Examples of the mechanisms involved in body fluid regulation by present-day crustaceans inhabiting variable salinity habitats are described using amphipod gammarids and the isopod Mesidotea (Saduria) entomon as models. Appropriately the species inhabiting the most demanding habitats have the greatest range and most sophisticated regulatory responses. Behaviour, micromorphology and physiology are all involved to a variable degree and, in the examples discussed, responses to salinity change seem finely tuned to countering the problems generated by particular environments. This applies both in the rapid responses to sudden alteration in salinity and to the longer term changes associated with acclimation to a new steady state condition. The isolation of populations and features of derived freshwater races are considered and the implications for the presumed physiological mechanisms of fossil forms discussed.
