Form and Function of Calcium Concretions in Unionids

Abstract

Unionid mussels brood hundreds of thousands of embryos to a fully-shelled glochidia stage larvae before releasing them to the freshwater environment. These embryos are developed within the enlarged water channels of brooding gills. The source of calcium for shell formation comes from the maternal animal, where calcium is stored as extracellular calcium phosphate concretions in the connective tissue of unionid gills as well as in the conspicuous adult shell. These concretions can compose up to 50% of the dry tissue weight of the gills in non-reproductive unionids. These concretions are mobilized during reproduction concomitant with shell production in the glochidia. Passage of 45Ca to the embryos has been demonstrated from maternal animals injected with radiolabel one to three months before embryo development begins. In addition, when gravid females are exposed to 1,000,000 CPM 45Ca over a 4 h period, either injected into the blood or placed directly into the external bath, uptake of label by glochidia is directly related to the amount of radiolabel present in maternal blood, and shows no relationship to the amount of label in the external bath. Conversely, glochidia isolated from the water channel and placed in labeled bath, accumulate roughly 10 times the observed label incorporated while in the gill. This value is similar to adsorption seen on mature shell. Thus calcium contribution is from maternal sources. These data indicate the water channel is a physiologically closed brood chamber adapted to provide a protective environment for developing embryos. Morphological evidence indicates the water channel becomes three separate compartments during brooding in Anodonta, such that water does not have to pass directly over the embryos for respiratory purposes. The calcium concretions themselves are rapidly mobilized during the reproductive period of the year suggesting tight neurohormonal control of mobilization. The organic matrix of the concretions contains at least one calcium-binding protein with molecular weight and calcium binding activity similar to calmodulin. This protein cross-reacts with calmodulin antibody and is the only soluble organic material thus far isolated with calcium binding properties.