Osmoregulation in the Graceful Crab, Cancer gracilis (Crustacea: Brachyura), from southern Puget Sound
Nicole A. Nelson, Kirk J. Ireson, Raphael D. W. Ritson-Williams, and Erik V. Thuesen
Although very abundant in Puget Sound at salinities below 30 ppt, Cancer gracilis Dana is often considered to be a coastal species unable to tolerate estuarine conditions. In order to investigate the osmotolerance of C. gracilis, crabs were maintained at 10, 13, 17, 25, and 38 ppt for an incubation period of two weeks. Cancer gracilis had a 100 % survival rate at salinities of 17 and 25 ppt, had reduced survival at 13 and 38 ppt, and did not survive at 10 ppt. Hemolymph drawn from crabs incubated for four hours in salinities ranging from 14-37 ppt shows C. gracilis to be a weak hyperosmoregulator at salinities from 14-20 ppt, and a weak hyposmoregulator at 33 and 37 ppt. Osmoregulation was also observed in C. gracilis following a 24-hour incubation period at 16 ppt. Regulation of Na+ and Ca2+ ion concentrations in the hemolymph was hyperosmotic at all experimental salinities. Levels of K+ remained isosmotic with the seawater, and Mg2+ was hyporegulated at all salinities. Carbonic anhydrase (CA), an enzyme that contributes to internal ionic regulation and pH balance, was also investigated. Enzyme assays were performed to obtain the CA activity of each gill. No difference was found in CA activity between gill pairs, indicating that there are no specialized performance patterns in the gills of freshly caught C. gracilis. These results stand in contrast to previous data gathered for strong osmoregulators which have increased CA activity in the posterior gill pairs that function in osmoregulation. The data for C. gracilis support the hypothesis that the patterns of CA distribution observed in the gills of strong osmoregulators by other investigators are an osmoregulatory adaptation to hyposaline environments.
The photograph below of Cancer gracilis was taken by Brian Kegel.
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