The Online Library of Knowledge

Ocean life

Deep-ocean life

Some deep-sea speciesSome deep-sea species 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. 

Gulper eelGulper eel

Bioluminescent fish and other marine species Bioluminescent fish and other marine species


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.

Bioluminescence sometimes acts as a "tag". A species of sea cucumber coats its attacker with sticky, glowing mucus. An even bigger predator can then more easily spot it and go after it.

© 2020 Q-files Ltd. All rights reserved. Switch to Mobile