Extracellular matrix and muscle fibers in the gills of freshwater bivalves

The supportive tissue in the gills of three species of freshwater bivalves (Corbicula fluminea, Dreissena polymorpha, and Toxolasma texasensis) is com posed of a similar extracellular matrix with interspersed muscle fibers. Th ese matrices consist of a periodic acid Schiff-positive tissue supported by small collagen fibrils as determined by morphological and biochemical exam ination. The periodicity observed in fibrils and the electrophoretic migrat ion patterns are consistent with other collagens. The ultrastructure of the muscle fibers associated with the connective tissue corresponds with a kno wn type of smooth muscle. The high degree of similarity between the tissues of these species suggests a similar, conserved function.