Isopoda is an order of crustaceans. Members of this group are collectively called isopods and include both aquatic species such as gribbles and terrestrial species such as woodlice. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax called the marsupium.
Isopods have various feeding methods: some are scavengers and detritivores, eating dead or decaying plant and animal matter; others are grazers or filter feeders, a few are predators, and some are internal or external parasites, mostly of fish. Aquatic species are mostly benthic, living on the bottom of water bodies, but some taxa can swim for short distances. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball (known as volvation) as a defense mechanism or to conserve moisture like species in the family Armadillidiidae, commonly called the pillbugs.
There are over 10,000 described species of isopod worldwide, with around 4,500 species found in marine environments, mostly on the seabed, 500 species in fresh water, and another 5,000 species on land. The order is divided into eleven suborders. The fossil record of isopods dates back to the Pennsylvanian epoch of the Carboniferous, at least 300 million years ago, where these isopods lived in shallow seas.
Etymology The name Isopoda is derived from the Greek roots (from , meaning "equal") and (from , the stem of , genitive , meaning "foot"). This refers to the fact that they have seven pairs of similarly shaped legs.
Description
Classified within the arthropods, isopods have a chitinous exoskeleton and jointed limbs. Isopods are typically flattened dorsoventrally (broader than they are deep), although many species deviate from this rule, particularly parasitic forms, and those living in the deep sea or in groundwater habitats. Their colour may vary, from grey to white, or in some cases red, green, or brown. Isopods vary in size, ranging from some Microcerberidae species measuring just to the deep sea giant isopod Bathynomus spp. of nearly . Giant isopods lack an obvious carapace (shell), which is reduced to a "cephalic shield" covering only the head. This means that the gill-like structures, which in other related groups are protected by the carapace, are instead found on specialised limbs on the abdomen.
The dorsal (upper) surface of the animal is covered by a series of overlapping, articulated plates which give protection while also providing flexibility. The isopod body plan consists of 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 is fused with the first segment of the thorax to form the cephalon. There are two pairs of unbranched antennae, the first pair being vestigial in land-dwelling species. The eyes are compound and unstalked and the mouthparts include a pair of maxillipeds and a pair of mandibles (jaws) with palps (segmented appendages with sensory functions) and lacinia mobilis (spine-like movable appendages).
The seven free segments of the thorax each bear a pair of unbranched pereopods (limbs). In most species these are used for locomotion and are of much the same size, morphology and orientation, giving the order its name "Isopoda", from the Greek equal foot. In a few species, the front pair are modified into gnathopods with clawed, gripping terminal segments. The pereopods are not used in respiration, as are the equivalent limbs in amphipods, but the coxae (first segments) are fused to the tergites (dorsal plates) to form epimera (side plates). In mature females, some or all of the limbs have appendages known as oostegites which fold underneath the thorax and form a brood chamber for the eggs. In males, the gonopores (genital openings) are on the ventral surface of segment eight and in the females, they are in a similar position on segment six.
One or more of the abdominal segments, starting with the sixth segment, is fused to the telson (terminal section) to form a rigid pleotelson. The first five abdominal segments each bear a pair of biramous (branching in two) pleopods (lamellar structures which serve the function of gas exchange, and in aquatic species serve as gills and propulsion), and the last segment bears a pair of biramous uropods (posterior limbs). In males, the second pair of pleopods, and sometimes also the first, are modified into sexual organs for use in transferring sperm. The endopods (inner branches of the pleopods) are modified into structures with thin, permeable cuticles (flexible outer coverings) which act as gills for gas exchange. In some terrestrial isopods, these resemble lungs.
Diversity and classification
Isopods belong to the larger group Peracarida, which are united by the presence of a special chamber under the thorax for brooding eggs. They have a cosmopolitan distribution and over 10,000 species of isopod, classified into 11 suborders, have been described worldwide. Around 4,500 species are found in marine environments, mostly on the sea floor. About 500 species are found in fresh water and another 5,000 species are the terrestrial woodlice, which form the suborder Oniscidea. In the deep sea, members of the suborder Asellota predominate, to the near exclusion of all other isopods, having undergone a large adaptive radiation in that environment. The largest isopod is 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. They can damage or kill their hosts and can cause significant economic loss to commercial fisheries. In reef aquariums, parasitic isopods can become a pest, endangering the fish and possibly injuring the aquarium keeper. Some members of the family Cirolanidae suck the blood of fish, and others, in the family Aegidae, consume the blood, fins, tail and flesh and can kill the fish in the process.
Taxonomy The World Marine, Freshwater and Terrestrial Isopod Crustaceans database subdivides the order into eleven suborders:
Asellota – This suborder contains the superfamily Aselloidea, a group that contains most of the freshwater isopods in the Northern Hemisphere, and the superfamilies Stenetrioidea, Gnathostenetroidoidea and Janiroidea, which are mostly marine. Janiroidea experienced a massive radiation in deep-sea ecosystems, with many families having taken bizarre forms. Calabozoida – A small suborder consisting of two marine species in the family Calabozoidae and one freshwater species in the family Brasileirinidae which is found in subterranean locations. Cymothoida – Chiefly marine isopods with over 2,700 species. Members are mostly carnivorous or parasitic. Includes the family Gnathiidae, the juveniles of which are parasitic on fishes. The previously recognised suborder Epicaridea is included as two superfamilies within this suborder and Cymothoida now includes part of the formerly recognised suborder Flabellifera. Also includes the former suborder Anthuridea, a group of worm-like isopods with very long bodies. Limnoriidea – Mainly tropical isopods, some of which are herbivorous. Microcerberidea – Tiny, worm-like isopods that live between particles on the bed of freshwater and shallow marine habitats. Oniscidea – Semi-terrestrial and terrestrial isopods fully adapted for life on land. There are over 4,000 species of oniscid woodlice inhabiting forests, mountains, deserts and the littoral zone. Phoratopidea – A single marine species, Phoratopus remex, which warrants its own suborder because of its unique characteristics. Phreatoicidea – Small suborder of freshwater isopods resembling amphipods, limited to South Africa, India, Australia and New Zealand. Sphaeromatidea – Benthic isopods mostly from the Southern Hemisphere with respiratory pleopods inside a branchial chamber. This suborder now includes part of the formerly recognised suborder Flabellifera. Tainisopidea – Freshwater isopods in a "relictual environment". Valvifera – A large group of benthic, marine isopods with respiratory pleopods inside a branchial chamber under the abdomen.
Evolutionary history Isopods first appeared in the fossil record during the Carboniferous period of the Paleozoic some 300 million years ago. They were primitive, short-tailed members of the suborder Phreatoicidea. At that time, Phreatoicideans were marine organisms with a cosmopolitan distribution. Nowadays, the members of this formerly widespread suborder form relic populations in freshwater environments in South Africa, India and Oceania, the greatest number of species being in Tasmania. Other primitive, short-tailed suborders include Asellota, Microcerberidea, Calabozoidea and the terrestrial Oniscidea.
The short-tailed isopods have a short pleotelson and terminal, stylus-like uropods and have a sedentary lifestyle on or under the sediment on the seabed. The long-tailed isopods have a long pleotelson and broad lateral uropods which can be used in swimming. They are much more active and can launch themselves off the seabed and swim for short distances. The more advanced long-tailed isopods are mostly endemic to the southern hemisphere and may have radiated on the ancient supercontinent of Gondwana soon after it broke away from Laurasia 200 million years ago. The short-tailed forms may have been driven from the shallow seas in which they lived by increased predatory pressure from marine fish, their main predators. The development of the long-tailed forms may also have provided competition that helped force the short-tailed forms into refugia. The latter are now restricted to environments such as the deep sea, freshwater, groundwater and dry land. Isopods in the suborder Asellota are by far the most species-rich group of deep sea isopods.
Biology Unlike amphipods within the same ecosystem, marine and freshwater isopods are entirely benthic. This gives them little chance to disperse to new regions and may explain why so many species are endemic with restricted ranges. Crawling is the primary means of locomotion, and some species bore into the seabed, the ground or timber structures. Some members of the families Sphaeromatidae, Idoteidae and Munnopsidae are able to swim pretty well, and have their front three pairs of pleopods modified for this purpose, with their respiratory structures limited to the hind pleopods. Most terrestrial species are slow-moving and conceal themselves under objects or hide in crevices or under bark. The semi-terrestrial sea slaters (Ligia spp.) can run rapidly on land and many terrestrial species can roll themselves into a ball when threatened, a feature that has evolved independently in different groups and also in the marine sphaeromatids.Proceedings of the United States National Museum
Terrestrial isopods While the majority of crustaceans are aquatic, isopods are one of the few groups with terrestrial members. Other terrestrial crustaceans include sandhoppers and landhoppers, land crabs, coenobitid hermit crabs, and potentially hexapods (such as insects) if they are considered to fall under Crustacea (Pancrustacea). Terrestrial isopods play an important role in many tropical and temperate ecosystems by aiding in the decomposition of plant material through mechanical and chemical means, and by enhancing the activity of microbes. Macro-detritivores, including terrestrial isopods, are absent from arctic and subarctic regions, but have the potential to expand their range with increased temperatures in high latitudes. Woodlice