ECOPHYSIOLOGY OF TRACE-METAL UPTAKE IN CRUSTACEANS

The uptake of trace metals from solution by crustaceans is often descr ibed as typically following one of two routes; one passive, the other depending on active transport. In the case of passive facilitated diff usion, the trace metal binds initially to a metal-binding protein in t he membrane of the epithelial surface, and then passes down a thermody namic gradient of metal-binding ligands of increasing metal affinities . Some trace metals may also follow routes for the uptake of major met al ions, as in the case of cadmium and calcium. This uptake is ultimat ely 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 concentrati on 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 dif ferent routes Varies between trace metals and between crustaceans, oft en according to their ecology. The physicochemistry of trace metal spe ciation in solution is important in releasing the free metal ion, typi cally the bioavailable form of a trace metal, but the physiological re sponses of particular crustaceans may interact to affect uptake rates. Such physiological responses include changes in activities of major i on pumps and integumental permeability, and appear to be a feature of common, but physiologically special, euryhaline crustaceans. (C) 1997 Academic Press Limited.