Abstract
We examined the reproductive biology and genetic structure of the polychaete Bathykurila guaymasensis (Annelida: Polynoidae), a species recorded from both whale‐falls and hydrothermal vents. The habitat of B. guaymasensis is quite different from other vent polynoids, being apparently a specialist feeder on Beggiatoa mat, rather than living commensally with mussels or on the sides of hot vent chimneys. Specimens were collected from whale‐fall sites in the Santa Cruz and Santa Catalina Basins, Southern California at depths of 1200–1600 m, and investigated using a combined morphological and molecular approach. The species exhibits marked sexual dimorphism, asynchronous gametogenesis, evidence for internal fertilization and lecithotrophic larval development – all characters shared with other hydrothermal vent polynoids. Two distinct, sympatrically distributed haplotype groups of B. guaymasensis were identified using mitochondrial cytochrome oxidase 1 gene sequences, indicating that there may be two species within the nominal designation B. guaymasensis . Broad sharing of haplotypes between the two whale‐fall sites suggests high dispersal rates among basins along the California margin.