Some polynoid polychaetes, found living commensally in mantle
cavities of deep-sea mussels on the Galapagos Rift vent area, are described as a
new genus and new species, Branchipolynoe symmytilida, and referred to the new
subfamily Branchipolynoinae. They are unusual in having well-developed arborescent
branchiae. A series of young, juveniles, and adults allows observation of
their growth stages.