TRANSPOSED HINGES - A POLYMORPHISM OF BIVALVE SHELLS

A study of hinge transposition in the bivalve families Carditidae, Con dylocardiidae, Crassatellidae, Astartidae, Trapezidae, Cardiidae, Tell inidae, and Veneridae, all in the subclass Heterodonta showed that all these families except Condylocardiidae, Cardiidae, Tellinidae. and Ve neridae 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 te eth always had normal cardinal teeth. Previous work on the hinge struc ture of various fossil and living bivalves, other than Sphaeriidae, su pports this observation. The anterior lateral teeth and the cardinal t eeth may be a single genetically linked unit in these families, and th e posterior lateral teeth another unit. Each unit is represented by tw o phenotypes, namely normal and transposed dentition. With a combinati on of the two units by two phenotypes, therefore, four types of dentit ion are expected in heterodont bivalves, other than the Sphaeriidae. T hey are Phenotype I--normal anterior lateral teeth & cardinal teeth an d normal posterior lateral teeth. Phenotype II--normal anterior latera l teeth & cardinal teeth and transposed posterior lateral teeth, Pheno type III- transposed anterior lateral teeth & cardinal teeth and norma l posterior lateral teeth, and Phenotype IV--transposed anterior later al teeth & cardinal teeth and transposed posterior lateral teeth. In c ontrast to a whole transposition of gastropods with anatomical and she ll 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 nearl y neutral. At present, there is no evidence to show that a transposed hinge is genetically controlled. Some evidence at the evolutionary lev el suggests that it is not a result of developmental noise or accident , but it may be a developmentally controlled directional asymmetry, wh ich may be an example of a genetically controlled discontinuous charac ter.