Activity of Na+,K+-ATPase in a ‘freshwater shrimp’, Palaemonetes argentinus (Caridea, Palaemonidae): ontogenetic and salinity-induced changes

Embryos, larvae, and adults of Palaemonetes argentinus tolerate a wide range of salinities (1 to 25‰). While osmoregulatory capacities have previously been demonstrated in all postembryonic stages, little is known about the occurrence of osmoregulation during the embryonic phase. We examined ontogenetic and salinity-induced changes in the activity of a key enzyme involved in osmoregulation, Na+,K+-ATPase. Its activity was studied in: (1) eggs at an early (SI), an intermediate (SII), and a late stage of embryonic development (SIII); (2) in newly hatched larvae (Zoea-I, ZI); and in homogenates of (3) whole adults and (4) isolated gill tissue. All stages were directly exposed to 1, 15, or 25‰, and Na+,K+-ATPase activity was chemically determined 24 h (embryos, larvae) or 48 h later (adults). Enzyme activity was detected in all developmental stages, being low in SI and SII, maximum in SIII, and intermediate in ZI and adults. Maximum salinityinduced activity changes prior to hatching (SIII) suggest that hyper-osmoregulatory functions are expressed by the end of the embryonic phase. The ontogenetic activity maximum at this stage, however, may also be related to the hatching process. Comparing different salinities, Na+,K+-ATPase activity in SIII was always highest at 15‰, whereas the activity in gills was higher at both 15 and 25‰ than at 1‰. While gills are absent in the embryonic and early larval stages, ion-transporting cells must be located elsewhere during these early ontogenetic stages, probably in the brachiostegites.