Behavioral and subcellular approach to understanding metal toxicity in fiddler crabs

Behavioral toxicities of Cd and Ag were examined in the fiddler crab, Uca pugnax at 5ppt and 30ppt salinities. Changes in competitive foraging (scooping on mud surface and on a protein-rich patch) and dominance (ability to control patch and percent successful attacks) of both dominant and submissive crabs were examined prior to and after exposure of dominant crabs to series of mumolar concentrations of Cd and Ag. Results show that exposure of crabs to metal concentrations ≤22.3muM had no observable adverse effects on competitive behaviors. At 44.5mum Cd or Ag and greater, competitive behavior of the dominant crab was drastically diminished as compared with pre-exposure, while that of submissive crabs improved remarkably. The protective effect of salinity against metal toxicity was first observed at the lowest observable adverse effect level (LOAEL) of 44.5muM Cd or Ag. The ability of fiddler crabs to tolerate Cd to a greater extent than Ag (88.9--44.5muM) at 5ppt, but tolerance to a greater Ag concentration (148.0--88.9muM) at 30ppt suggests that Ag is more toxic than Cd at low salinity, but it is less toxic than Cd at high salinity. This shows that salinity plays an important role in determining metal toxicity, presumably, by altering metal speciation.; The combined toxicity of Cd and Ag was examined by exposing dominant crabs to Cd and Ag in mixtures at the two salinities. At LOAEL, results suggest that the combined toxicity of Cd and Ag is additive at 5ppt, but may be synergistic at 30ppt. Analysis of tissue burden showed that the bioaccumulation of Cd and Ag increased on a whole body basis as well as in all subcellular fractions analyzed (i.e. organelles, enzymes, metallothioneins and insolubles) with the increase in metal exposure concentration. Unlike Cd, bioaccumulation of Ag was not affected by salinity. This may because neutral Ag chloro complexes are bioavailable whereas Cd chloro complexes are not. This study demonstrates that the toxicity of Cd and Ag is dependent on the physicochemical condition (e.g. speciation), and on metal ability to bind to intracellular fractions, specifically enzymes.