| Summary: | The four named species of <i>Branchipolynoe</i> all live symbiotically in mytilid mussels (<i>Bathymodiolus</i>) that occur at hydrothermal vents or methane seeps. Analyses using mitochondrial (COI and 16S) and nuclear (ITS) genes, as well as morphology, were conducted on a collection of <i>Branchipolynoe</i> from Pacific Costa Rican methane seeps and West Pacific hydrothermal vents. This revealed five new species of <i>Branchipolynoe,</i><i> </i>and these are formally described. The new species from Costa Rica live in three species of <i>Bathymodiolus</i> mussels (also new) at depths ranging from 1000 to 1800 m. <i>Branchipolynoe kajsae</i> n. sp. and <i>Branchipolynoe halliseyae </i>n. sp. were found in all three undescribed <i>Bathymodiolus </i>species<i>, </i>while<i> Branchipolynoe eliseae</i> n. sp. was found in <i>Bathymodiolus</i> spp. 1 and 2,<i> </i>and<i> Branchipolynoe meridae</i> n. sp. was found in <i>Bathymodiolus</i> spp. 1 and 3<i>. </i>Hence, <i>Bathymodiolus</i> sp. 1 hosted all four of the new species, while the other two <i>Bathymodiolus </i>hosted three each. Most mussels contained only one specimen of <i>Branchipolynoe</i>; where there was more than one, these were often a female and smaller male of the same species. The newly discovered species from the West Pacific, <i>Branchipolynoe</i> <i>tjiasmantoi</i> n. sp.<i>, </i>lives in unidentified <i>Bathymodiolus</i> at depths ranging from 674 to 2657 m from hydrothermal vents in the North Fiji (Fiji) and Lau Basins (Tonga) and also from New Zealand, Vanuatu, and the Manus Basin (Papua New Guinea). The phylogenetic and biogeographical implications of this diversity of <i>Branchipolynoe </i>are discussed.
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