Presence of two phylogenetically distinct groups in the deep-sea mussel Acharax (Mollusca: Bivalvia: Solemyidae)

Abstract

The family Solemyidae represents ancestral protobranch bivalves with the shallow-water genus Solemya and the deep-sea genus Acharax. All known members of this family host symbiotic sulfur-oxidizing bacteria in their gill filaments. Analysis of 18S rRNA gene sequences of Acharax specimens from methane-seeps off Makran (Pakistan), Java (Indonesia), the Aleutian Trench and off the Oregon, Costa Rica, and Peru margins revealed that Solemya spp. and Acharax spp. are well-separated genetically. This supports the current systematic distinction based on morphological criteria. We found 2 clearly distinct clusters within the genus Acharax, with specimens from the Makran, Oregon and Peru (MOP) margins in one (MOP-Acharax) cluster, and those from Java, the Aleutian Trench and Costa Rica (JAC) in the other (JAC-Acharax) cluster. The separation of MOP- and JAC-Acharax clusters from each other and from Solemya (S. reidi and S. velum) is well-supported by phylogenetic calculations employing maximum likelihood and maximum parsimony. Compared to genetic distances among other protobranch groups, distances between the MOP- and JAC-Acharax clusters would justify the affiliation of these clusters to separate species. This implies that species differentiation in Acharax based on shell morphology is likely to underestimate true species diversity within this taxon. Furthermore, our results support the hypothesis that genetic separation of Solemya and Acharax is congruent with the phylogeny of their bacterial endosymbionts. © Inter-Research 2006.