Below a depth of about 200 metres (about 660 feet) there is little light, and below 1000 metres (3300 feet) the water is completely black and very cold. Phyto­plankton cannot survive here, and the amount of animal life is greatly reduced. Some scavenging deepwater animals feed on the dead plant and animal matter that rains down through the water from the surface waters above. Others, such as the hatchetfish, travel up towards the surface to feed, then return to the depths.

Bioluminescence

Despite the blackness of the water, there is still some light in the depths of the ocean. Some deepwater animals are able to produce light from their body tissues, which are sometimes concentrated into areas as special light-producing organs. This feature is known as bioluminescence. Fish, squid, jellyfish and even tiny deep-sea copepods all produce their own light. These lights may act as a lure to prey animals, or as a signal to others of the same species in the search for mates. They may also be used as search beams, or “flashed” on and off to confuse or temporarily blind an attacker.

Some fish have organs that produce light. Viperfish use a light on their dorsal (back) fin to lure their prey towards their fang-like teeth. Barracudinas have loosely hinged jaws and expanding stomachs that allow them to consume prey even larger than themselves.

Some animals travel between surface and twilight zones, feeding in one zone and resting in the other. They include the hatchetfish and the lanternfish, which journey to the surface at night to feed, stinging jellyfish-like creatures called siphonophores, and certain kinds of shark.

Bathypelagic zone

Deeper still, and below 1000 metres (3300 feet) is the near-freezing world of the bathypelagic zone. Even in these completely black, cold depths there is animal life, although it is extremely sparse.

At great depths, food comes along only rarely so the animals that live here need to save energy between meals. Gulper eels wait to ambush their prey, rather than dash after it. With their massive jaws, taking up about a quarter of their total body length, they are well equipped to capture any size of animal. Their stomachs can stretch to hold fish even larger than themselves—a useful feature since it may be long a time before another meal comes along.

Sperm whale

Although, like all mammals, it must breathe air, the sperm whale is able to dive to depths of at least 1000 metres (3300 feet)—and almost certainly much deeper than that, perhaps as far as 3000 metres (10,000 feet). It can spend up to 90 minutes below water in search of its prey, giant squid. From the scratch marks often seen on the whale's skin, we know that the contest is violent.

The sperm whale has a special organ in its head which helps it to descend and ascend such long distances. This organ, called the spermaceti, is filled with a waxy substance. When the whale dives, the cold water cools the wax, causing it to solidify and make the head heavier. On ascent, blood flow to the head warms and melts the wax, making the head lighter.