A wealth of information on osmotic and ionic reaulation in Estuarine and Marine Animals has been accumulated over the past decades. Beyond early studies of whole-animal responses to changes in envi- ronmental salinities, efforts have been made later on to identify, to localize and to characterize the organs and structures responsible for the control of the characteristics of the cell's environmental fluid. When considering the problem of cell volume control in animals facing media of fluctuating salinities, we are indeed dealing with two different categories of mechanisms. A first one is...