Nautiliniellidae (Annelida) from Costa Rican cold seeps and a western Pacific hydrothermal vent, with description of four new species

Four genera and five species of Nautiliniellidae (Phyllodocida; Annelida), representing four new species and one new record, were found in association with bivalves belonging to Mytilidae (Bathymodiolus spp.), Solemyidae (Acharax sp.) and Vesicomyidae (Calyptogena and Vesicomya) from hydrothermal vents of the western Pacific (Lau back-arc basin) and cold seeps of the eastern Pacific (Costa Rica). Iheyomytilidicola lauensis n. sp. is characterized by the presence of unidentate hooks; Laubierus alvini n. sp. has long dorsal cirri and notoacicula not protruding in any segment; Natsushima sashai n. sp has bifurcate chaetae with distal teeth markedly different in length and width between each other; and Shinkai fontefridae n. sp. differs in having posterior hooks with subdistal spines. Sexual dimorphism is described for Shinkai and Natsushima species and confirmed by DNA sequencing (Cytochrome c oxidase subunit I sequences). In addition, one specimen of N. bifurcata Miura {&} Laubier, 1990 from the Gulf of Cadiz has been studied to compare it morphologically and genetically with N. sashai n. sp. Scanning electron and optical microscopy images are provided for each species, as well as pictures of live animals. Morphological features described and documented for the first time include the presence of internal longitudinal chambers within chaetae and possible evidence that the simple chaetae, characteristic of nautiliniellids, may be derived from compound chaetae.