CRUSTACEAN

any member of the subphylum Crustacea (phylum Arthropoda), a group of invertebrate animals consisting of nearly 30,000 species distributed worldwide. Crustaceans differ from other arthropods by having two pairs of antenna-like appendages in front of the mouth and other paired appendages near the mouth that act as jaws. The majority of crustaceans are marine forms, the most notable of which are shrimp, lobsters, crabs, and barnacles. Some crustaceans, including most crayfish, live in freshwater habitats. Still others, such as sand fleas, land crabs, and sow bugs, live in moist terrestrial environments. The class Malacostraca contains the larger and better-known crustaceans, including lobsters, shrimp, crabs, and crayfish, and encompasses more than two-thirds of all living species in the subphylum. There is little general agreement on the status or interrelationships of the other groups of crustaceans, chief among which are the Branchiopoda (fairy shrimps, water fleas), Cephalocarida (horseshoe shrimps), Ostracoda (mussel, or seed, shrimps), Mystacocarida (mustache shrimps), Copepoda (copepods), and Cirripedia (barnacles). It is believed that the ancestral crustacean resembled either a cephalocarid or a branchiopod. The typical adult crustacean body is composed of a series of segments, called somites, which either are fused or are linked to each other by flexible areas that form movable joints. The typical form of a somite is seen in the segments that make up the abdomen, or tail, of a lobster or crayfish. The first four somites fuse with the presegmental region, or acron, to form the head (cephalon). The two segments that are in front of the mouth carry two pairs of appendages called antennules and antennae, which are used as sensory organs in the adult and as aids to swimming and feeding in larvae. The feeding and chewing mandible is found on somites behind the mouth. The carapace, or shell, begins at the maxillary somite and varies in thickness and size among the orders. It must be periodically molted because the hard structure prohibits growth. Varying from a thin, undifferentiated sac, as in the barnacles, to a massive shell, as in crabs and lobsters, the carapace is composed of an epicuticle and a chitinous cuticle and can be stiffened by deposits of calcium salts. Ingrowths of the integument, called apodemes, provide an internal skeleton. When the integument is thin, gas exchange occurs through the inner wall of the carapace. Gills formed by modified parts of the appendages serve as respiratory organs in some species. The great diversity in the shapes of crustaceans' appendages or limbs results from modifications of a peduncle, or stalk (the protopodite), that bears two branches, the exopodite and the endopodite. On the inner and outer margins of the protopodite there may be additional lobes that can function as gills or assist in mastication. The antennules, or first antennae, are also considered to be appendages. The biramous (or double-branched) antennae, because of their position near the mouth, are also able to assist the mandibles in pushing food. Mandibles, in most cases, have masticatory lobes on the protopodites, with distinctive cutting and grinding capabilities. There is little differentiation in the limbs behind the head region of most species, but some are specially adapted for such functions as walking and swimming or as additional mouthparts or weapons. A continuation of the carapace lines the gut, which generally runs straight through the body. Ceca, or tubular outgrowths, increase the functions of digestion and absorption. Blood flows through channels without definite walls and is pumped by rhythmic body motions or by the heart, a long tube with valvular openings in each somite. The nervous system consists of a ganglionic mass, or brain, which is united with a double chain of segmentally arranged ganglia that has nerve connections to the eyes and the antennules. The eyes may consist of a small group of three or four simple eyes (ocelli), or they may be compound eyes that have visual elements separated by pigment, similar to those of insects. The sexes are separate in most crustaceans. Frequently, the males have prehensile appendages for clasping the female while mating. Crustaceans of most species hatch from eggs and then pass through a series of free-swimming larvae stages. In less-primitive crustaceans, the earlier transformations may take place within the egg, resulting in an acceleration of the appearance of the appendages, the beginning of the carapace, and the segmentation of its posterior parts. In the oceans crustaceans play a part not unlike that taken by insects on land. The most familiar crustaceans are the larger crabs and lobsters that are used as food by humans, but these play a relatively small role in oceanic life compared to amphipods and isopods (class Malacostraca), which are ubiquitous in most seas. These small crustaceans serve as scavengers, feeding on all kinds of animal and vegetable debris and forming, in their turn, the food of many fishes. An even more important group of crustaceans is the copepods (class Maxillopoda), which form an important constituent of plankton, the assemblage of tiny floating animal and vegetable life in the surface waters of the ocean. Copepods feed upon diatoms and other microscopic plant plankton and themselves form the food of many economically important fishes. any member of the subphylum Crustacea (phylum Arthropoda), a group of invertebrate animals consisting of some 39,000 species distributed worldwide. Crabs, lobsters, shrimps, and wood lice are among the best-known crustaceans, but the group also includes an enormous variety of other forms without popular names. Crustaceans are generally aquatic and differ from other arthropods in having two pairs of appendages (antennules and antennae) in front of the mouth and paired appendages near the mouth that function as jaws. Because there are many exceptions to the basic features, however, a satisfactory inclusive definition of all the Crustacea is extraordinarily hard to frame. Additional reading A major reference on all aspects of the class is Dorothy E. Bliss (ed.), The Biology of Crustacea, 10 vol. (198285). Alfred Kaestner, Invertebrate Zoology, vol. 3 (1970; originally published in German, 2nd ed., 1967), gives an excellent survey of morphology, physiology, embryology, and ecology. Other overviews are provided by the section Crustacea in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, vol. 2 (1982), pp. 173326; Raymond C. Moore (ed.), Treatise on Invertebrate Palaeontology, pt. Q, Arthropoda 3 (1961), and pt. R, Arthropoda 4, 2 vol. (1969); and Robert D. Barnes, Invertebrate Zoology, 5th ed. (1987). Robert H. Gore and Kenneth L. Heck (eds.), Crustacean Biogeography (1986), is an important discussion on distribution. Patsy A. McLaughlin, Comparative Morphology of Recent Crustacea (1980), gives clear diagrams of external and internal anatomy. Frederick R. Schram, Crustacea (1986); and Frederick R. Schram (ed.), Crustacean Phylogeny (1983), include recent theories of crustacean evolutionary relationships.