The crustaceans (Crustacea) are a large group of arthropods (55,000
species), usually treated as a subphylum. They include various familiar
animals, such as lobsters, crabs, shrimp and barnacles. The majority
are aquatic, living in either fresh water or marine environments,
but a few groups have adapted to terrestrial life, such as terrestrial
crabs, terrestrial hermit crabs and woodlice. The majority are motile,
although a few taxa are parasitic and live attached to their hosts
(including sea lice, fish lice, whale lice, tongue worms, and Cymothoa
exigua), and adult barnacles live a sessile life, attached head-first
to the substrate.
The scientific study of crustaceans is known as carcinology. Other
names for carcinology are malacostracology, crustaceology and crustalogy,
and a scientist who works in carcinology is a carcinologist, crustaceologist
Crustaceans have three distinct body parts: head, thorax, and abdomen
(or pleon), although the head and thorax may fuse to form a cephalothorax.
The head bears two pairs of antennae, one pair of compound eyes
and three pairs of mouthparts. The thorax and pleon bear a number
of lateral appendages, including the gills, and the tail ends with
a telson. Smaller crustaceans respire through their body surface
by diffusion, and larger crustaceans respire with gills or, with
In common with other arthropods, crustaceans have a stiff exoskeleton
which must be shed to allow the animal to grow (ecdysis). Various
parts of the exoskeleton may be fused together; this is particularly
noticeable in the carapace, the thick dorsal shield seen on many
crustaceans. Crustacean appendages are typically biramous; this
includes the second pair of antennae, but not the first, which is
uniramous. There is some doubt whether this is an advanced state,
as had been traditionally assumed, or whether it may be a primitive
state, with the branching of the limbs being lost in all extant
arthoropod groups except the crustaceans. One piece of evidence
supporting the latter view is the biramous nature of trilobite limbs.
Although a few are hermaphroditic, most crustaceans have separate
sexes, which are distinguished by appendages on the abdomen called
swimmerets or, more technically, pleopods. The first (and sometimes
the second) pair of pleopods are specialised in the male for sperm
transfer. Many terrestrial crustaceans (such as the Christmas Island
red crab) mate seasonally and return to the sea to release the eggs.
Others, such as woodlice lay their eggs on land, albeit in damp
conditions. In many decapods, the eggs are retained by the females
until they hatch into free-swimming larvae.
Those crustaceans that have hard exoskeletons reinforced with calcium
carbonate, such as crabs and lobsters tend to preserve well as fossils,
but many crustaceans have only thin exoskeletons. Most of the fossils
known are from coral reef or shallow sea-floor environments, but
many crustaceans live in open seas, on deep sea-floors or in burrows.
Crustaceans tend, therefore, to be rarer in the fossil record than
trilobites. Some crustaceans are reasonably common in Cretaceous
and Caenozoic rocks, but barnacles have a particularly poor fossil
record, with very few specimens from before the Mesozoic era.
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