The Ecological Distribution of British Species of Idotea (Isopoda)

Abstract

Descriptions of the ecology of Idotea are often generalized, and there appears to be no comprehensive work on the habits of individual species. The present account of the ecology of Idotea in the Isle of Man, together with a discussion of relevant literature, includes extended observations on the commoner species, and illustrates that each species has a fairly characteristic range of habitat. Collections were made on various types of rocky shore, and in some localities quantitative sampling was attempted with the aid of a wire frame enclosing an area of 0.1 m2. Rock pools were examined, as were regions of the shore subject to the influence of freshwater streams. Surf plankton was collected by dragging a D-net along the edge of the sea on various sandy beaches, and the faunas of surface drift weed and buoys were sampled from a small boat. Dredgings were made off the south coast of the Isle of Man in depths down to about 10 fathoms. Both a naturalist's dredge and a D-net were used, the latter serving mainly to skim accumulations of algae, with accompanying Idotea, from a sandy bottom. Seven species of Idotea were found in the Isle of Man-I. emarginata (Fabricius), I. neglecta G. O. Sars, I. baltica (Pallas), I. linearis (Pennant), I. granulosa Rathke, I. pelagica Leach and I. viridis (Slabber). Collinge (1917) records two other species from Britain: I. metallica Bosc. and I. sarsi Collinge. The first of these does not seem to be a British resident, the few specimens which have been recorded being chiefly found amongst Lepas on floating wood (Tattersall 1911), whilst the second, I. sarsi, is synonymous with I. baltica. I have examined the type specimens of I. sarsi in the British Museum, and they cannot be distinguished from I. baltica (Naylor 1955 c).