REPRODUCTIVE-BIOLOGY AND POPULATION-STRUCTURE OF THE DEEP-SEA HYDROTHERMAL VENT WORM PARALVINELLA-GRASSLEI (POLYCHAETA, ALVINELLIDAE) AT 13-DEGREES-N ON THE EAST PACIFIC RISE

Paralvinella grasslei is a polychaetous annelid living in the harsh, u nstable and heterogeneous environmental conditions found at deep-sea h ydrothermal vent sites in the eastern Pacific. The aim of this work wa s to examine the possible influence of the reproductive biology of P. grasslei on the structure of its populations. Maximum observed oocyte size inside the oviduct is 275 mu m, and fecundity is relatively low. Examination of gametes and young specimens suggested a direct benthic development for this species. The population structure of P. grasslei at 13 degrees N/EPR (EPR = East Pacific Rise) revealed a discontinuous recruitment which seems to be synchronized within vent sites and fiel ds. The data also suggested the occurrence of discrete breeding period s. P. grasslei probably reproduces several times a year, with an appar ent periodicity. Tidal signals could be a possible cue for the coordin ation of the reproductive cycle. The life-history of P. grasslei is di scussed in light of the reproductive biology of other terebellomorph p olychaetes, and seems to be well adapted for colonizing the unstable e nvironment of hot vents. Two main hypotheses can explain the dissemina tion processes of this species along axial oceanic ridges. The influen ce of near-bottom currents occurring along the central ''graben'' of t he East Pacific Rise can be considered to account for part of the tran sport of larvae and juveniles, but the observations of polychaete erpo chaetes on the test of hydrothermal bythograeid crabs and evidence tha t crab migrations occur between vents also support the possibility of zoochory for the dissemination of alvinellid polychaetes.