path: Research & Methods > Organisms > 17. Arthropoda (Crustacea and Insecta) > Crustacea (Crustaceans) > 07. Isopoda > Asellidae
In the UK there is a single genus of freshwater Asellidae, Asellus (however a question still remains over the correct classification of two other brackish genera Jaera and Sphaeroma). This genus contains 4 species, cavaticus which is rarely recorded and exists in underground streams, communis which occurs in a single lake in Northumberland, aquaticus and meridianus which are common and widely distributed. The taxonomic classification is Kingdom, animalia; Phylum, crustacean; Class, malacostraca; Order, isopoda; Family, Asellidae. The Asellidae typically have a head with antenna, mandibles and 2 maxilla; the thorax has 8 thoracic segments with a maxilliped and 7 pereopods ;the abdomen has 6 abdominal segments with 5 pleopods (the first is absent in the female) a uropod and a telson. Size may range from 8 millimetres in juveniles to 1-2 centimeteres in the adult. This genus may also be known by the names ‘hoglouse’, ‘sowbug’ and ‘water louse’.
Known as one of the largest families of freshwater isopods as they can exist in both epigean and hypogean fresh water habitats in North America and Europe They are prevalent throughout the UK, less common in Northern Scotland and apparently absent from the western isles. A. meridianus appears to be the only species to have colonized the offshore islands. These two species can be found in habitats ranging from clear streams to stagnant ponds, but generally they prefer to occupy the benthos of rivers in the slow moving areas rather than riffles.
These species are omnivores, generally feeding on decomposing plants and associated bacteria and fungi but also on some living plants such as Elodea, algae, filamentous diatoms and Stonewort. The juveniles tend to feed on faecal detritus and are predated on by flatworms, leeches, fish and waterfowl. During the breeding season they form pairs and the larger males guard the females and mate with them by grasping them using their fourth pereopods. Mating occurs after the female has moulted and shed the posterior part of the cuticle. Internal fertilisation occurs and the young grow within a ventral brood pouch and then emerge as miniature copies of the adults.
Life cycles are generally controlled by temperature and food availability and in Britain there are two generations. Young born in spring that become sexually mature in summer may only have a relatively short life span of 3-6 months, however the later autumn generation over winter and breed in spring and have a longer life cycle of up to a year. These species are used as indicators of water quality with A. aquaticus being particularly tolerant of a variety of environmental conditions such as organic pollution, high salinities, low pH and heavy metal pollution. Increased domination of A. aquaticus may indicate eutrophication.
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added: 31 August 2004
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