Ecophysiology of salt acclimation in crustaceans : a mini review.

With their marine, fresh water and euryhaline representatives, crustaceans exhibit almost any of the known possible patterns of osmo-ionoregulation. That group therefore appears as a choice material to tackle the question of ecophysiology of salt acclimatisation from a comparative point of view. In crustaceans, osmo-ionoregulation can be effected in two different ways the significance of which is always to avoid water movements at the cellular level. The first one, of general occurrence and considered as a prerequisite for adaptation to salinity changes, is to maintain the intracellular fluid isosmotic to the extracellular fluid, either the body fluids, or that of the environment. The second one is to control the concentration of the extracellular fluids at a more or less constant level regardless of the external salinity. This review will focus on the second way where mechanisms are active essentially in boundary epithelia. The gills will be shown to be the prominent structure responsible for the blood NaCl balance and regulation in marine, marine euryhaline and brackish water species. The review will therefore deal mostly with recent physiological and ultrastructural data on gill tissue and provide information leading to a characterization of the particular mechanisms and driving forces at work at that level. It will refer largely to experiments using perfused preparations of gills isolated from the chinese crab Eriocheir sinensis, taken as a model. The applicability of the chinese crab model to other crustaceans will be considered. It will be shown also that the cuticle lining the epithelium is largely involved in ionic regulation in crustaceans. It does contribute indeed to reduce ionic leaks in regulators and yet allows for the entry of ions across specific <<channels>> at the sites where active uptake takes place. An attempt is made to understand how both the cuticle and the epithelium fit in a working epithelio-cuticular complex.