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Temporal range: Carboniferous to Holocene 300–0  Ma
Eurydice pulchra.jpg
Eurydice pulchra , a carnivorous isopod found on sandy shores
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Superorder: Peracarida
Order: Isopoda
Latreille, 1817 [1]

Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. 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.


Isopods have various feeding methods: some eat 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 mostly live on the seabed or bottom of freshwater bodies of water, but some taxa can swim for a short distance. 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 as a defense mechanism or to conserve moisture.

There are over 10,000 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 Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago, when isopods lived in shallow seas. The name Isopoda is derived from the Greek roots iso- (from ἴσος ísos, meaning "equal") and -pod (from ποδ-, the stem of πούς poús, meaning "foot"). [2] [3]


The woodlouse Oniscus asellus
showing the head with eyes and antennae, carapace and relatively uniform limbs Porcellio scaber - male side 2 (aka).jpg
The woodlouse Oniscus asellus
showing the head with eyes and antennae, carapace and relatively uniform limbs

Classified within the arthropods, isopods have a chitinous exoskeleton and jointed limbs. [4] Isopods are typically flattened dorsoventrally (broader than they are deep), [5] although many species deviate from this rule, particularly parasitic forms, and those living in the deep sea or in ground water habitats. Their colour may vary, from grey to white, [6] or in some cases red, green, or brown. [7] Isopods vary in size, ranging from some Microcerberidae species of just 0.3mm to the deep sea Bathynomus spp. of nearly 50 cm (20 in). [3] 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. [3] [8] 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 eight segments (pereonites), and an abdomen (pleon) with six segments (pleonites), some of which may be fused. [5] 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). [9]

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. [9]

One or more of the abdominal segments, starting with the sixth segment, is fused to the telson (terminal section) to form a rigid pleotelson. [9] [10] [11] 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), [3] [12] 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 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. [9] In some terrestrial isopods, these resemble lungs. [3]

Diversity and classification

Numbers of marine Isopoda (except Asellota and crustacean symbionts) in biogeographic regions Numbers of marine Isopoda (except Asellota and crustacean symbionts) in biogeographic regions.png
Numbers of marine Isopoda (except Asellota and crustacean symbionts) in biogeographic regions
Representative marine isopod forms Representative marine isopod forms.png
Representative marine isopod forms

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. [3] [13] 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. [14] 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. [14] The largest isopod is in the genus Bathynomus and some large species are fished commercially for human food in Mexico, Japan and Hawaii. [15]

Some isopod groups have evolved a parasitic lifestyle, particularly as external parasites of fish. [9] They can damage or kill their hosts and can cause significant economic loss to commercial fisheries. [16] 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. [17]

The World Marine, Freshwater and Terrestrial Isopod Crustaceans database subdivides the order into eleven suborders: [1]

Evolutionary history

Isopods first appeared in the fossil record during the Carboniferous period of the Paleozoic some 300 million years ago. [23] 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. [14]

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. [14]


Unlike the amphipods, 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 to 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 Flabellifera can swim to a limited extent 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. [9]

Feeding and nutrition

Anilocra (Cymothoidae) parasitising the fish Spicara maena, Italy Spicara flexuosa + Anilocra.jpg
Anilocra (Cymothoidae) parasitising the fish Spicara maena , Italy

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. [9]

Isopods are detritivores, browsers, carnivores (including predators and scavengers), parasites, and filter feeders, and may occupy one or more of these feeding niches. Only aquatic and marine species are known to be parasites or filter feeders. [24] [25] Some exhibit coprophagia and will also consume their own fecal pellets. [25] Terrestrial species are in general herbivorous, with woodlice feeding on moss, bark, algae, fungi and decaying material. In marine isopods that feed on wood, cellulose is digested by enzymes secreted in the caeca. Limnoria lignorum , for example, bores into wood and additionally feeds on the mycelia of fungi attacking the timber, thus increasing the nitrogen in its diet. Land-based wood-borers mostly house symbiotic bacteria in the hindgut which aid in digesting cellulose. There are numerous adaptations to this simple gut, but these are mostly correlated with diet rather than by taxonomic group. [9]

Parasitic species are mostly external parasites of fish or crustaceans and feed on blood. The larvae of the Gnathiidae family and adult cymothoidids have piercing and sucking mouthparts and clawed limbs adapted for clinging onto their hosts. In general, isopod parasites have diverse lifestyles and include Cancricepon elegans , found in the gill chambers of crabs; Athelges tenuicaudis , attached to the abdomen of hermit crabs; Crinoniscus equitans living inside the barnacle Balanus perforatus ; cyproniscids, living inside ostracods and free-living isopods; bopyrids, living in the gill chambers or on the carapace of shrimps and crabs and causing a characteristic bulge which is even recognisable in some fossil crustaceans; and entoniscidae living inside some species of crab and shrimp. [9] [26] Cymothoa exigua is a parasite of 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. [27]

Reproduction and development

In most species, the sexes are separate and there is little sexual dimorphism, but a few species are hermaphroditic and some parasitic forms show large differences between the sexes. [9] Some Cymothoidans are protandrous hermaphrodites, starting life as males and later changing sex, and some Anthuroideans are the reverse, being protogynous hermaphrodites that are born female. Some Gnathiidans males are sessile and live with a group of females. [24] 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. [9]

The eggs, which may number up to several hundred, are brooded by the female in the marsupium, a chamber formed by flat plates known as oostegites under the thorax. This is filled with water even in terrestrial species. [9] The eggs hatch as mancae, a post-larval stage which resembles the adult except for the absence of the last pair of pereopods. The lack of a swimming phase in the life cycle is a limiting factor in isopod dispersal, and may be responsible for the high levels of endemism in the order. [14] As adults, isopods differ from other crustaceans in that moulting occurs in two stages known as "biphasic moulting". [3] First they shed the exoskeleton from the posterior part of their body and later shed the anterior part. The giant Antarctic isopod Glyptonotus antarcticus is an exception, and moults in a single process. [28]

Terrestrial isopods

Armadillidium vulgare 001.jpg
Armadillidium vulgare on the move ...
Armadillidium vulgare 000.jpg
... and rolled into a ball

The majority of crustaceans are aquatic and the isopods are one of the few groups of which some members now live on land. [29] [30] The only other crustaceans which include a small number of terrestrial species are amphipods (like sandhoppers) and decapods (crabs, shrimp, etc.). [29] 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. [31] Macro-detritivores, including terrestrial isopods, are absent from arctic and sub-arctic regions, but have the potential to expand their range with increased temperatures in high latitudes. [32]

The woodlice, suborder Oniscidea, are the most successful group of terrestrial crustaceans [9] 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 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. [9]

Many species can roll themselves into a ball, a behaviour used in defence that also conserves moisture. Members of the families Ligiidae and Tylidae, commonly known as rock lice or sea slaters, are the least specialised of the woodlice for life on land. They inhabit the splash zone on rocky shores, jetties and pilings, may hide under debris washed up on the shore and can swim if immersed in water. [9]

Related Research Articles

Copepod Subclass of crustaceans

Copepods are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic, some are benthic, and some continental species may live in limnoterrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds, and puddles, damp moss, or water-filled recesses (phytotelmata) of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Copepods are sometimes used as biodiversity indicators.

Malacostraca Largest class of crustaceans

Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments, and divided into a head, thorax, and abdomen.

Amphipoda Order of malacostracan crustaceans

Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from 1 to 340 millimetres and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far described. They are mostly marine animals, but are found in almost all aquatic environments. Some 1,900 species live in fresh water, and the order also includes terrestrial animals and sandhoppers such as Talitrus saltator.

Cymothoidae family of crustaceans

The Cymothoidae are a family of isopods in the suborder Cymothoida and are found in both marine and freshwater environments. Cymoithoids are ectoparasites, usually of fish, and among their number they include the bizarre "tongue-biter" which attaches to a fish's tongue causing it to atrophy, and replaces the tongue with its own body. Ceratothoa oestroides is one of the most devastating ectoparasites in Mediterranean aquaculture. Around 40 genera and more than 380 species of cymothoid are recognised. Species of the Cymothoidae are generally found in warmer waters and rarely in the cool and cold climates.

Woodlouse Suborder of Crustacea

A woodlouse is a crustacean from the monophyletic suborder Oniscidea within the isopods. They are called that from being found in old wood.

Giant isopod genus of crustaceans

A giant isopod is any of the almost 20 species of large isopods in the genus Bathynomus. They are abundant in the cold, deep waters of the Atlantic, Pacific, and Indian Oceans. Bathynomus giganteus, the species upon which the generitype is based, is often considered the largest isopod in the world, though other comparably poorly known species of Bathynomus may reach a similar size. The giant isopods are noted for their resemblance to the much smaller common woodlouse, to which they are related.

Asellota suborder of isopod crustaceans

Asellota is a suborder of isopod crustaceans found in marine and freshwater environments. Roughly one-quarter of all marine isopods belong to this suborder. Members of this suborder are readily distinguished from other isopods by their complex copulatory apparatus. Other characteristics include six-jointed antennal peduncle, the styliform uropods, the fusion of pleonites 5, 4 and sometimes 3 to the pleotelson, and absence of the first pleopod in females.

Cymothoida suborder of crustaceans

Cymothoida is the name of a suborder of isopod crustaceans with a mostly carnivorous or parasitic lifestyle. It contains more than 2,700 described species in four superfamilies. Members of the suborder are characterised by their specialised mouthparts which include a mandible with a tooth-like process which is adapted for cutting or slicing.

Crustacean larva crustacean larval and immature stages between hatching and adult form

Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow. The larvae of crustaceans often bear little resemblance to the adult, and there are still cases where it is not known what larvae will grow into what adults. This is especially true of crustaceans which live as benthic adults, more-so than where the larvae are planktonic, and thereby easily caught.

Agnaridae family of crustaceans

Agnaridae is a family of woodlice. They were formerly considered part of the Trachelipodidae, but were moved from that family to Porcellionidae in 1989, and then placed as a separate family in 2003.

<i>Aega psora</i> species of crustacean

Aega psora is a species of isopod crustacean that parasitises a number of fish species in the North Atlantic. It is a serious ectoparasite of larger species of fish, particularly when they are injured.

The Arcturidae are a family of marine isopod crustaceans in the suborder Valvifera. Members of the family resemble woodlice and are found globally in cooler areas in shallow seas.

Cymothoa elegans is a species of parasitic isopod in the genus Cymothoa. It has rarely been recorded, with all records coming from the north coast of Java. They are in the arthropod phylum and can more closely be classified as crustaceans.

Crustacean Subphylum of arthropods

Crustaceans form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, shrimps, prawns, krill, woodlice, and barnacles. The crustacean group can be treated as a subphylum under the clade Mandibulata; because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the clade Pancrustacea other than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans.

<i>Uromunna sheltoni</i> species of crustacean

Uromunna sheltoni is a species of isopod first described by Brian Kensley in 1977. U. sheltoni is included in the genus Uromunna and family Munnidae. No subspecies are listed. The species was first collected by Peter Shelton of the University of Cape Town, for whom it is named.

<i>Deto echinata</i> species of crustacean

Deto echinata, the horned isopod, is a species of air-breathing isopod, or woodlouse, in the family Detonidae. It inhabits seashores in southern Africa and on some oceanic islands.

Rhyscotidae is a family of woodlice, terrestrial crustaceans of the order Isopoda.

<i>Elthusa californica</i> species of crustacean

Elthusa californica is a species of isopod in the family Cymothoidae of the Isopoda Order. Elthusa californica is a

<i>Helleria brevicornis</i> species of crustacean

Helleria brevicornis, the sole species of the monotypic genus Helleria, is a terrestrial woodlouse endemic to the islands and coastal regions of the northern Tyrrhenian sea. H. brevicornis is of interest due to its endemism, unique ecology and basal position in the suborder Oniscidea.


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