Ocean ecology: the dim and dark depths | WWF
Some sunlight reaches below 200m, but not enough to allow photosynthesis.
Deep-sea angler fish collected by a scientific trawl at a depth of over 1,000m in the Porcupine Seabight, North East Atlantic Ocean. © WWF / Ian HUDSON

Only animals and bacteria live in these dim and dark waters, with the food chain based on detritus falling from surface waters.

The twilight zone

The twilight, or mesopelagic, zone, and extends from 200m to 1,000m down. Animals living here have various adaptations for living in the dimly lit waters. Some species have enormous eyes to find food. To avoid being eaten, many are transparent, including squid and crustaceans. Some fish have silvery reflective scales to help make them 'invisible'.

The dark depths
Almost no light penetrates below 1,000m. The water is cold, reaching 3ºC, and contains very little oxygen. The pressure is enormous, up to 1,000 times that on the surface. These dark waters include the bathypelagic (1,000-4,000m), abyssopelagic (4,000m to the ocean floor) and hadopelagic (water in ocean trenches) zones.

Aliens in our midst
Despite the fact that 60% of our planet is covered by water over 1,600m deep, we know very little about pelagic and other deep-sea life. However, the few expeditions to these depths have revealed strange, almost alien-looking animals - such as fish with giant teeth and gaping jaws, long tentacle-like feelers, and even glowing patches.

The deep sea is much poorer in productivity than shallower regions. The fish tend to be much smaller than on the surface, with minimal bone structure and more jelly-like flesh. They are therefore slower and less agile than fish living near the surface. They also tend to grow much more slowly than surface fish. Some take many years to reach sexual maturity.

All teeth and mouth
Life in the deep-sea is scarce, so predators need large teeth and mouths to cope with whatever crosses their path. The fangtooth fish, for example, has the largest teeth of all marine animals in relation to body size, while gulpers can swallow prey at least as large as themselves.

Not completely dark
There is some light at these depths - from bioluminescence. An estimated 90% of deep-sea animals produce their own light to communicate, find food, and even to avoid capture.

For example, some shrimp squirt out a bioluminescent liquid to distract predators. There are fish with bioluminescent ‘headlamps’ for finding food, while others, such as the deep-sea angler fish, use bioluminescent lures to catch prey. Some of these animals produce the light themselves, while others have recruited specialized bacteria to do the job.

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Sperm whales (<i>Physeter catodon</i>) are the deepest diving mammals, hunting for ... rel= © WWF / Hal WHITEHEAD

The greatest migration on Earth
Despite the very different ecologies of the epipelagic, mesopelagic, and bathypelagic zones, animals do move between them - including the largest migration on Earth, which happens twice a day.

Every night, billions of animals swim up from the depths to feed on plankton in surface waters. When the sun rises, they move back down into the relative safety of the darkness. Some species also rise to the surface to spawn.

In addition, some surface dwellers dive to great depths to hunt for food. Sperm whales, for example, breathe air at the ocean surface, and dive down to 2,000m to hunt giant squid. The pressure at these depths is at least 100 times greater than at the surface, squeezing their lungs to 1% of their surface volume.

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