BIVALVES UNHINGED: HINGE MORPHOLOGY AND BIOMECHANICS IN THE VENERIDAE

Hinges enable bivalves to perform the critical motion of opening and closing the paired valves, while resisting shear during burrowing and predation. The diversity in hingeplates across the family Veneridae echoes the functional and morphological variety that defines this, the most diverse family in the Bivalvia (~750 extant species in 135 genera). Despite the biomechanical importance of the hinge, its complex shape has made it difficult to analyze in traditional morphometric frameworks. Thanks to our campaign to 3D scan all of Bivalvia using X-ray computed micro-tomography (microCT), we have unprecedented access to the minute details of hingeplate morphology. In particular, we focused on the toothbank (i.e. the region of the hinge containing the primary dentition in the form of the cardinal teeth). We quantified toothbank morphology by using homologous points and semilandmark curves to automatically generate surface semilandmarks on 3D scans of 150 species in 132 genera. This method allowed us to compare more distantly related and morphologically disparate taxa than a strict landmark analysis would. We used Procrustes-PCA (Principal Components Analysis) to create a morphospace of hinge shapes, within which we assessed potential effects of partitioning by taxonomic association, body size, and shell ornamentation. Our preliminary results revealed biomechanical trends among hingeplate shape, tooth placement, and tooth “topography” (the projection of the teeth above the hingeplate). Specifically, we found that these trends correlate more strongly to shell ornamentation and life habits than taxonomic groupings or shell size/outline shape. Compared to unornamented shells, highly ornamented shells have larger hinge plates with higher reliefs, and shallower burrowers tend to have thicker teeth and correspondingly wider sockets than deeper burrowers.