Oceans

The mighty oceans

Oceans cover 71% of the Earth's surface. The different oceans merge into one another, forming the largest habitat on earth.
The 3 major oceans of the world are the Pacific, the Atlantic and the Indian Ocean. The Arctic Ocean surrounds the North Pole while the Southern Ocean (really the southern portion of the Atlantic and Pacific Oceans) surrounds the continent of Antarctica.

Great ocean currents swirl around the Earth, many of them thousands of kilometres long. Some are warm currents, some are cold. These currents have an enormous effect on the world's weather systems. Oceans also have layers of water at different temperatures.

Life forms - from the smallest to the biggest
Oceans support the greatest variety of life on earth, from microscopic plankton to giant whales. The deepest parts of the oceans have barely begun to be explored, and new life forms are being discovered every year by deep ocean submersible machines.

The floor of the ocean is called the benthic habitat while the water itself is called the pelagic habitat. Both support a diversity of animal life. Coral reefs, which grow in warm tropical and subtropical seas, are perhaps the richest marine habitat in terms of the diversity of life they shelter.
Oceans of the world. / ©: WWF-Canon
Oceans of the world.
© WWF-Canon

Wildlife in Oceans

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Oceans support the greatest variety of life on earth. / ©: WWF
Oceans support the greatest variety of life on earth.
© WWF
Marine food chains
The oceanic food chain begins with microscopic drifting plants called phytoplankton. Phytoplankton are found close to the surface of the water where there is adequate sunlight for photosynthesis.

Phytoplankton are eaten by tiny floating animals known as zooplankton. Zooplankton include the larvae of crabs, jellyfish, corals and worms, as well as adult animals like tiny shrimps, copepods and euphausiids (krill). They keep buoyant with the help of gas-filled chambers and oil droplets which reduce their density.

Moving up the food chain, zooplankton provide food for fish. Big fish eat smaller fish and at the very top of the food chain are large predatory fish like sharks, mammals like seals, and seabirds. A very large fish, the whale shark, and some very large mammals, the baleen whales, feed directly on zooplankton.

Millions of people on all continents depend on fish for food. That is why it is so important that fish populations are conserved. Overfishing by huge modern fishing fleets is threatening the entire ocean food chain.
Phytoplankton & Zooplankton. rel=
Phytoplankton & Zooplankton.
© WWF
The spectacular coral reefs
Many people mistakenly believe that coral reefs are made of rock. In fact, coral reefs are made by millions of tiny animals called coral polyps. They have a soft, sac-like body with one end closed and the other opening at a mouth surrounded by tentacles with stinging cells. Each coral polyp secretes a hard limestone skeleton. Millions of these cases make up the structure of the reef.

Coral reefs are so rich in animal life they have been called the ‘rainforests of the sea’. A great variety of small colourful fish live around the reef as well as animals like nudibranchs (sea slugs), sea anemones, sea urchins and starfish.
Ocean fish
There are more than 21,000 species of fish, of which most live in the oceans. Scientists classify fish into 2 main groups - fishes without jaws (hagfish and lampreys) and fishes with jaws.

The jawed fishes are further divided into those with skeletons made of cartilage (sharks, chimaeras, skates and rays) and those with bony skeletons (all the rest, including herring, cod, flounders and barracudas).

Fish have evolved to live in all kinds of marine habitat, from the cold dark ocean depths to surface waters. Some fish - sharks, marlins, swordfish, and tunas - are very fast swimmers and efficient predators which cover thousands of km each year.

Other kinds of fish such as the flat flounders and plaice move slowly over the ocean bed where they feed on molluscs and worms. Colourful coral reef fish tend to stay close to their territory on the reef.
Black Sting ray. / ©: WWF-Canon
Black Sting ray.
© WWF-Canon
Animals on the ocean floor
The deep ocean floor supports a special community of animals. Although seaweeds grow from the sea floor near coasts, it is too dark at the bottom of the deep ocean for plants to live. Some organisms living here look a little like plants but actually they are animals like sponges, sea mats, sea anemones, and sea squirts.

Dead plankton, fish and fish faeces fall to the ocean floor where they are consumed by the decomposers.

Some creatures crawl over the bottom or burrow beneath it. They include lobsters, crabs, prawns, starfish, brittlestars, featherstars, and many kinds of sea slugs and worms. Finally, there are the bottom-feeding fish such as rays, turbots, halibuts and gurnards.

Sea Anemone. / ©: WWF-Canon
Sea Anemone.
© WWF-Canon
Creatures of the deep
Much of the ocean is so deep that no light penetrates. Many fish that live at such depths have developed light-emitting organs that signal other fish or attract prey. The lantern fish has a row of lights long its body and the angler fish swims around with a lighted rod that attracts other fish, which are then quickly swallowed!
Angler Fish. / ©: WWF-Canon
Angler Fish.
© WWF-Canon
Conservation concern
Despite their huge size, the oceans have been greatly affected by human activity. Pollution and overfishing are 2 major concerns.

WWF has hundreds of projects addressing conservation of the world's oceans and marine wildlife. WWF campaigns aim to help the recovery of threatened species of marine fishes such as sharks, marlins and tunas, and to encourage sensible, sustainable fishing practices which do not overuse or waste fish.

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