Studies on bivalve molluscs of the genus Crassinella in the Northwestern Gulf of Mexico: anatomy, ecology and systematics.

Nine nominal species of the genus Crassinella have been described in the western Atlantic, but only two biological species are thought to exist there. The variations in size, shape, color and sculpture of these two species are discussed. Crassinella lunulata (Conrad) ranges from Cape Cod to the Gulf of Mexico, Antilles and possibly to Brazil. C. martiniquensis (d'Orbigny) is known only from the Caribbean and Gulf of Mexico. Little is known of the ecology of the latter species, reported from only three stations, at depths of 18 to 25 fathoms, from muddy bottoms, where coarser particles are sparse. C. lunulata is found mostly offshore at depths of one to 35 fathoms, and also in deeper passes in the bays along the coast of the Northwestern Gulf of Mexico. It is usually found wher shell bottom predominates, with little mud. A study of the anatomy of C. lunulatus shows that there is only a single mantle suture, between the two siphonal areas; a single eulamelle-branchian demibranch on each side. The kidney contains granules which seem to increase in size with the size of the animal, and these are thought to be retained secretion products. The red color of the blood is thought to be hemaglobin, free in the plasma. The pigment is also described as contained in cells of the mantle, and in the cytoplasm of the eggs. A peculiar mantle gland, previously unreported, is present in C. lunulatus and Crassatella speciosa (A. Adams) and may be characteristic of the family. A few early embryos found in the gills of these two species suggest that they brood the early developmental stages, at least. The anatomy of Crassinella lunulata differs from that of Crassatella speciosa in several important respects, which are thought to represent generic rather than familial differences.