Deep sea animals are those that live at various depths below the Photic Zone, the region where there is sunlight. They include fish and invertebrates of various kinds, such as squid and crustaceans, but there are other, unique kinds of creature. All are adapted by evolution to the strange conditions in which they live.
Beyond the Photic Zone
A lightless world
The Photic Zone goes down to a few hundred metres, but light is merely a glimmer well before this.
The absence of light means that sea fish and other animals of the sea cannot find any plants or algae growing there to eat.
They feed on debris dropping from higher levels, much of which is animal remains.
Much of the sea life that lives here is predatory upon other deep sea fish.
Fish and invertebrates
The distance from the surface means that there are no air-breathing creatures, such as dolphins, save for the occasional deep-diving sperm whale.
Most are fish, but there are invertebrates, creatures without a skeleton, such as squid, and some crustaceans, creatures with shells.
Some creatures can be enormous, such as the giant squid that lurk in these depths and which sometimes approach the surface.
The deepest layers
These are the ocean trenches, long depressions in the ocean floor where tectonic plates are in collision.
Life finds it hard to thrive here, but there are worm-like creatures that scavenge for scraps and dead fish falling from above.
Hydrothermal vents, sometimes found in trenches, have a strange fauna.
These volcanic outlets on the seabed are home to a range of microscopic creatures, bizarre animals whose body chemistry is like nothing else on earth.
One of the main fish adaptations is bioluminescence, the ability of creatures to provide their own light.
The viper fish is a scary fish that has rows of lights to attract other creatures, and one of them is attached to a long dangling lure near its mouth.
Lights attract not only predators, but mates.
Dr Beebe, the first person to descend into the depths, spotted a crustacean that squirted a bright fluid in the face of predators to blind them while it escaped
Other adaptations enable sea creatures to cope with the enormous pressures at great depth.
The deeper a sea creature lives, the more jelly-like its flesh is, and the more minimal its bone structure.
These ocean creatures have adapted to equalize the pressure inside their bodies with the pressure outside.
Scientists have discovered that the metabolic rate of deep sea creatures is slower than it is for animals of similar size at higher levels, which is to cope with the shortage of food so deep down