Members of the large crustacean order, the Isopoda, occur in a wide range of habitats, from the deep sea to the sea-shore, through rivers and ponds, to terrestrial habitats of gardens and woodland. Their body shape throughout all these diverse habitats is broadly similar to that typified by the woodlouse, being dorso-ventrally flattened, and divided into a broad head bearing lateral eyes, a thorax of seven segments, and an abdomen of six segments.
A number of marine isopods have adopted a parasitic way of life, either as juveniles or adults, with very little change to their morphology. A number of other isopod groups, however, have become such highly specialised parasites, that their morphology, particularly of the females, has become so distorted as to make them almost unrecognisable as isopods:-
The Bopyridae live in the branchial chambers of crabs and prawns, the Entoniscidae live internally in the viscera of crabs, the Dajidae on mysids and euphausiids, and the Cryptoniscidae in barnacles.
Several parasitic species are to be found in the coastal and off-shore areas of the central North sea and are readily identified from these images of specimens recorded in the area.
The curious isopods of this genus are benthic, but give rise to a small parasitic larval stage known as a praniza, that are commonly found on fishes along the Yorkshire coast.
A large species growing to 45mm in length, it is found clinging and feeding on fishes. Possibly many individuals become dislodged from their hosts during the trauma and crush of trawling operations, thus distorting evidence of the true status of this species, but on present records it appears to be a very rare isopod in the North Sea.
Usually found attached to the abdomen of hermit crabs, Pagurus, but may occur also in the branchial cavity of the same host.
This species occurs in the branchial cavity of the common Hippolyte varians,orientated with its dorsum towards the gills, and the ventral surface and brood chamber facing externally. The presence of the parasite produces a swollen aspect to the carapace of the host.
Similar in morphology to Bopyroides, this species is frequent in the branchial chambers of species of the burrowing decapod Upogebia, that is present in large numbers off the Yorkshire coast.
This appears to be a rare species on the Yorkshire coast, and is found in the branchial chamber of species of Galathea, the squat lobster. Infected hosts are unlikely to be overlooked due to the carapace distortion caused by the parasite.
A common species along the coast, infecting the long-clawed porcelain crab, Pisidia longicornis. There is usually no distortion of the carapace of the host, or any other external evidence of infection.