Salinity effects on cadmium accumulation in various tissues of the tropical fiddler crab Uca rapax

Pollution of water and sediments by industrial discharges of heavy metals, mainly cadmium (Cd), threatens the biota of mangrove ecotones around the mouth of the Tuy river, in northern-central Venezuela. Short-term lethal Cd concentrations were determined in adult male Uca rapax collected from a clean site and kept in 75% seawater (SW). The 96-h LC50 obtained, 380 micromol (=43 mg) Cd liter(-1), is remarkably higher than values previously reported for other crustacean, mollusc or fish species. Tissue accumulation of Cd was examined in crabs acclimated to 125, 75 or 25% SW and exposed to sublethal CdCl2 levels ranging from 9.0 to 225 micromol liter(-1) (i.e. 1.0-25 mg Cd liter(-1)). The metal contents of carapace, gills, hepatopancreas, muscle and hemolymph rose pronouncedly at the highest concentration tested and particularly in the crabs treated in dilute (=25%) SW. In the latter, analyses of the relative distribution of whole-body Cd burden showed that about 80% is in the carapace, 2.4% in the gills, 4% in hepatopancreas, 3% in muscle, 9.3 in other viscerae, 1% is bound to the dried solids of hemolymph and ca. 0.2% appears to be in solution in this fluid. Exposure of crabs to Cd in dilute SW enhances the rates of entry or uptake of the metal, possibly mediated by Ca2+ transport mechanisms, and may exceed the detoxifying capacity of organs such as the hepatopancreas. Thus, Cd may be more harmful to crab populations experiencing frequent fluctuations of ambient salinity, such as are usual in mangrove swamps.