The distribution of the freshwater Malacostraca in the area of the Mersey and Weaver River Authority

Freshwater Malacostraca distribution data acquired through the routine biological work of a river authority is of general scientific interest. The distribution of Asellus aquaticus and Gammarus pulex is related to pollution and natural factors. Although widespread in the area, both species are absent from an extensive part of the major river system. A. meridianus appears to be less successful in competition with A. aquaticus and is comparatively scarce, being mostly limited to isolated pools. The distribution of Crangonyx pseudogracilis in canals and static waters is also related to pollution and natural factors. Orchestia cavimana, a semi-terrestrial amphipod, is shown to be an overlooked species which is common in canals.