GEOGRAPHIC CO-OCCURRENCE AND CHARACTER DISPLACEMENT IN FOSSIL AND MODERN MARINE BIVALVES

Character displacement and species sorting are two processes thought to promote the geographic co-occurrence of closely related species. Determining the timing, magnitude, and then mechanism of ecological change that enables the geographic range overlap of closely related, and therefore presumably ecologically similar species, requires spatially explicit data on past trait distributions. Many studies use phylogeny to create this historical context. This study uses the detailed modern and fossil record for a set of closely-related marine bivalves to empirically determine the present and past distribution of burrowing ability–an ecological trait that promotes habitat partitioning. I compared shell outlines–a known proxy for burrowing ability–of five species of Venerid bivalves from the genus Chione. In one notable result, the present-day allopatric populations of C. californiensis and C. subimbricata differ in burrowing ability, where C. californiensis burrows more slowly and to a shallower depth than C. subimbricata. Where the two species are sympatric, C. californiensis burrows the same as in allopatry, but C. subimbricata burrows more slowly and shallowly than in allopatry–equivalent to the burrowing ability of C. californiensis. This remarkable convergence in burrowing ability counters the expectation that ecological traits promoting competition should differ in areas of co-occurrence, and instead suggests a filtering for similar traits imposed by the shared environment. A very preliminary examination of the fossil populations (Pliocene and Pleistocene) suggests a stability in the spatial distribution of these burrowing traits has existed over the past 5 My. Other Chione species show shared ecological ability between their overlapping populations through time, which suggests a process such as environmental filtering may commonly correlate with geographic co-occurrence in the clade.