Testing for allocation strategies and evolutionary tradeoffs in the bivalve shell

ABSTRACT Tradeoffs are a fundamental aspect of evolution, such that organismal morphologies often reflect a combination of developmental constraints and functional requirements. The net result is a striking variety of allocation strategies—where and how resources are shunted into aspects of the body plan to meet these competing demands. The bivalve shell records aspects of the animal's allocation strategies as the relative investment in the carbonate valves and the internal soft parts, generally reflected by the volume of the internal cavity. The carbonate volume can be further partitioned among different components of the shell, including the sculptural elements thought to evolve partly under ecological controls. Assuming there are general limits to energy acquisition, bivalve individuals may show different allocations between the volumes of the inner cavity and shell, and possibly for the sculpture. Using 3D scans of 385 valves representing 339 extant marine species occurring in the Florida Keys, USA, we find a variety of allocation strategies but that tradeoffs in shell construction are uncommon. Instead, species with relatively high allocations to their shell thickness also have prominent sculpture, suggesting a structural relationship. Neither outer shell mineralogy nor microstructure appears to partition the tradeoff space, suggesting varied pathways to the fabrication of both convergent and divergent shell forms. Framing new morphological and physiological analyses of bivalves by allocation strategies recorded in their well-fossilized shells opens new opportunities for understanding an under-studied, macroevolutionary dynamic: how tradeoffs have shaped the class's 500 million-year evolutionary and ecological history.