Population structure and reproductive biology of two sympatric hydrothermal vent polychaetes,Paralvinella pandorae and P. palmiformis

The alvinellid polychaetes Paralvinella pandorae Desbruyères and Laubier and P. palmiformis Desbruyères and Laubier occur at deep-sea hydrothermal vents along the Juan de Fuca and Explorer Ridges in the northeast Pacific Ocean. The population structure and reproductive biology of both species were studied in samples taken from three vent sites during six cruises in 1983 and 1984. Size-frequency analyses of two P. pandorae populations produced unimodal histograms, suggesting continuous or semi-continuous juvenile recruitment; in a third population two possible size classes were evident. Histograms of P. palmiformis displayed size-class peaks, which most likely reflected periodic recruitment of juveniles. Both species are gonochoric and gametes develop free in the coelom. Due to the simultaneous presence of a full range of gametogenic stages in P. pandorae populations, including spermatozoa in males, and to the continuous or semi-continuous recruitment pattern suggested by the size-frequency histograms, continuous reproduction is proposed for this species. In P. palmiformis a discrete, possibly synchronized, breeding cycle is thought to occur. Although maximum fecundity of P. pandorae is very low, continual reproduction over a long period of time could enhance its reproductive potential. The estimate of maximum fecundity for P. palmiformis is comparable to estimates for other polychaetes that undergo non-planktotrophic larval development. Maximum observed oocyte size was 215 and 260 m in P. pandorae and P. palmiformis, respectively. It is proposed that P. pandorae broods its young, while P. palmiformis probably undergoes demersal lecithotrophic larval development. The continual production of brooded young by P. pandorae could maintain a vent population, but severely limit dispersal to other vents. Demersal lecithotrophic larvae of P. palmiformis could repopulate vents, and potentially be carried by bottom currents to other vent sites.