Anatomy and Morphology
Krill are crustaceans and have a chitinous exoskeleton made up of three segments: the cephalon (head), the thorax, and the abdomen. The first two segments are fused into one segment, the cephalothorax. 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.
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