Exploring the depuration of metals in the green shore crab (Carcinus maenas): Studies with radiolabelled calcium, zinc, and nickel

The depuration of newly accumulated metal from the crab Carcinus maenas (inter-moult stage) was studied, with a particular focus on the carapace, in light of recent findings that it is a major site for direct uptake and incorporation of Ca, Zn, and Ni from the external sea water. Crabs were exposed for 24 h to calcium ([Ca] = 389 mg L^-1 or 9.7 mmol L^-1), zinc ([Zn] = 82 μg L^-1 or 1.25 μmol L^-1), and nickel ([Ni] = 8.2 μg L^-1 or 0.14 μmol L^-1) with the addition of radio-labeled metal (⁴⁵Ca, ⁶⁵Zn, ⁶³Ni) in sea water (12 °C, 32 ppt), then transferred to clean sea water for ≥48 h. After 24 h of metal exposure, the carapace accounted for ≥85 % of the total body burden of all three newly accumulated metals. For Ca, depurations from the carapace and whole crab were negligible, though levels in soft tissues (gills, hemolymph, and muscle) fell quickly. In contrast, newly accumulated Zn levels in carapace and gills declined by ~60 % over 48 h, while muscle, hepatopancreas, and hemolymph burdens increased, reflecting shifts from gills and/or carapace to internal tissues. Newly accumulated Ni concentrations in the carapace and gills declined by ~50 % over 48 h, reaching >75 % loss by 10 days. As for Zn, Ni levels increased in the hemolymph and initially in the hepatopancreas, indicative of internal redistribution. Acute temperature increase (12 °C to 22 °C) had negligible effects on depuration rates (Q₁₀ values ~1.0). Depuration from the carapace was unchanged in recently euthanized crabs, or when the carapace was shielded with a membrane. We conclude that physicochemical processes are the rate-limiting steps for metal elimination from the carapace, and that metals newly incorporated in the carapace can leave only through the internal surface of the carapace; the external surface is not a depuration site.