Bilateral Asymmetry in the Shell Morphology and Microstructure of Early Ontogenetic Stages ofAnomia simplex

Abstract

Scanning electron microscopic examination of the valves of Anomia simplex larvae and postlarvae provides details of bilateral asymmetry in shell morphology and microstructure. Central provincular denticles and larger anterior and posterior hinge teeth develop in both valves. The umbo of the left valve becomes increasingly prominent with larval development, whereas no umbo is evident in the right valve. Thus, right and left valves of the prodissoconch differ markedly in height.In the left valve of the dissoconch, union of antero- and posterodorsal shell margins above the hinge causes lateral, exterior displacement of the umbo. The internal ligament extends ventrally to the right valve. Thin antero- and posterodorsal shell margins of the right valve extend centrally against the substrate and eventually unite exterior to the ligament. An increasingly larger byssal foramen, with a flexible, organic covering, is formed in the right valve.The outer layer of the left valve of the dissoconch is foliated calcite, whereas the outer layer of the right valve is composed of short, calcitic prisms. Inner shell layers consist of crossed lamellar and complex crossed lamellar microstructure, as well as myostracal prisms.