Reproductive biology of free-living and commensal polynoid polychaetes at the Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge)

We examined the reproductive biology of a polynoid polychaete commensal in mussel mantle cavities (Branchipolynoe cf, seepensis) and of a polynoid pol ychaete that is free-living among those same mussels (Opisthotrochopodus n. sp.). Specimens of each species were collected from 2 different sites (Eif fel Tower and Sintra; similar to 400 m apart) within the Lucky Strike hydro thermal field. Both species exhibit sexual dimorphism, with varying numbers of pairs of nephridial papillae. In B. cf. seepensis, sexual dimorphism al so includes a difference in size, with females larger than males. Contrary to our expectations, the reproductive biology of both species is very simil ar. Sperm heads are elongate, suggesting some mode of sperm transfer or sto rage. Females of both species contain mature sperm and there is evidence of internal fertilization. Maximum oocyte diameters are large (>390 mu m), fr om which we infer non-planktotrophic development; oogenesis is intraovarian and suggests rapid oocyte development. Oocyte size-frequency distributions and population size structure suggest a pattern of asynchronous gametogene sis. We found no substantive evidence for site-specific variations in repro ductive biology in either species.