Spanning the depths or depth-restricted: Three new species of Bathymodiolus (Bivalvia, Mytilidae) and a new record for the hydrothermal vent Bathymodiolus thermophilus at methane seeps along the Costa Rica margin

Bathymodiolus Kenk and Wilson, 1985 includes fourteen currently recognized species from deep-sea chemosynthetic environments in the Pacific, Atlantic, and Indian oceans. In this study, phylogenetic analyses of mytilid mussels sampled from seeps along the Costa Rica margin reveal the presence of three new Bathymodiolus species, sampled from depths across ~1000–1900 m. Bathymodiolus billschneideri n. sp., B. earlougheri n. sp., and B. nancyschneiderae n. sp. differ genetically from congeneric species of Bathymodiolus and show some stratification by depth. The depth ranges of Bathymodiolus billschneideri n. sp., B. nancyschneiderae n. sp., and B. earlougheri n. sp. were ~1400–1900 m, 1000–1100 m, and ~1000–1900m, respectively. Bathymodiolus billschneideri n. sp. and B. earlougheri n. sp. were found to be closely related to Bathymodiolus thermophilus Kenk and Wilson, 1985, while B. nancyschneiderae n. sp. exhibited closer relationships to non-East-Pacific taxa. Bathymodiolus thermophilus was the first Bathymodiolus mussel discovered, sampled from a vent along the Galápagos Rift Zone in 1985 and later recorded along much of the East Pacific Rise. This study confirms the presence of B. thermophilus at the Costa Rica margin, representing the first DNA and morphological samples of B. thermophilus at a seep environment. Analysis of habitat evolution suggests that B. thermophilus and its closest relative, B. antarcticus Johnson and Vrijenhoek, 2013, are of seep ancestry. •Three new species of deep-sea Bathymodiolus mussels are described from seeps along the Costa Rica margin in the East Pacific.•The newly described Bathymodiolus species show some stratification by depth.•The presence of Bathymodiolus thermophilus is confirmed at the Costa Rica margin.•Analysis of habitat evolution suggests that B. thermophilus and its closest relative B. antarcticus are of seep ancestry.