COSMOPOLITAN COMPROMISES AND TROPICAL TRADE-OFFS – THE RELATIONSHIP BETWEEN LATITUDINAL AND MORPHOLOGICAL “RANGE” IN A DIVERSE BIVALVE FAUNA

The taxonomic, functional, and morphological richness of life on Earth is distributed unevenly across the globe, following gradients dictated by both biotic and abiotic factors. As those underlying factors shift in distribution and intensity, we might expect to see corresponding shifts in the distribution of biodiversity. In order to predict, or detect, such shifts in the fossil record and going into the future, we need to understand these axes of biodiversity alongside environmental variation in the present day; here we analyze morphological patterns and their correlates along a latitudinal gradient. The bivalve shell is a tightly functionally controlled structure that interacts with both the external environment and the soft parts of the animal itself. Thus, shell morphology is a compromise among anatomical constraints, biochemical resource allocation, and functional requirements dictated by the environmental conditions the animal lives in, and so is an ideal target for analyses of morphological diversity across latitude. We use micro-CT scans of the highly diverse modern Florida Keys bivalve fauna (c. 360 species) to quantify internal biovolume, shell-material volume, and internal and external surface areas, which enables us to evaluate the interplay between these parameters and the potential role of environmental regulation of shell construction, comparing tropical specialists to wider-ranging generalist taxa. This set of 3D measurements provides a framework for detailed investigation of the controls on bivalve morphological diversity across both spatial and temporal scales. Early analyses suggest that Keys-resident taxa that range further north have shells that are thinner relative to their biovolume, less complex external surface areas, and potentially are allocating fewer resources to shell building, than those co-occurring taxa that are restricted to the tropical latitudes. There is also a narrowing of variance of interspecific shell form with increasing northern latitudinal extent - indicating convergence in aspects of morphology regardless of taxonomic affinity, and potentially, trade-offs in morphology for greater environmental range.