecedented political and financial investment in ‘building with nature’. To address the crisis facing freshwater biodiversity like the Giant freshwater stingray, WWF has just published an Emergency Recovery Plan - together with scientists from across the world - as a call to action to the world’s decision makers to value, protect and restore freshwater habitats before it’s too late.
#7 Grey Crowned crane
The Grey Crowned crane, Balearica regulorum, calls attention for its beautiful colours and a majestic golden plumage on their head unfolding like a crown. They are also known for displaying playful behaviours such as dancing. Mainly a form of socialization and pair formation, other species of cranes are also famous for their dance moves. You can find the Grey Crowned crane “performing” in the eastern and southern regions of Africa, where they are also a culturally important species – considered icons of Africa’s wetlands and savannahs, besides being the national bird of Uganda. However, it’s not all glamour for the Grey Crowned crane. Listed as Endangered by the IUCN list of threatened species, they are threatened primarily by illegal trade, disturbance, and habitat loss. The Global Wetland Outlook 2018 states that up to 87% of wetlands have been lost since 1700. The Grey Crowned crane and the other 14 living species of cranes need wetlands as stepping stone corridors – where they can find a safe harbour to sleep, rest, nesting, and rearing the young. As cranes are among the most threatened bird families in the world, WWF has joined the International Year of Crane 2020, announced by the International Crane Foundation and the Crane Working Group of Eurasia. The Amur River basin is the flyway and breeding grounds of no less than six species of cranes, and WWF will be working with partners in the region to strengthen the conservation of both wetlands and the majestic cranes.
#6 Rainbow mussel
The Rainbow mussel, Villosa iris, is a fascinating freshwater species not only for its beautiful inside shell, which is iridescent – they are also the master of disguise! “Hidden” in the North American rivers and streams, this tiny mussel uses mimicry to increase its odds of success: the mussel lures fishes by looking and moving like a crayfish, which is, in reality, a few flaps of fleshy skin coming out of its shell. Once the fish is close enough to take a bite of the “crayfish”, the Rainbow mussel releases a cloud of tens of thousands of parasitic spawn! Aiming for a safe place to transition into juvenile mussels, the mussel larvae latch to the fish’s gills, fins, and skin. Their new “home” (and “ride”) also provide for their food – they feed on the minuscule amounts of nutrients from their host. Most of the time, however, the fooled host-fish is not harmed by this interaction.
#5 Golden Mahseer
The Golden Mahseer, Tor putitora, is one of the world’s most iconic freshwater fish – highly prized by anglers, it is viewed as a God fish by local communities in some parts of South Asia, such as India and Bhutan. But its numbers are declining across South and South East Asia due to a range of threats from dam blocking its migratory routes to pollution and unsustainable fishing. It is now classified as Endangered on the IUCN Red List of threatened species. Local communities in India have revered fish species as symbols of divine power, offering them protection and building temple sanctuaries along the river banks, the first established nearly 1200 years ago. The Golden Mahseer benefits from this protection in several stretches of the river Ganges associated with these temples, where fishing is not allowed, and local communities, pilgrims, and temple authorities help to monitor and protect the mahseer population. The Mahseer also has cultural, religious and economic significance in Bhutan, where it is known as the Tiger of the River (you can find out more in this great documentary on the “Tiger of the River” produced by WWF Bhutan). In 2015, as a response to the pressures on Bhutan’s river systems, the Fisheries Conservation Foundation, WWF Bhutan, and the Ministry of Agriculture and Forests started a partnership to assess the migration pattern of the God fish using radio telemetry technology. This year, the 2 International Mahseer Conference is being held in Thailand from 11th to 15th February, where researchers and conservationists - including WWF - will share knowledge, collaborate and promote the conservation of the species.
#4 Yangtze Finless porpoise
The Yangtze Finless porpoise, Neophocaena asiaeorientalis asiaeorientalis, as the name suggests, lives in the Yangtze River and is the world’s only freshwater porpoise – and one of only five surviving species of freshwater cetacean. Unfortunately, there are only an estimated 1,012 finless porpoises left in China’s greatest river and they are listed as Critically Endangered on the IUCN red list of threatened species due to a host of threats, including water pollution, overfishing, boat traffic, sand mining, and river fragmentation. Many finless porpoises have also been killed after becoming accidentally entangled in gill nets – something which contributed to the extinction of the Yangtze freshwater dolphin, the Baiji (Lipotes vexillifer). But there is still hope for the Yangtze finless porpoise. Led by the Chinese government, conservation efforts – involving WWF, partners and local communities – have helped to drastically slow the annual decline in the finless porpoise population, including relocating animals to safer habitats away from the river’s mainstem to create new breeding populations. Huge amounts of work still need to be done to secure the survival of the species and WWF will continue to be at the forefront through its global River Dolphin Rivers initiative. This initiative is working on the development of smart river pingers in an effort to prevent future entanglement in gillnets. For many years, the finless porpoise had little to smile about but thanks to the combined efforts of the government, communities and conservationists, there is now a chance that the species can turn the corner and that its population will finally start to increase.
#3 Hellbender Salamander
The Hellbender salamander, Cryptobranchus alleganiensis, a.k.a. giant salamander, is, non-surprisingly, the largest salamander in North America – and the third-largest in the world. But these amphibian giants are fascinating not only for their size: they have lungs, but they mostly breathe through their wrinkly skin; they can swim, but usually walk underwater through their sturdy limbs. With tiny eyes on the top of their heads and light-sensitive cells all over their bodies, especially on their tails, they can keep their (giant) selves completely hidden from large fish under rocks and use their keen sense of smell to hunt crayfish. Besides playing this important role in their ecosystem, they are also bioindicators of stream quality, as they have a low tolerance for poor water quality. It is not a good sign for the North American rivers that their populations are considered Near Threatened by the International Union for Nature Conservation (IUCN), mainly due to the worsening condition of their freshwater habitats. Find out more about this species in this cool video by David Herasmtschuk.
#2 Matamata turtle
The matamata turtle, Chelus fimbriata, is one of the world’s largest freshwater turtles – and undeniably one of the weirdest too. Found in the Amazon and Orinoco basins, their unmistakable appearance is the combination of a leaf-shaped head, a long pointy snout, a rough tuberculate flat shell, and an elongated, thick neck with protuberances. The bizarre look is the result of a unique adaptation for feeding and blending in - they are incredibly specialized feeders that wait for dinner to come to them. When unknowing fish approach them to see if the protuberances on their neck are edible, they quickly stretch their neck out and open their mouth to create a vacuum that swallows their prey whole.
#1 European eel
For the first week we are highlighting the European eel, Anguilla anguilla. There can be few species more mysterious than the European eel, Anguilla Anguilla. A species that breeds just once in its lifetime and must travel thousands of kilometres to do so. European eels, like other Anguillid eels exhibit a number of life stages: they start their lives in the ocean as leptocephalus (larvae) which float back to estuaries where they turn into glass eels, and then elvers before travelling upstream into our rivers where they turn into yellow eels. The final stage of an European eel’s life is as a silver eel; silver eels are able to migrate thousands of kilometres to the Sargasso sea to spawn. No one has witnessed their spawning and so the spawning location remains a hypothesis. European eels are a Critically Endangered species, threatened by dams and infrastructure, habitat destruction and illegal trafficking. Find out more about this enigmatic and mysterious freshwater species in this great article.
If you’d like to find out about other Anguillid species, check out this blog from Kathy Hughes, our very own Freshwater Habitats and Species ACAI lead. Kathy like’s eels so much she takes holidays especially to swim with them.