F Two Euryhaline Crab Ectes Similis: Energy Nd Osmoregulation as a Salinity

crab Callinectes swimming crabs th southern coasts of Fowler, 1984; Milli estuarine habitat is characterized by fluctuating salinities and temperatures, which can impose substantial ionic and osmotic stress on inhabitants. Although both C. sapidus and C. similis co-occur in estuarine waters from 30 to 15 ‰ salinity, C. sapidus also inhabits more dilute waters, as low as 0 ‰, and generally prefers low salinities (Perry, 1984; Hsueh et al. 1993). In contrast, C. similis is rarely found below 15 ‰ in Alabama estuaries (Hsueh et al. 1993). Similar salinity-limited distributions have been observed for these and other Callinectes species (Tagatz, 1967; Norse, 1978; Paul, 1982). Diet and habitat characteristics other than salinity were similar in both C. sapidus and C. similis in Alabama estuaries (Hsueh, gge sal . s esp otic (King, 1965; 1983; Mangum et al. 1985). On the basis of measurements of hemolymph osmolarity, Engel (1977) concluded that C. sapidus is the better hyperosmoregulator at lower salinities because it maintained higher hemolymph concentrations of Na+ and Cl2 than C. similis. Hyperosmoregulators are known to maintain high hemolymph osmolarity by active ion pumping in the gills, from the more dilute surrounding sea water into the more concentrated hemolymph (Mantel and Farmer, 1983; Lucu, 1990). The physiological mechanism for active ion uptake involves the coordinated function of a number of membraneThe Journal of Experimental Printed in Great Britain © Th