Manganese handling in the shore crab Carcinus maenas: Influence of hypoxia and calcium concentration

Manganese leaching from reduced sediments during oxygen depletion events may expose benthic organisms to elevated manganese concentrations. Various aspects of manganese handling in Carcinus maenas were investigated. Exposure to up to 400 mg Mn L^-1 for 4 d caused no mortality. Crabs accumulated ⁵⁴Mn from water almost linearly with time, reaching a whole-body concentration factor of 21 after 22 d. After uptake from food or water, major parts of the body burden were eliminated with half-lives of 96 and 44 d, respectively. Most of the manganese entering the haemolymph (by feeding or injection) was removed within 24 h, a substantial part being transferred to the exoskeleton. Exposure to 50 and 300 μg Mn⁺⁺ L^-1 in the water led to increased manganese concentrations in most tissues. Hypoxic conditions (19 % oxygen saturation) had only moderate effects on manganese accumulation in midgut gland and exoskeleton, and none in gills. Investigations at field locations confirmed that smaller crabs have higher manganese concentrations in their exoskeleton than larger ones. The most important findings in these experiments are: 1) manganese and calcium compete for uptake over the gills into the haemolymph and 2) manganese in the haemolymph may be translocated to the exoskeleton, thus supporting the possibility that the different behavior of smaller and larger crabs during postmoult calcification of the exoskeleton - by uptake of Ca⁺⁺ from the sea water - may explain their different manganese body burdens (of which ≈95 % is found in the exoskeleton).