Krill are small shrimp-like crustaceans about an inch or so long, found in all the oceans of the world. In areas with nutrients, they occur in huge swarms, with more than 10,000 krill per cubic meter. They feed on phytoplankton and to a lesser extent zooplankton.
Krill are ecologically very important, because they are near the bottom of the food chain. They make up the largest part of the diet of many animals. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of over 500,000,000 tonnes (490,000,000 long tons; 550,000,000 short tons), roughly twice that of the human race. They are eaten by many animals, including whales, seals, penguins, birds, fish, squid, and whale sharks. Of this, over half is eaten by each year, and is replaced by growth and reproduction.
The order Euphausiacea is divided into two families, Bentheuphausiidae and Euphausiidae. the first has one species, the second has 85 species. Well known species are the Antarctic krill (Euphausia superba), Pacific krill (Euphausia pacifica) and Northern krill (Meganyctiphanes norvegica).
Except for Bentheuphausia amblyops, krill are bioluminescent. They have organs called photophores which emit light. The light is made by an enzyme-catalysed chemiluminescence reaction. A luciferin (a kind of pigment) is activated by a luciferase enzyme. Krill probably get this substance as part of their diet, which contains dinoflagellates. Krill photophores are complex organs with lenses and focusing abilities, and can be rotated by muscles. The exact function of these organs is not known. The possibilities include mating, social interaction or as a form of camouflage to compensate for their shadow against the overhead light.
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