Ecophysiology of Trace Metal Uptake in Crustaceans

The uptake of trace metals from solution by crustaceans is often described as typically following one of two routes; one passive, the other depending on active transport. In the case of passive facilitated diffusion, the trace metal binds initially to a metal-binding protein in the membrane of the epithelial surface, and then passes down a thermodynamic gradient of metal-binding ligands of increasing metal affinities. Some trace metals may also follow routes for the uptake of major metal ions, as in the case of cadmium and calcium. This uptake is ultimately driven by an energy-requiring pump in the epithelial cell membrane. This may be apical and directly transfer the metal ion into the cell, or as in the case of sodium, a basal ATPase setting up a concentration gradient from the medium to the interior of the cell allowing entry down a metal-transporting channel. Carrier proteins and channels may indeed be variations of the same theme. The relative importance of different routes varies between trace metals and between crustaceans, often according to their ecology. The physicochemistry of trace metal speciation in solution is important in releasing the free metal ion, typically the bioavailable form of a trace metal, but the physiological responses of particular crustaceans may interact to affect uptake rates. Such physiological responses include changes in activities of major ion pumps and integumental permeability, and appear to be a feature of common, but physiologically special, euryhaline crustaceans.