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The Australian freshwater mussel Hyridella depressa (Lamarck, 1819) has extensive deposits of calcium phosphate granules in its interstitial connective tissues. The structure, distribution and elemental profile of these granules were documented by light and electron microscopy. For the elemental study, granules in cryo-prepared mantle tissue were examined by X-ray microanalysis. The granules were bright orange and formed a conspicuous cover over the mantle and palps, and were also abundant in the visceral mass. By contrast, they were not common in the gills and foot. The granules were typically distributed in discrete clusters and, in places, dominated the tissue space. Iron was a particularly important component of the granules and may account for their colour. The granules also contained other common elements, including Mg, Mn and Al, and trace elements, including Cu, Zn and Pb. Considering the important position of the Hyriidae in understanding the evolution and phylogeny of the Unionoida, emphasis was placed on comparison of the calcium granules of H. depressa to those in the Margaritiferidae and Unionidae. Granule distribution in H. depressa was most similar to that described for margaratiferids and contrasted with that described for unionids. The impressive capacity to accumulate extensive calcium deposits in their tissues is a unique feature of the Unionoida, but the rationale underlying production of these excess calcium stores is not understood. It is suggested that the granules may be a by-product of biomineralization processes in the Unionoida associated with the highly efficient calcium uptake system these bivalves evolved in conjunction with colonization of freshwater environments.

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