Maternal calcium contribution to glochidial shells in freshwater mussels (Eulamellibranchia: Unionidae)

Abstract

AbstractPrevious work has demonstrated that calcium concretions, located in the gills of freshwater mussels, disappear during the reproductive season. While calcium is being mobilized, there is no increase in calcium content in the blood. The current study documents the passage of maternal calcium to glochidia, for the production of calcium carbonate shells while the larvae are brooding in the water channels of the lateral gills; 45Ca was injected into Anodonta grandis 2 mo before the onset of reproduction and the label was distributed through the animal but was concentrated largely in the calcium concretions. Following glochidia development, 45Ca was found incorporated in the shell of the embryos. To determine the extent of the maternal passage of calcium to the embryos, Ligumia subrostrata containing embryos undergoing shell formation were exposed to 45Ca, either by injection into the blood or by inclusion of label in the bathing medium. The amount of label found in the developing glochidia is proportional to the 45Ca content in the blood of the maternal animal. Isolated glochidia directly exposed to label will adsorb 45Ca onto their shells, but there is little addition of label to glochidia in intact maternal gills during the bath exposure experiments. The results indicate that the maternal contribution of 45Ca is over 90%, with direct contribution from the bathing medium accounting for less than 8%. These results indicate that the branchial water channel in which the embryos are developing is isolated from bath 45Ca. This study presents morphological evidence that the physiological isolation of glochidial shells from bath 45Ca is due to a rearrangement of water channel morphology, and that the chamber, in which the embryos develop, has no contact with the bathing medium.