Isopoda

Isopoda is an order of crustaceans known collectively as isopods, encompassing aquatic forms like gribbles and terrestrial ones like woodlice. Their bodies feature a rigid, segmented exoskeleton, two pairs of antennae, seven pairs of jointed thoracic limbs, and five pairs of branching abdominal appendages used for respiration. Females carry their developing young in a brood pouch under the thorax called the marsupium. The name Isopoda comes from Greek, meaning "equal foot," a reference to their seven pairs of similarly shaped legs.

Isopods display a range of feeding habits: many are scavengers or detritivores that consume dead or decaying matter, while others graze, filter feed, or act as predators. Some are parasites, living internally or externally, mostly on fish. Aquatic isopods are primarily benthic, dwelling on the bottom of water bodies, though a few can swim short distances. Terrestrial species crawl and are typically found in cool, damp environments. Certain types, such as those in the family Armadillidiidae (commonly called pillbugs), can roll into a ball—a behavior known as volvation—for defense or to retain moisture.

Over 10,000 described isopod species exist worldwide: about 4,500 live in marine environments, mostly on the seabed; around 500 inhabit fresh water; and another 5,000 are terrestrial. The order is divided into eleven suborders. The fossil record of isopods extends back to the Pennsylvanian epoch of the Carboniferous, at least 300 million years ago, when these animals lived in shallow seas.

Classified among arthropods, isopods have a chitinous exoskeleton and jointed limbs. They are usually flattened dorsoventrally (broader than deep), though many exceptions occur, especially among parasitic species and those living in deep sea or groundwater habitats. Coloration varies from gray to white, and sometimes red, green, or brown. Size ranges from just 0.3 millimeters in some Microcerberidae species to nearly 50 centimeters in deep sea giant isopods of the genus *Bathynomus*. Giant isopods lack a prominent carapace, which is reduced to a "cephalic shield" covering only the head; their gill-like structures are found on specialized abdominal limbs instead. The dorsal surface is covered by overlapping, articulated plates that offer both protection and flexibility. The body plan includes a head (cephalon), a thorax (pereon) with seven segments (pereonites), and an abdomen (pleon) with six segments (pleonites), some of which may be fused. The head fuses with the first thoracic segment to form the cephalon. There are two pairs of unbranched antennae, with the first pair vestigial in land-dwelling species. Eyes are compound and unstalked, and mouthparts include a pair of maxillipeds and a pair of mandibles with palps and a lacinia mobilis. Each of the seven free thoracic segments bears a pair of unbranched pereopods (limbs), usually similar in size and shape for locomotion—hence the name "Isopoda." In some species, the front pair are modified into clawed gnathopods. The coxae of these limbs are fused to the tergites to form epimera. Mature females have oostegites on some limbs that form a brood chamber; males have gonopores on segment eight, females on segment six. One or more abdominal segments fuse with the telson to form a rigid pleotelson. The first five abdominal segments each bear a pair of biramous pleopods used for gas exchange and, in aquatic species, as gills and for propulsion. The last segment bears a pair of biramous uropods. In males, the second pair of pleopods (and sometimes the first) are modified for sperm transfer. The endopods of the pleopods have thin, permeable cuticles that act as gills; in some terrestrial isopods, these resemble lungs.

Isopods belong to the larger group Peracarida, united by a brood chamber under the thorax. They have a cosmopolitan distribution, with over 10,000 described species in 11 suborders. About 4,500 species are marine, mostly on the sea floor; 500 are freshwater; and 5,000 are terrestrial woodlice (suborder Oniscidea). In the deep sea, the suborder Asellota dominates, having undergone a large adaptive radiation. The largest isopods are in the genus *Bathynomus*, and some large species are fished commercially for human food in Mexico and Japan. Some isopod groups have evolved a parasitic lifestyle, particularly as external parasites of fish, where they can damage or kill hosts and cause significant economic loss to commercial fisheries.