Calcium concretions in the gills of a freshwater mussel serve as a calcium reservoir during periods of hypoxia

Abstract

Abstract The gill of the freshwater mussel, Ligumia subrostrata , is composed of 25% calcium concretions on a dry weight basis. These concretions are located extracellularly in the connective tissue of the gill. The concretions have a lamellar structure and occur singly, multiply, and in clumps. The concretions are composed primarily of calcium phosphate and an organic matrix. The concretions are more numerous toward the base of the gill and less frequent in the tip of the gill. The average size of the concretions is 2–3 μm. The concretions are not solubilized in animals exposed to prolonged hypoxic conditions. Instead, as calcium increases in the blood under hypoxic conditions the percentage of the gill dry weight attributed to concretion material increases and the calcium content of the concretion material is elevated. The calcium content of the concretions also is inversely correlated with blood pH. This study indicated that the concretion material in the gills was not mobilized under hypoxic conditions, but served as a calcium reservoir during periods in which the animal was liberating calcium from other sites.