TRANSPOSED HINGES: A POLYMORPHISM OF BIVALVE SHELLS

Abstract

A study of hinge transposition in the bivalve families Carditidae, Condylocardiidae, Crassatellidae, Astar-tidae, Trapezidae, Cardiidae, Tellinidae, and Veneri-dae, all in the subclass Heterodonta showed that all these families except Condylocardiidae, Cardudae, Tellinidae and Venendae rarely have transposed hinges. Of the 3767 valves studied, any valve with transposed anterior lateral teeth had transposed cardinal teeth on the same valve, and any valve with normal anterior lateral teeth always had normal cardinal teeth Previous work on the hinge structure of various fossil and living bivalves, other than Sphaemdae, supports this observation. The anterior lateral teeth and the cardinal teeth may be a single genetically linked unit in these families, and the posterior lateral teeth another unit. Each unit is represented by two pheno-types, namely normal and transposed dentition. With a combination of the two units by two phenotypes, therefore, four types of dentition are expected in heterodont bivalves, other than the Sphaemdae They are Phenotype I—normal anterior lateral teeth & cardinal teeth and normal posterior lateral teeth Phenotype II—normal anterior lateral teeth & cardinal teeth and transposed posterior lateral teeth, Phenotype III—transposed anterior lateral teeth & cardinal teeth and normal posterior lateral teeth, and Phenotype IV—transposed anterior lateral teeth & cardinal teeth and transposed posterior lateral teeth. In contrast to a whole transposition of gastropods with anatomical and shell asymmetry, Phenotypes II and III show that a partial transposition is fairly common within the bivalves. The partial transposition may be caused by their symmetrical organization and may be selectively nearly neutral. At present, there is no evidence to show that a transposed hinge is genetically controlled. Some evidence at the evolutionary level suggests that it is not a result of developmental noise or accident, but it may be a developmentally controlled directional asymmetry, which may be an example of a genetically controlled discontinuous character.