Malacostracan

peacock mantis shrimp

malacostracan, any member of the more than 29,000 species of the class Malacostraca (subphylum Crustacea, phylum Arthropoda), a widely distributed group of marine, freshwater, and terrestrial invertebrates. Lobsters, crabs, hermit crabs, shrimp, and isopods are all malacostracan crustaceans.

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 by their Paleozoic ancestors (542 million to 251 million years ago). 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 terrestrial 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.