MALACOSTRACAN

(class Malacostraca), any member of a widely distributed crustacean group of marine, freshwater, and terrestrial invertebrates. Malacostracans typically are free-living, but they do form symbiotic, commensal, or parasitic relationships with other invertebrates and marine vertebrates. They account for more than two-thirds of all extant crustacean species. About 22,000 species have been described. The class includes some of the most highly evolved invertebratesthe lobster, crab, and shrimp (order Decapoda), the sand flea (Amphipoda), the pill bug (Isopoda), the mantis shrimp (Stomatopoda), and many other forms. Some are commercially important. The malacostracans include the largest of all arthropods, the giant crab (Macrocheira kaempferi) of Japan, whose outstretched limbs may extend 3.6 m (about 12 feet) or more. The class also includes many forms smaller than 1 mm (0.04 inch). Malacostracans typically have compound eyes (i.e., containing many facets). A carapace, or shield, covers the upper surface of the cephalothoraxthe combined head and thorax, or midsection. The thorax has eight segments and eight pairs of appendages, the abdomen six segments and six pairs of appendages. any member of the 22,000 species of the class Malacostraca (subphylum Crustacea), a widely distributed group of marine, freshwater, and terrestrial invertebrates. Malacostracans are the most numerous and most successful of the four major classes of Crustacea. Their members constitute more than two-thirds of all living crustacean species. They exhibit the greatest range of size (less than one millimetre, or 0.04 inch, to a limb spread of more than three metres, or 10 feet) and the greatest diversity of body form. Malacostracans are abundant in all permanent waters of the world: in the seas from the tropics to the poles and from the tidal zone to the abyss; in surface and subterranean fresh waters of all continents except Antarctica (where they once existed); and terrestrially on all continental landmasses and all tropical and temperate islands. The success of malacostracans can be attributed primarily to their increased body size and to the evolution of more functional body regions and more sophisticated food-gathering appendages than possessed either by their Paleozoic ancestors (570 to 245 million years ago) or by the next largest living crustacean class, the Maxillopoda. This evolutionary thrust has been marked by the development of ambulatory legs and specializations for benthic life and by the brooding of eggs and suppression of free-living larval development. Especially significant has been a shift of food-gathering limbs from head to thorax and of swimming appendages and respiratory organs from head to thorax and finally to the abdomen. This rearward shift freed the antennae for the development of specialized organelles sensitive to odours, sounds, vibrations, and physical contact and added more appendages (maxillipeds) to the mouthpart field. Such changes have enabled malacostracans to utilize efficiently the new food resources that have accompanied the evolution and proliferation of vascular plants from the late Paleozoic to the present. Additional reading E.L. Bousfield, Shallow-water Gammaridean Amphipoda of New England (1973), describes and illustrates the range, ecology, life history, and behaviour of 125 species of malacostracans.