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 concerned with the control of
the osmolality of the intracellular fluid, hence with the processes
directly implicated in the maintenance of cell volume and shape. They
have been extensively described in several recent review papers. The
second category includes the processes controllin the charac- teristics
of the cell's environmental fluid in order to minimize the amplitude of
the osmotic shocks the cells may have to cope with upon acclimation to
media of changed salinities. They are localized in particular organs and
structures: the so-called "caZt-transporting" epithelia. Up to now, most
of the studies on salt-transportino epithe- lia in estuarine and marine
animals used the black box approach, so that little or sometimes nothing
is still known on the physiological, the biochemical and the biophysical
basis of the transporting mecha- nisms as well as on the
structure-function relationships.