page_title: Krill
section_title: Anatomy and morphology
hierarchical_section_title: Krill / Anatomy and morphology
caption_attribution_description: The gills of krill are externally visible, contrary to those of shrimps. The image shows the gills of a krill of the species Euphausia pacifica.
context_page_description: Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.
  Krill are considered an important trophic level connection – near the bottom of the food chain. They feed on phytoplankton and zooplankton, yet also are the main source of food for many larger animals. In the Southern Ocean, one species, the Antarctic krill, Euphausia superba, makes up an estimated biomass of around 379,000,000 tonnes, making it among the species with the largest total biomass. Over half of this biomass is eaten by whales, seals, penguins, squid, and fish each year. Most krill species display large diel vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.
  Krill are fished commercially in the Southern Ocean and in the waters around Japan. The total global harvest amounts to 150,000–200,000 tonnes annually, most of this from the Scotia Sea. Most of the krill catch is used for aquaculture and aquarium feeds, as bait in sport fishing, or in the pharmaceutical industry.
context_section_description: Krill are crustaceans and have a chitinous exoskeleton made up of three tagmata: the cephalon (head), the pereion (fused to the cephalon to form a cephalothorax), and the pleon. This outer shell of krill is transparent in most species. Krill feature intricate compound eyes; some species adapt to different lighting conditions through the use of screening pigments. They have two antennae and several pairs of thoracic legs called pereiopods or thoracopods, so named because they are attached to the thorax; their number varies among genera and species. These thoracic legs include feeding legs and grooming legs. Additionally all species have five pairs of swimming legs called pleopods or "swimmerets", very similar to those of a lobster or freshwater crayfish. Most krill are about 1–2 centimetres (0.4–0.8 in) long as adults; a few species grow to sizes on the order of 6–15 centimetres (2.4–5.9 in). The largest krill species is the bathypelagic Thysanopoda spinicauda. Krill can be easily distinguished from other crustaceans such as true shrimp by their externally visible gills.
  Except for Bentheuphausia amblyops, krill are bioluminescent animals having organs called photophores that can emit light. The light is generated by an enzyme-catalysed chemiluminescence reaction, wherein a luciferin (a kind of pigment) is activated by a luciferase enzyme. Studies indicate that the luciferin of many krill species is a fluorescent tetrapyrrole similar but not identical to dinoflagellate luciferin and that the krill probably do not produce this substance themselves but acquire it 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 precise function of these organs is as yet unknown; possibilities include mating, social interaction or orientation and as a form of counter-illumination camouflage to compensate their shadow against overhead ambient light.
caption: The gills of krill are externally visible
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