Evolutionary convergence of bivalved shells: A comparative analysis of constructional constraints on their morphology

Abstract

SYNOPSIS. Bivalved, exoskeletal shells have evolved independently in brachiopods, several groups of molluscs, ostracodes, conchostracans, phyllocarids, and the Paleozoic rugose coral Calceola. Composed of a variety of usually composite organic and mineral materials, they may be rigid or somewhat flexible. Shell growth can occur only by accretion at the margins and over the inner surfaces of the valves. Isometric growth produces logarithmic spiral cones, paradigms from which real shells depart slightly or very far, in allometric response to physiological or mechanical demands of function. Shells which do not grow are molted and replaced at regular intervals in ostracodes and phyllocarids. The bivalved shell is the simplest of lever skeletons, its two elements being articulated in most cases about a fixed axis. Its role in support of soft tissues is complemented in all groups either by hydrostatic organs, as in the molluscan foot and brachiopod lophophore, or by an inner, articulated chitinous exoskeleton, as in the bivalved arthropods. Common constraints imposed by growth processes and the mechanics of articulation prescribe the observed close convergence of the hinge, adductor muscles, and structures that maintain valve alignment. Divergent adaptations accomodate varied shell functions, as protectivecovers (one adductor adequate), as digging tools (two adductors and/or substantial hinge teeth required), in channeling feeding currents, and as hydrofoils. The bivalved shell facilitates a wide range of adaptations in aquatic environments, but it places stringent limits on size and mobility. Such extensive convergence reflects the shell's ease of construction,its multiple functions, and the limited variety of viable skeletal designs. © 1988 by the American Society of Zoologists.