Evidence of a possible selective sweep in Anadara trapezia (Bivalvia: Arcidae), including the apparently introduced population in Western Australia

The arcid clam Anadara trapezia has an extensive Pleistocene range in Western Australia, but it is currently found in that state only at King George Sound, near Albany. This is thousands of kilometres west of other known populations of the species, which is widely distributed in eastern Australia. Whether the King George Sound population is a recent introduction or relictual is uncertain, so sequences of adenosine triphosphate synthase subunit α intron (ATPSα), a nuclear gene, and the mitochondrial cytochrome c oxidase subunit I (COI) were collected. Sequences were obtained from Oyster Harbour in King George Sound and Burrill Lake, New South Wales and compared to GenBank data. There was a low level of localised variation in both ATPSα and COI in Oyster Harbour, suggesting either recent establishment or ongoing gene flow. Indeed, no differences were observed between any COI sequences, suggesting the possibility of a selective sweep in A. trapezia mitochondrial DNA that has occurred so recently that variation has not been regenerated. Such a situation is rarely observed and is not previously recorded from Mollusca. Tracking variants as they arise could be highly useful for investigating their spread in this important bivalve species’ natural populations.