Reproductive biology and population structure of the deep-sea hydrothermal vent worm Paralvinella grasslei (Polychaeta: Alvinellidae) at 13N on the East Pacific Rise

Paralvinella grasslei is a polychaetous annelid living in the harsh, unstable and heterogeneous environmental conditions found at deep-sea hydrothermal vent sites in the eastern Pacific. The aim of this work was 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 μm, 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°N/EPR (EPR = East Pacific Rise) revealed a discontinuous recruitment which seems to be synchronized within vent sites and fields. The data also suggested the occurrence of discrete breeding periods. P. grasslei probably reproduces several times a year, with an apparent periodicity. Tidal signals could be a possible cue for the coordination of the reproductive cycle. The life-history of P. grasslei is discussed in light of the reproductive biology of other terebellomorph polychaetes, and seems to be well adapted for colonizing the unstable environment of hot vents. Two main hypotheses can explain the dissemination processes of this species along axial oceanic ridges. The influence of near-bottom currents occurring along the central graben of the East Pacific Rise can be considered to account for part of the transport of larvae and juveniles, but the observations of polychaete erpochaetes on the test of hydrothermal bythograeid crabs and evidence that crab migrations occur between vents also support the possibility of zoochory for the dissemination of alvinellid polychaetes.