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

thumb|left|The [[woodlouse Oniscus asellus showing the head with eyes and antennae, carapace and relatively uniform limbs]]

thumb|left|A [[Phreatoicidea|phreatoicidean isopod, from Australia]]

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 .

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 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. 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. 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.

Some isopod groups have evolved a parasitic lifestyle, particularly as external parasites of fish. 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:

  • Cymothoida – Chiefly marine isopods with over 2,700 species. The previously recognised suborder Epicaridea is included as two superfamilies within this suborder and Cymothoida now includes part of the formerly recognised suborder Flabellifera.
  • 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.
  • Tainisopidea – Freshwater isopods in a "relictual environment". 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.

Terrestrial isopods

The majority of crustaceans are aquatic, though the isopods are one of the few groups with terrestrial members. The other terrestrial crustaceans are sandhoppers (Amphipoda) along with land crabs and some hermit crabs (Decapoda). 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

thumb|[[Porcellio scaber and Oniscus asellus]]

The woodlice of the suborder Oniscidea are the most successful group of terrestrial crustaceans, and show various adaptations for life on land. They are subject to evaporation, especially from their ventral area, and as they do not have a waxy cuticle, they need to conserve water, often living in a humid environment and sheltering under stones, bark, debris or leaf litter. Desert species like Hemilepistus reaumuri are usually nocturnal, spending the day in a burrow and emerging at night. Moisture is achieved through food sources or by drinking, and some species can form their paired uropodal appendages into a tube and funnel water from dewdrops onto their pleopods. In many taxa, the respiratory structures on the endopods are internal, with a spiracle and pseudotrachaea, which resemble lungs. In others, the endopod is folded inside the adjoining exopod (outer branch of the pleopod). Both these arrangements help to prevent evaporation from the respiratory surfaces. Some exhibit coprophagia and will also consume their own fecal pellets. Cymothoa exigua is a parasite of multiple fish species, such as the spotted rose snapper Lutjanus guttatus in the Gulf of California; it causes the tongue of the fish to atrophy and takes its place in what is believed to be the first instance discovered of a parasite functionally replacing a host structure in animals.

Reproduction and development

In most species, the sexes are separate (dioecy) and there is little sexual dimorphism, but a few species are hermaphroditic and some parasitic forms show large differences between the sexes. Males have a pair of penises, which may be fused in some species. The sperm is transferred to the female by the modified second pleopod which receives it from the penis and which is then inserted into a female gonopore. The sperm is stored in a special receptacle, a swelling on the oviduct close to the gonopore. Fertilisation only takes place when the eggs are shed soon after a moult, at which time a connection is established between the semen receptacle and the oviduct.

See also

References

  • World List of Marine Freshwater and Terrestrial Isopod Crustaceans