BEYOND THE HALF-SHELL: MORPHOLOGICAL INTEGRATION AND MODULARITY OF VENERID BIVALVE SHELLS

Morphological integration is a measure of relationships among morphological characters at the level of the organism. Patterns of morphological integration are descriptions of how the phenotype of a group of organisms is actually assembled from recognized characters. Variation of patterns within a monophyletic clade is therefore representative of adaptive histories, as well as phylogenetic, developmental and functional constraints. In this study we use a method based on homologous landmarks to visualize patterns of integration among landmarks directly on the valves of a diverse collection of venerid taxa. Viewing patterns of morphological integration within this diverse clade, and over geological time, will allow us to address the role of phenotypic construction in morphological constraint, adaptive radiation, and phylogenetic diversification.

Patterns of integration are calculated as partial correlations among landmarks and may be visualized directly on the configuration of landmarks as graphs, where landmarks represent nodes, and edges are representative of significant correlations. Two networks are derived for a sample of individuals; first a network that incorporates growth and development, and reflects the common dependence of many landmarks on underlying developmental patterns, and second, a network that is size-independent, and possibly reflects spatial relationships that are determined early in development, or that are maintained for strictly functional reasons. Preliminary results from the left valves of chionine bivalves indicate modular structure of the hinge. We will also use paired valves for this exercise, because many landmark relationships are probably related to the symmetry/asymmetry of the bivalve shell. Furthermore, our network approach allows us to examine the effects of varying strengths of integration, an important concept for understanding constraint and selection, as well as the presence, nature and extent of modularity in the shell.