Evolutionary Diversification of the Marine Bivalve Clade Galeommatoidea.

Abstract

This dissertation investigates the diversification and morphological evolution of a major extant marine invertebrate lineage - the bivalve superfamily Galeommatoidea. It is inspired by the increasing realization among macroevolutionary biologists that the interplay between abiotic and biotic factors has shaped global biodiversity through time and that biotic interactions cannot be ignored if we wish to reconcile theory with natural systems. Galeommatoidea is a particularly apt group for understanding the interactions between abitoic and biotic diversification drivers because it contains large numbers of obligate commensal as well as free-living species and is therefore amenable to comparative approaches. I examined the ecological and evolutionary patterns of free-living and commensal galeommatoidean species on three levels: 1) on a microevolutionary level, focusing on commensal species that occupy multiple hosts; 2) on a regional level, for a faunal assemblage of galeommatoidean taxa that span three well-defined biogeographic provinces in southern Australia; 3) on a global level, for the entire superfamily. My ecological synthesis (Ch.2) suggests that the free-living lifestyle is strongly correlated with living in hard-bottom habitats while the commensal lifestyle is an adaptation for living in sediments. Commensal associations with bioturbating hosts allow the small-bodied clams to attain refuges at depth from predation while remaining oxygenated through their hosts' bioturbation. A case study on Neaeromya rugifera (Ch.3) indicates that clam populations occupying different hosts differ significantly in shell morphologies, but do not show host-specific genetic structuring. Regional phylogeographic analyses of an endemic Australian galeommatoidean species (Ch.4) show that the interaction of the Middle Miocene Climate Transition with the specific geography of the southern coastline of Australia was the primary cladogenic driver in this group. Macroevolutionary study of Galeommatoidea (Ch.5) reveals that a major free-living clade exhibits higher rates of lineage diversification compared to the commensals, possibly driven by complex ecological interactions in coral reef ecosystems. However, commensal species exhibit higher morphological disparity and intercladal convergence, likely reflecting host-specific morphological adaptations. Taken together, my multi-level study demonstrates that the present diversity of Galeommatoidea is shaped by the inseparable interactions between abiotic and biotic factors.