CHARMANTIER, G.: Ontogeny of osmoregulation in crustaceans : The embryonic phase.

Three patterns of ontogeny of postembryonic osmoregulation have been recognized in crustaceans : (1) osmoregulation varies little with development stage and the adults are weak regulators or osmoconformers; (2) the adult type of efficient osmoregulation is established in the first postembryonic stage; (3) metamorphosis marks the appearance of the adult type of osmoregulation, generallyalong with the occurrence of ion-transporting cells and Na-K ATPase in diverse organs. This review will concentrate on the ontogeny of osmoregulation in embryos, particularly in species belonging to type 2 : they are euryhaline and so are their hatchlings. Embryos are osmotically protected against variations in environmental salinity levels, either by closed incubating pouches (in some cladocerans and isopods), semi-closed pouches (where osmolality is at least partially controlled by the diverted female urine, in some amphipods), or more generally by egg membranes (in species whose eggs are directly exposed to the ambient medium). In some species, temporary (e.g. dorsal organs in amphipods) or definitive (e.g. gills in crayfish) osmoregulatory organs, where ionocytes are located, develop in embryos and the activity of Na-K ATPase increases concomitantly. The ability to osmoregulate is thus acquired during the embryonic development, resulting in osmoregulating hatchlings, which is a necessary and key adaptation for species spending their entire life-cycle at low (particularly in fresh water), high or variable salinity.