Interaction of trace metals Zn, Cd, and Mn, with Ca concretions in the gills of freshwater unionid mussels

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Abstract
The interaction of Zn, Mn, and Cd with Ca concretions in the gills of unionid mussels was investigated. Raising blood levels of these ions to twice their normal values for a week does not result in significant deposition of these metals into gill concretions. The interaction of Zn with the concretions was examined in more detail both in vivo and in vitro. Isolated concretions bind Zn as rapidly as Ca. However short-term in vivo labelling studies using radiolabelled Zn and Ca verify that Zn is not added to concretions while Ca is being rapidly accumulated. These studies demonstrate that calcium concretions from the gills of unionid mussels do not indiscriminately bind divalent cations, but precisely regulate the species of divalent cation incorporated. This is further evidenced by the lack of relationship between blood or pond water cation levels and the levels of specific cations found in the concretions. Since the concretions are mobilized during the reproductive period, and serve as a source of glochidial calcium, it would be disadvantageous for these concretions to bind divalent cations for detoxification purposes.