Crustaceans (Crustacea) form a very large group of
arthropods, usually treated as a
subphylum, which includes such familiar animals as
crabs,
lobsters,
crayfish,
shrimp,
krill and
barnacles. The 50,000 described species range in size from
Stygotantulus stocki at , to the
Japanese spider crab with a leg span of up to and a mass of . Like other
arthropods, crustaceans have an
exoskeleton, which they
moult to grow. They are distinguished from other groups of arthropods, such as
insects,
myriapods and
chelicerates by the possession of
biramous (two-parted) limbs, and by the
nauplius form of the
larvae. Most crustaceans are free-living
aquatic animals, but some are
terrestrial (e.g.
woodlice), some are
parasitic (e.g.
fish lice,
tongue worms) and some are
sessile (e.g.
barnacles). The group has an extensive
fossil record, reaching back to the
Cambrian, and includes
living fossils such as
Triops cancriformis, which has existed apparently unchanged since the
Triassic period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being
shrimp and prawns.
Krill and
copepods are not as widely fished, but may be the animals with the greatest
biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a
scientist who works in carcinology is a
carcinologist. ==List of crustaceans of Montana==