Ontogeny of osmoregulation in the palaemonid

Osmoregulation was studied in the zoeal stages I and VI, the first decapodid, the first juvenile, and in adults of the palaemonid shrimp Palaernonetes argentinus. The larvae hatch in freshwater creeks or in adjacent brackish coastal lagoons of the warm temperate southwestern coast of the Atlantic Ocean; larval development is possible in low salinities. To cope with these demanding environments, the capacity for osmoregulation is well developed at hatching, increasing only slightly throughout development. All the postembryonic developmental stages hyper-regulated at low salinity (1 to 10Ym), hyper-osmoconformed at l?%, and osn~oconformed at higher salinities (26%0; up to 32%0 in adults). The type of osmoregulation did not change dunng developnlent from larval hatching through the adult phase. The ecological implications and the evolutionary sigruficance of osmoregulation in early llfehistory stages of P. argentinus and other aquat~c rustaceans are discussed.