Group

Isopod

Isopoda, Latreille, 1816

marinebrackishfreshwater

About Isopod

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.

Appearance

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 .3 millimetres (0.012 in) to the deep sea giant isopod Bathynomus spp. of nearly 50 cm (20 in). 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.

Diet

Isopods have a simple gut which lacks a midgut section; instead there are caeca connected to the back of the stomach in which absorption takes place. Food is sucked into the esophagus, a process enhanced in the blood-sucking parasitic species, and passed by peristalsis into the stomach, where the material is processed and filtered. The structure of the stomach varies, but in many species there is a dorsal groove into which indigestible material is channelled and a ventral part connected to the caeca where intracellular digestion and absorption take place. Indigestible material passes on through the hindgut and is eliminated through the anus, which is on the pleotelson.

Reproduction

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. Some Cymothoidans are protandrous hermaphrodites, starting life as males and later changing sex, and some Anthuroideans are the inverse, being protogynous hermaphrodites that are born female. Some Gnathiidan males are sessile and live with a group of females. 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.

Where to see it

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