A COMPARATIVE-STUDY OF 2 SYMPATRIC SPECIES WITHIN THE GENUS CALLINECTES - OSMOREGULATION, LONG-TERM ACCLIMATION TO SALINITY AND THE EFFECTSOF SALINITY ON GROWTH AND MOLTING

Juvenile blue crabs, Callinectes sapidus Rathbun, and lesser blue crab s, C. similis Williams, were exposed to a range of salinities for 67 d ays to determine if salinity exerted species specific differential eff ects 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 af fected by salinity. C. similis juveniles exposed to 5 parts per thousa nd grew significantly slower than those exposed to 10 parts per thousa nd, with those at 30 parts per thousand exhibiting intermediate growth rates. Salinity had no effect on growth rates of C. sapidus by any me asure of weight or carapace width. C. sapidus grew at faster rates tha n 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 w eight) of C. similis was greater than that of C. sapidus. However, int ermoult periods of C. similis were longer and exhibited a more pronoun ced effect of salinity than those of C. sapidus. Feeding rates and hem olymph osmolalities were measured at the beginning and end of the 67 d ay exposure period. Weight-adjusted feeding rate of C. sapidus increas ed significantly after 67 days exposure to low salinity, whereas that of C. similis decreased significantly. Each species exhibited a declin e in hemolymph osmolality at low salinities by the end of the exposure period. In order to determine if either species exhibits an ontogenet ic shift in ability to regulate hemolymph osmolality, juvenile and adu lt 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 exposu re. Adult C. sapidus maintained higher hemolymph osmolalities than juv eniles when exposed to low salinities (less than or equal to 25 parts per thousand). Hemolymph osmolalities of adult C. similis exposed to l ow salinity varied with salinity of collection site. Those from a high salinity site (30 parts per thousand) exhibited hemolymph osmolalitie s no different than juveniles when exposed to salinities of 2.5 and 10 parts per thousand. Those from a lower salinity site (22 parts per th ousand) exhibited greater hemolymph osmolalities than juveniles, osmor egulating at levels insignificantly different from adult C. sapidus. R esults of this study indicate that although previously published studi es may have overestimated the effects of low salinity on C. similis re lative to it's more euryhaline congener C. sapidus, effects of salinit y alone are probably sufficient to limit this species' distribution to waters of 10 parts per thousand or greater. (C) 1997 Elsevier Science B.V.