THE KINETICS OF ZINC AND CADMIUM IN THE HEMOLYMPH OF THE SHORE CRAB CARCINUS-MAENAS (L.)

The trace metals zinc and cadmium are taken up from solution by the sh ore crab Carcinus maenas into the haemolymph. Both metals are accumula ted in the haemolymph at a rate proportional to the external dissolved metal concentration (and hence the rate of metal uptake into the crab ). Newly taken up zinc is added sequentially over time to the substant ial content of zinc present in the haemolymph, replacing existing zinc already present, but only at extremely high ambient concentrations (1 000 mu g Zn l(-1)) did the incoming zinc replace significant amounts o f existing haemolymph zinc. However, even after long-term exposure, cn . 55% of the zinc in the haemolymph remained too tightly bound to be r eplaced by incoming zinc. In the haemolymph, newly taken up cadmium, i n contrast to zinc, rapidly reaches an equilibrium concentration when the rate of removal matches the rate of cadmium uptake into the haemol ymph. This equilibrium concentration of new cadmium in the haemolymph increases with an increase in the external dissolved cadmium concentra tion. The rapid removal of cadmium from the haemolymph, therefore, ind icates that in most circumstances, the haemolymph acts as a transient store for accumulated cadmium. The rate of zinc accumulation in (and u ptake into) the haemolymph was not affected by pre-exposure of crabs t o zinc, indicating the lack of induction of a zinc-binding system with in the haemolymph or in a tissue able to receive zinc from the haemoly mph. Cadmium was unable to replace haemolymph zinc, indicating either that zinc is bound too tightly for replacement by cadmium or that it o ccupies different binding sites in the haemolymph to cadmium. The long half-life of zinc in the haemolymph (ca. 21 days) probably reflects t he turnover of haemocyanin. In Carcinus maenas the high concentration of zinc in the haemolymph (ca. 35 mu g Zn ml(-1)) acts as a store for zinc. Haemolymph volume of C.maenas was estimated to be 0.32 +/- 0.10 mi per gramme total fresh weight. (C) 1998 Elsevier Science B.V.