crustacean , primarily aquatic arthropod of the subphylum Crustacea. Most of the 44,000 crustacean species are marine, but there are many freshwater forms. The few groups that inhabit terrestrial areas have not been particularly successful in an evolutionary sense; most require very humid environments in order to survive.

Types of Crustaceans

The most important classes of Crustacea are Branchiopoda, which includes the brine shrimp; Maxillopoda, which includes the barnacles and copepods; Ostracoda, which includes the mostly very small seed shrimp; and Malacostraca, which includes the familiar shrimp , crayfish , lobsters , and crabs . Most of the smaller marine crustaceans can be found in plankton (see marine biology ) and thereby occupy an important position in the marine food chain. For example, the crustacean subclass Copepoda supplies the food of the crustacean crustacean order Euphausiacea, the euphausids or krill, shrimplike creatures that are the food of baleen whales and other marine animals. Other copepods supply food for small fish, and still others exist as parasites on the skin and gills of fish. Best known of the smaller freshwater crustaceans are members of the genus Daphnia (water fleas), the fairy shrimp (a phyllopod that swims inverted), and Cyclops (a copepod). The order Isopoda includes the only large group of truly terrestrial crustaceans. Known as woodlice, sow bugs, or pillbugs, these small animals can be found under the bark of trees, beneath stones and rocks, and in other damp places. When disturbed they curl up armadillolike, withdrawing into the exoskeleton.

Crustacean Anatomy

All crustaceans have bilaterally symmetrical bodies covered with a chitinous exoskeleton, which may be thick and calcareous (as in the crayfish) or delicate and transparent (as in water fleas). Since it does not grow, the exoskeleton must be periodically molted when the animal undergoes metamorphosis (typically from free-swimming larva to adult) or simply outgrows its shell. The free-swimming larva characteristic of crustaceans, called a nauplius larva, has an unsegmented body, a median eye, and three pairs of appendages.

Like other arthropods, adult crustaceans have segmented bodies and jointed legs; the segments are usually grouped into a recognizable head, thorax, and abdomen. In the majority of larger crustaceans the head and thorax are fused into a cephalothorax, which is protected by a large shieldlike area of the exoskeleton called the carapace. The head bears two pairs of antennae, usually one median eye and two lateral eyes, and three pairs of biting mouthparts—the mandibles and the two pairs of maxillae. Crustacean appendages have undergone extensive adaptation for various tasks such as swimming, sensory reception, and walking. Many species have the first pair of thoracic appendages modified into claws and pincers. The gills are generally attached at the bases of the thoracic appendages, and the beating of the appendages creates a flow of water over the gills that facilitates respiration. Reproduction is sexual, and in most forms the sexes are separate. In many species the eggs are brooded beneath the abdominal segments of the female.

Classification

Crustaceans constitute the subphylum Crustacea of the phylum Arthropoda .

Crustacea, a group of invertebrate animals belonging to the phylum Arthropoda. It includes not only large shellfish such as crabs, shrimps, and lobsters, but also smaller animals like copepods and krill that are the most numerous and important animals of the plankton. They have a hard outer skeleton or carapace, which in many species is hardened with calcium carbonate, and they have to shed this in order to grow. In crabs mating can only occur after the female has moulted and still has a soft carapace. In some species like the shore crab (Carcinus maenas) once a crab matures it no longer moults, and so can only mate once. In the spring the male common spider crabs (Maia squinado) assemble in huge piles in the centre of which are the moulting females. In contrast the edible crab (Cancer pagurus) and species of lobster (Homarus) continue to moult and grow, so mating is less of a crisis event and individual animals can attain much larger sizes. The heaviest on record is a 1.06-metre (3-ft 6-in.) long American lobster, which weighed 20.14 kilograms (44 lb 6 oz). The record for size is held by a giant spider crab (Macrocheira kaempferi) found off Japan, which had a leg span of over 3.7 metres (12 ft). Many of the larger crustaceans spend their larval stages in the plankton. The planktonic larvae of lobsters are highly transparent and leaflike, and are called phyllosoma; they spend nearly a year in the plankton during which time they may drift across an ocean.

Many types of crustacean live permanently in the plankton. One group, the copepods, make up 70% of the animals caught in nets and are probably the most numerous invertebrates on the planet. They are a key link in the food web between the phytoplankton and larger animals such as fish and the smaller seabirds. Another important planktonic group are the euphausiids. These shrimplike animals range in adult size from 1 to 3 centimetres (0.5–1.5 in.).

There are many important commercial species of crustaceans. Crabs and lobsters are trapped in baited pots and prawns are trawled from the seabed. In tropical countries sea-water ponds are carved out of mangrove swamps to make ponds in which prawns are cultured. The stocking of these ponds relies heavily on larvae from the wild populations that are introduced when the ponds are opened to the sea. Nearly 6.5 million tonnes of crustaceans are caught each year, and a nearly another million tonnes are produced by aquaculture. In all crustaceans contribute 7% to global fisheries.

M. V. Angel

Crustacean

The Crustacea are a subphylum of the animal phylum Arthropoda. This is a large and diverse group with more than forty thousand species, including crabs, shrimp, lobsters, crayfish, barnacles, and many near-microscopic members of the zooplankton community. The subphylum is characterized especially by having mandibles and compound eyes and living in mostly aquatic habitats, although the "pillbugs" found under rocks and boards are also crustaceans, and many crabs spend much of their time on land.

The Crustacea are named for their hard, crusty exoskeletons , well known to anyone who has dined on lobster or crab. The hardness of the exoskeleton comes partly from chitin , but moreover from a heavy deposit of calcium carbonate. The edible blue crab, for example, has as much calcium carbonate in its exoskeleton as four sticks of chalk. The rigid exoskeleton requires crustaceans to molt, or shed it periodically, in order to grow. Some crustaceans can mate only during the brief time just after they have molted and the new exoskeleton is still soft. This is also a time of great vulnerability to predators, so crustaceans often seek a place to hide before molting.

Some crustaceans resemble miniature adults from the moment they hatch, but many species have larval forms with little or no resemblance to the adult. These larvae, and some adult crustaceans, such as krill and cope-pods, are very important members of the freshwater and oceanic plankton community and are a major source of food for corals, fish, baleen whales, and other animals. A few crustaceans turn the tables on these predators by parasitizing the skin of fishes. These parasitic crustaceans are often wormlike and scarcely recognizable as relatives of shrimp and crabs.

see also Animalia; Arthropod; Lakes and Ponds; Ocean Ecosystems; Plankton

Kenneth S. Saladin

Bibliography

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Fort Worth, TX: Saunders College Publishing, 1994.

Crustacea (crustaceans; phylum Arthropoda) Diverse subphylum of mandibulate arthropods, the body usually divided into three parts: head, thorax, and abdomen. In some crustaceans (e.g. crayfish) the head and thorax may be joined to form the cephalothorax. The head bears two pairs of antennae, one pair of mandibles, and two pairs of maxillae. The limbs are biramous, and are adapted for a wide range of functions. Closely placed setae on the limbs function as filters in filter-feeding species. Respiratory gills are situated on the appendages, but vary greatly in location and number; they are absent only in very small species. In addition to the antennae, sense organs include a pair of compound eyes, and a small, dorsal, median, nauplius eye, comprising three or four closely applied ocelli (clusters of photoreceptors). The nauplius eye, characteristic of crustacean larvae, is absent in many adults; and some groups lack the compound eyes. Mainly marine, but there are many fresh-water species, and a relatively small number have invaded the land. Four classes of crustaceans have an important fossil record. The Malacostraca (crabs, lobsters, woodlice, etc., Cambrian to Recent) includes the earliest crustaceans of the subclass Phyllocarida. The Branchiopoda (similar to modern water fleas, Lower Devonian to Recent) are valuable index fossils in non-marine strata. The Cirripedia (barnacles) occur from Upper Silurian to Recent, and the Ostracoda from Lower Cambrian to Recent. The living class Cephalocarida (e.g. Hutchinsonella) is thought to be closest to the ancestral crustacean stock, but the group is without any unequivocal fossil representative.

Crustacea (crabs, lobsters, shrimps, slaters, woodlice, barnacles; phylum Arthropoda) Diverse class of arthropods which have two pairs of antennae, one pair of mandibles, and two pairs of maxillae. The limbs are biramous, and are adapted for a wide range of functions. Closely placed setae on the limbs function as filters in filter-feeding species. Respiratory gills are situated on the appendages, but vary greatly in location and number; they are absent only in very small species. In addition to the antennae, sense organs include a pair of compound eyes, and a small, dorsal, median, nauplius eye, comprising three or four closely applied ocelli. The nauplius eye, characteristic of crustacean larvae, is absent in many adults; and some groups lack the compound eyes. Nitrogenous excretion is via a pair of maxillary glands. Most of the 31 400 species are marine, but there are many freshwater species, and a relatively small number have invaded the land. A few marine species are parasites of other Crustacea; and one group, the Cyamidae, are ectoparasites of whales (whale lice). The first representatives of the group are known from Cambrian rocks.

Crustacea A phylum of arthropods containing over 35 000 species distributed worldwide, mainly in freshwater and marine habitats, where they constitute a major component of plankton. Crustaceans include shrimps, crabs, lobsters, etc. (see Decapoda) and the terrestrial woodlice, all of which belong to the class Malacostraca; the barnacles (class Cirripedia); the water fleas (see Daphnia), fairy shrimps, and tadpole shrimps (class Branchiopoda); and the copepods (see Copepoda). The segmented body usually has a distinct head (bearing compound eyes, two pairs of antennae, and various mouthparts), thorax, and abdomen, and is protected by a shell-like carapace. Each body segment may bear a pair of biramous appendages used for locomotion, as gills, and for filtering food particles from the water. Appendages in the head region are modified to form jaws and in the abdominal region are often reduced or absent. Typically, the eggs hatch to produce a free-swimming nauplius larva. This develops either by a series of moults or undergoes metamorphosis to the adult form.

Crustacea(crabs, lobsters, shrimps, slaters, woodlice, barnacles) A diverse subphylum of Arthropoda, comprising animals which have two pairs of antennae, one pair of mandibles, and two pairs of maxillae. The limbs are biramous, and adapted for a wide range of functions. Most of the 31 400 species are marine, but there are many freshwater species, and a relatively small number have invaded the land. A few marine species are parasites of other Crustacea and one group (Cyamidae) are ectoparasites of whales (whale lice). The first crustaceans are known from Cambrian rocks.

crustacean Any member of the class Crustacea, comprising c.30,000 species of arthropods. The class includes the decapods (crabs, lobsters, shrimps and crayfish), isopods (pill millipedes and woodlice) and many varied forms, most of which have no common names. Most crustaceans are aquatic and breathe through gills or the body surface. They are typically covered by a hard exoskeleton. They range in size from the Japanese spider crab up to 3m (12ft) across to the ocean plankton, as little as 1mm (0.04in) in diameter.

crus·ta·cean / krəˈstāshən/ • n. any arthropod of the phylum Crustacea, having a hard shell and usu. aquatic, including crabs, lobsters, and shrimps. • adj. of or related to the crustaceans.

crustacea Zoological class of hard‐shelled marine arthropods (shellfish) including crabs, crayfish, lobster, prawns, scampi, shrimps.

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