Evolutionary Convergence of Bivalved Shells: A Comparative Analysis of Constructional Constraints on their Morphology

SYNOPSIS. Bivalved, exoskeletal shells have evolved independentlyin brachiopods, several groups of molluscs, ostracodes, conchostracans,phyllocarids, and the Paleozoic rugose coral Calceola. Composedof a variety of usually composite organic and mineral materials,they may be rigid or somewhat flexible. Shell growth can occuronly by accretion at the margins and over the inner surfacesof the valves. Isometric growth produces logarithmic spiralcones, paradigms from which real shells depart slightly or veryfar, in allometric response to physiological or mechanical demandsof function. Shells which do not grow are molted and replacedat regular intervals in ostracodes and phyllocarids. The bivalvedshell is the simplest of lever skeletons, its two elements beingarticulated in most cases about a fixed axis. Its role in supportof soft tissues is complemented in all groups either by hydrostaticorgans, as in the molluscan foot and brachiopod lophophore,or by an inner, articulated chitinous exoskeleton, as in thebivalved arthropods. Common constraints imposed by growth processesand the mechanics of articulation prescribe the observed closeconvergence of the hinge, adductor muscles, and structures thatmaintain valve alignment. Divergent adaptations accomodate variedshell functions, as protectivecovers (one adductor adequate),as digging tools (two adductors and/or substantial hinge teethrequired), in channeling feeding currents, and as hydrofoils.The bivalved shell facilitates a wide range of adaptations inaquatic environments, but it places stringent limits on sizeand mobility. Such extensive convergence reflects the shell'sease of construction,its multiple functions, and the limitedvariety of viable skeletal designs.