The functional morphology of Bentholyonsia teramachii (Bivalvia: Lyonsiellidae): clues to the origin of predation in the deep water Anomalodesmata

Abstract

Bentholyonsia teramachii has hitherto been described from Japan, but a conspecific specimen is herein recorded from Western Australian shelf waters at a depth of ∼100 m. Hitherto placed in the Lyonsiidae, B. teramachii is herein relocated in the Lyonsiellidae, alongside a second genus – Lyonsiella. Features of the anatomy of Bentholyonsiateramachii, including highly sensory siphons, reduced ctenidia unable to collect and transport particulate food, non-sorting labial palps, and a simplified Type II stomach and muscular gut all suggest that the species is a predator like Lyonsiella formosa. Other features of the anatomy, such as pallial taenioid muscles, radial mantle glands, and the structure of the ctenidia all point to a link between B. teramachii and the Lyonsiellidae and thus with the Verticordiidae and Parilimyidae and, therefore, with some of the oldest anomalodesmatans, the Pholadomyidae. In the possession by B. teramachii of a unique pair of antero-dorsal suspensory muscles, the link with fossil pholadomyoideans is reinforced. Bentholyonsia thus represents another genus of predatory bivalve, not only expanding our view of the prevalence of this feeding mode in the deep sea by this class but also helping us show how it evolved, in anatomical terms, from a suspension- (or deposit-) feeding ancestor.