A comparative study of two sympatric species within the genus Callinectes: osmoregulation, long-term acclimation to salinity and the effects of salinity on growth and moulting

Abstract

Juvenile blue crabs, Callinectes sapidus Rathbun, and lesser blue crabs, C. similis Williams, were exposed to a range of salinities for 67 days to determine if salinity exerted species specific differential effects on growth and moulting. Growth was measured in terms of carapace width, wet weight, dry weight and ash-free dry weight. Growth rates of the two species (% increase in dry weight/day) were differentially affected by salinity. C. similis juveniles exposed to 5‰ grew significantly slower than those exposed to 10‰, with those at 30‰ exhibiting intermediate growth rates. Salinity had no effect on growth rates of C. sapidus by any measure of weight or carapace width. C. sapidus grew at faster rates than C. similis at low salinity as measured by wet and dry weight, and at all treatments as measured by carapace width. Growth per moult (wet weight) of C. similis was greater than that of C. sapidus. However, intermoult periods of C. similis were longer and exhibited a more pronounced effect of salinity than those of C. sapidus. Feeding rates and hemolymph osmolalities were measured at the beginning and end of the 67 day exposure period. Weight-adjusted feeding rate of C. sapidus increased significantly after 67 days exposure to low salinity, whereas that of C. similis decreased significantly. Each species exhibited a decline in hemolymph osmolality at low salinities by the end of the exposure period. In order to determine if either species exhibits an ontogenetic shift in ability to regulate hemolymph osmolality, juvenile and adult C. sapidus and C. similis were collected and exposed to a range of salinities for measurement of hemolymph osmolalities. These crabs were collected and exposed separately from those used in the 67 day exposure. Adult C. sapidus maintained higher hemolymph osmolalities than juveniles when exposed to low salinities (≤25‰). Hemolymph osmolalities of adult C. similis exposed to low salinity varied with salinity of collection site. Those from a high salinity site (30‰) exhibited hemolymph osmolalities no different than juveniles when exposed to salinities of 2.5 and 10‰. Those from a lower salinity site (22‰) exhibited greater hemolymph osmolalities than juveniles, osmoregulating at levels insignificantly different from adult C. sapidus. Results of this study indicate that although previously published studies may have overestimated the effects of low salinity on C. similis relative to it`s more euryhaline congener C. sapidus, effects of salinity alone are probably sufficient to limit this species' distribution to waters of 10‰ or greater.