Short- and long-term salinity challenge, osmoregulatory ability, and (Na+, K+)-ATPase kinetics and α-subunit mRNA expression in the gills of the thinstripe hermit crab Clibanarius symmetricus (Anomura, Diogenidae)

The evolutionary history of the Crustacea reveals ample adaptive radiation and the subsequent occupation of many osmotic niches resulting from physiological plasticity in their osmoregulatory mechanisms. We evaluate osmoregulatory ability in the intertidal, thinstripe hermit crab Clibanarius symmetricus after short-term exposure (6 h) or long-term acclimation (10 days) to a wide salinity range, also analyzing kinetic behavior and alpha-subunit mRNA expression of the gill (Na+, K+)-ATPase. The crab strongly hyper-regulates its hemolymph at 5 and 15 parts per thousand S (Salinity, g L-1) but weakly hyper-regulates up to 27 parts per thousand S. After 6h exposure to 359 , 60S and 45 parts per thousand S, C. symmetricus slightly hypo-regulates its hemolymph, becoming isosmotic after 10 days acclimation to these salinities. (Na+, K+)-ATPase specific activity decreases with increasing salinity for both exposure periods, reflecting physiological adjustment to isosmoticity. At low salinities, the gill enzyme exhibits a single, low affinity ATP binding site. However, at elevated salinities, a second, high affinity, ATP binding site appears, independently of exposure time. (Na+, K+)-ATPase alpha-subunit mRNA expression increases only after 10 days acclimation to 5 parts per thousand S. Our findings suggest that hemolymph hyper-regulation is effected by alterations in enzyme activity during shortterm exposure, but is sustained by increased mRNA expression during long-term acclimation. The decrease in gill (Na+, K+)-ATPase activity seen as a consequence of increasing salinity appears to underlie biochemical adjustments to hemolymph isosmoticity as hypo-regulatory ability diminishes.