Temporal range: 300–0Ma
|Eurydice pulchra, a carnivorous isopod of sandy shores|
Isopoda is the name of an order of peracarid crustaceans. Different species of isopods live in marine or fresh water or on land and include familiar animals such as the sea slater and woodlouse. They are mostly small, dull-coloured animals with rigid, segmented, exoskeletons and fourteen jointed limbs. They are mostly scavengers but some are predators and others ectoparasites, mostly of fish. The females brood their young in a pouch under their thorax. The name Isopoda derives from the Greek roots ἴσος (iso-, meaning "same") and ποδός (podos, meaning "foot"). The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago. There are about 10,000 species worldwide, a large proportion in the sea, some in freshwater and others on land. The woodlice are the most successful group of terrestrial crustaceans.
Isopods are typically flattened dorsoventrally, although many species deviate from this rule, particularly parasitic forms, those from the deep sea or from ground water habitats. Isopods lack an obvious carapace, which is reduced to a "cephalic shield" covering only the head. This means that the gills, which in other related groups are protected by the carapace, are relocated to specialised limbs on the abdomen. The dorsal (upper) surface of the animal is covered in a series of overlapping, articulated plates which provide protection and enable flection. The isopod body plan consists of a head, a thorax with eight segments and an abdomen with six segments. The head is fused with the first thoracic segment to form the cephalothorax. 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 consist of a pair of maxillipeds and a pair of mandibles 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 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 do not bear gills, as do 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 and form a brood chamber for the eggs. In males, the gonopores are on the ventral surface of segment eight and in the females, they are in a similar position on segment six.
The abdomen is formed from six segments, the last one or more of which are fused to the telson (posterior segment) to form a rigid pleotelson. The first five segments each bears a pair of biramous (branching in two) pleopods and the last segment, a pair of biramous uropods (posterior limbs). In males, the second pair of pleopods, and sometimes the first also, are modified for use in transferring sperm. The inner branch of each pleopod is modified into a structure with a thin, permeable cuticle which acts as a gill for gas exchange. In terrestrial isopods, these resemble lungs, and are readily visible on the underside of a woodlouse.
Diversity and classification
Isopods belong to the larger group Peracarida, which are united by the presence of a special brood pouch for brooding eggs. Around 10,215 species of isopod have been described worldwide, classified into 11 suborders. Around 4,500 species of isopods are found in marine environments, mostly on the sea floor. Some 500 species are found in fresh water; another 5,000 species are the woodlice in the suborder Oniscidea, which are thus by far the most successful group of terrestrial crustaceans. 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.
A number of isopod groups have evolved a parasitic lifestyle. The suborder Cymothoida is exclusively parasitic, while the polyphyletic suborder Flabellifera is partly parasitic. Cymothoa exigua, for example, is a parasite of the spotted rose snapper fish Lutjanus guttatus in the Gulf of California; it eats the tongue of the fish, and takes its place, in the only known instance of a parasite functionally replacing a host structure.
Unlike the amphipods, some of which are pelagic, 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.
Feeding and nutrition
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 oesophagus and passes into the stomach where hydrolysis takes place and the material is processed and sorted. 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. Indigestible material passes on through the hindgut and is eliminated through the anus which is located on the pleotelson.
The majority of isopods are scavengers and omnivores, but a few are predators and some have adopted a parasitic lifestyle. 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 raising the nitrogenous content of 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.
Many parasitic species are external parasites of fish and feed on blood. The larvae of the Gnathiidae family and adults cymothoidids have piercing and sucking mouthparts and clawed limbs adapted for clinging onto their hosts. Other 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 free-living isopods and bopyrids, living under the carapace of shrimps and crabs and causing a characteristic bulge which is even recognisable in some fossil crustaceans.
Reproduction and development
In most species, the sexes are separate and there is little sexual dimorphism, but a few species are hermaphrodite and some parasitic forms show large differences between the sexes. Males have a pair of penes, 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 seminal 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 seminal receptacle and the oviduct.
The eggs, which may number up to several hundred, are brooded in the brood chamber of the female, which is filled with water even in terrestrial species. Isopod larvae hatch as mancae, a post-larval stage which resembles the adult except for the absence of the last pair of pereiopods. The lack of a swimming phase in the lifecycle is a limiting factor in isopod dispersal, and may be responsible for the high levels of endemism in the order. As adults, isopods differ from other crustaceans in that they replace their exoskeletons (in the process called ecdysis) in two phases known as "biphasic moulting".
Adaptations for life on land
Terrestrial isopods, suborder Oniscidea, 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 by other means. They do this by living in a humid environment, under stones, bark or debris or hidden in leaf litter. Desert species are usually nocturnal, spending the day in an underground burrow and emerging at night. Moisture is obtained 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 gills. Many species can roll themselves into a ball, a behaviour used in defence but which also helps to conserve moisture. Members of the families Ligiidae and Tylidae, commonly known as rock lice or sea slaters, are the least specialised for life on land. They inhabit the splash zone on rocky shores, jetties and pilings and hide under the debris washed up on the shore, and can swim if immersed in seawater.
- M. Schotte, C. B. Boyko, N. L. Bruce, J. Markham, G. C. B. Poore, S. Taiti & G. D. F. Wilson. "World List of Marine, Freshwater and Terrestrial Isopod Crustaceans". World Register of Marine Species. Retrieved October 7, 2010.
- Richard Brusca (August 6, 1997). "Isopoda". Tree of Life Web Project.
- Frederick R. Schram (1970). "Isopod from the Pennsylvanian of Illinois". Science 169 (3948): 854–855. doi:10.1126/science.169.3948.854. PMID 5432581.
- S. J. Keable, G. C. B. Poore & G. D. F. Wilson (October 2, 2002). "Australian Isopoda: Families". Australian Museum.
- Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition. Cengage Learning. pp. 661–667. ISBN 978-81-315-0104-7.
- Martin,Joel W.; Davis, George E. (2001). An Updated Classification of the Recent Crustacea (PDF). Natural History Museum of Los Angeles County. pp. 132 pp.
- R. C. Brusca & M. R. Gilligan (1983). "Tongue replacement in a marine fish (Lutjanus guttatus) by a parasitic isopod (Crustacea: Isopoda)". Copeia 3 (3): 813–816. doi:10.2307/1444352. JSTOR 1444352.
- Ronald L. Shimek (2002). "Pills, parasites, and predators; isopods in the reef aquarium". Reefkeeping 1 (4).
- Shields, Jeffrey. "Epicaridea: The parasitic isopods of Crustacea". Virginia Institute of Marine Science. Retrieved 2014-03-23.