Phylogenetic relationships of deep-sea mussels of the genus Bathymodiolus (Bivalvia: Mytilidae)

We examined phylogenetic relationships among three Bathymodiolus species in Japanese waters and Bathymodiolus spp. from the Manus Basin by two different approaches. Two-dimensional gel electrophoresis allowed us to compare 263-407 (average=318) proteins, giving comprehensive information on genetic distances among the species. The neighbor-joining tree presented two clusters: (1) B. japonicus and B. platifrons and (2) B. septemdierum and B. sp. Members of the first cluster contain methanotrophic endosymbiotic bacteria and members of the second cluster contain thioautotrophic endosymbionts. DNA sequencing of a fragment (415 bp) of mitochondrial cytochrome c oxidase subunit I (COI) provided a neighbor-joining tree with the same topology as that derived from protein analysis. Inspection of intraspecific variation in COI in B. japonicus and B. platifrons revealed no genetic differentiation between mussel populations of either species from cold-water seeps versus hydrothermal vents, suggesting high adaptability of these Bathymodiolus species to deep-sea chemosynthetic environments. Our results indicated genetic exchanges between mussels from distant localities, suggesting that a limited dispersal capability of the larvae is not the likely factor leading to speciation events in these Bathymodiolus species.