crustacean (krŭstā`shən), 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

Crustaceans can be divided according to size into two main groups. The larger group, which is divided into the classes Branchiopoda, Copepoda, Ostracoda, and Cirripedia, includes the familiar shrimp shrimp, small marine decapod crustacean with 10 jointed legs on the thorax, well-developed swimmerets on the abdominal segments, and a body that is compressed laterally.
..... Click the link for more information. , crayfish crayfish or crawfish, freshwater crustacean smaller than but structurally very similar to its marine relative the lobster , and found in ponds and streams in most parts of the world except Africa. Crayfish grow some 3 to 4 in. (7.6–10.
..... Click the link for more information. , lobsters lobster, marine crustacean with five pairs of jointed legs, the first bearing large pincerlike claws of unequal size adapted to crushing the shells of its prey.
..... Click the link for more information. , and crabs crab, crustacean with an enlarged cephalothorax covered by a broad, flat shell called the carapace. Extending from the cephalothorax are the various appendages: five pairs of legs, the first pair bearing claws (or pincers), are attached at the sides; two eyes on
..... Click the link for more information. . The smaller group includes species that are either microscopic or range up to a few inches (about 5 cm) in size. Most of the smaller marine forms can be found in plankton (see marine biology marine biology, study of ocean plants and animals and their ecological relationships. Marine organisms may be classified (according to their mode of life) as nektonic, planktonic, or benthic. Nektonic animals are those that swim and migrate freely, e.g.
..... Click the link for more information. ) and thereby occupy an important position in the marine food chain. 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 wood lice, 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 Arthropoda (ärthrŏp`ədə) [Gr.,=jointed feet], largest and most diverse animal phylum.
..... Click the link for more information. .

crustacean

Any member of the 45,000 arthropod species in the subphylum Crustacea. Distributed worldwide, crustaceans are distinguished by having two pairs of antenna-like appendages in front of the mouth and other paired appendages near the mouth that act like jaws. Most species are marine, including shrimps and barnacles. Some, including crayfishes, live in freshwater habitats; others (e.g., sand fleas, land crabs, and sow bugs) live in moist terrestrial environments. The typical adult body is composed of a series of segments (somites) either fused or linked to each other by flexible areas that form movable joints. The carapace (shell) varies in thickness among species and must be periodically molted to allow growth. Many species of marine crustaceans are scavengers, and many (including copepods and krill) are significant components of the diets of larger organisms. See also decapod.