Isopods are crustaceans belonging to the order Isopoda. With over 10,000 described species, they occupy the marine environment as much as fresh water and terrestrial habitats. Most of these animals are small in size, with segmented exoskeletons and jointed limbs. The fact that the legs are of the same type brought them the name isopoda, which from Greek translates into “same foot”.
Isopods are part of the larger group Peracarida, with the shared characteristic of having a special brood pouch which plays an important role in brooding eggs. There have been described about 10,215 Isopoda species, and these are classified into 11 suborders. Out of these, around 4,500 species occupy the marine environments, living mostly on the sea floor, with the rest being disproportionally scattered – 500 in fresh water and 5,000 species on land. At lower sea levels, species from the suborder Asellota are predominant, having adapted to the conditions of that environment.
Most isopods are scavengers and have an omnivore diet. Land species are mostly herbivore, while marine species feed on algae, bacteria, detritus, with some species feeding on small animals on the bottom of the sea floor. Some species have adapted to living a parasitic lifestyle. All species from the suborder Cymothoida live as parasites, while in the case of the suborder Flabellifera, only a part of the species are parasitic.
Parasitic species are mostly external, and they fix themselves unto fish, feeding on blood. Some have specialized piercing and sucking mouthparts, and some have clawed limbs that help them attach unto the host. An interesting case is that of Cymothoa exigua parasites the spotted rose snapper fish. It destroys the tongue of the fish and then it replaces it, without causing any further damage to the fish. This is the only known case of a parasite that functionally replaces an organ of the host.
While most terrestrial Isopoda species are pelagic, in the case of marine freshwater species, they are exclusively benthic. Their primary mechanism of locomotion is crawling, with some species using appendages to bore holes into the sea bed. There are some species that have limited swimming capabilities, using their modified first three pairs of pleopods. The slow locomotion explains why they rarely disperse over new regions, and it also explains the vast amount of isopods that are endemic to restricted areas.