Classification
Antarctic Krill are tiny shrimp-like crustaceans. Krill is a term used to describe about 85 species of open-ocean crustaceans known as euphasids.
Appearance
Krill look like smaller versions of more familiar crustaceans such as prawns or lobsters, and range in size from small tropical species less than a centimeter long to little known deep-sea giants that can reach lengths of up to 14 centimeters. All species have a recognisable crustacean shape; that is, they have an elongated head-trunk region (cephalothorax) and a muscular, segmented tail (abdomen) to which are attached five pairs of paddle-like swimming legs.
Diet
Antarctic Krill are mainly herbivorous, feeding on the phytoplankton (microscopic suspended plants) of the Southern Ocean, though planktonic animals (zooplankton) may also form part of their diet.
Development
Krill, like all crustaceans, grow by moulting; that is, they cast off the old confining shell and expand in size while the shell is still soft. What seems to be unique in krill is that is the ability to use this process in reverse (in other words, to shrink) when food is absent.
Breeding
In summer, female Antarctic krill lay up to 10 000 eggs at a time, sometimes several times during a season, into the surface waters of the Southern Ocean. The eggs are thought to sink to a depth of 2000 metres before hatching. They then begin their long ‘developmental ascent’ (up to 10 days), during which the newly hatched larvae journey up towards the sunlit waters to feed.
Distribution
Antarctic krill (Euphasia superba) are one of the most abundant and successful animal species on Earth.
Behavior
As Krill come to resemble adults they begin to aggregate into huge schools or swarms; sometimes stretching for kilometres in every direction, with many thousands of krill packed into each cubic meter of water turning it red or orange. The first seamen who ventured into waters round Antarctica were confused by discoloured patches on the surface of the water. Captain Cook may even have mistaken one of these patches for shallow water, leading him to suspect he was near land.
No one is quite sure how krill, old or young, survive the Antarctic water. They do not seem to build up fat reserves, so must either use some food available under the ice such as the algae which grows on the underside of pack ice, detirus on the seafloor or the other animals in the water or utilise some internal store other than fat.