Spatial and temporal dynamics of reproduction and settlement in the Pompeii worm Alvinella pompejana (Polychaeta: Alvinellidae)

Abstract

The tubicolous polychaete Alvinella pompejana (Desbruyeres & Laubier) inhabits deep- sea hydrothermal vents. To assess its reproductive and recruitment strategies in response to such extreme conditions, populations of A. pompejana were sampled at various spatial scales: among hydrothermal vents within the 13°N/East Pacific Rise vent field and among vent sectors along the southern East Pacific Rise. Additional samples were collected at 1 vent 31 d apart to determine short- term temporal variations. Such a sampling strategy allowed us to conduct an integrated study of both the spatial variation of the reproductive dynamics (i.e. fecundity, oocyte distribution) and the popula- tion structure of this polychaete. Despite a 1:1 sex ratio for each population, a male-biased sex ratio was found at sexual maturity, which could maximise reproductive success in this spermathecae- bearing species. Oocyte size distributions in mature females distinguished 5 maturity stages related to the presence of 1 to 5 distinct oocyte size groups. Coelomic fecundity, which is among the highest known in polychaetes (max. 978 000 oocytes), depends on the maturity stage, but not on the size of the female. The analysis of oocyte distributions strongly highlighted that reproduction is semi- continuous. Mature oocytes are however discretely recruited in the gonoducts from a massive stock of coelomic oogonia and spawned in pulses that may be synchronised between reproductive females. Size-frequency distributions of organisms displayed polymodal structure, suggesting discontinuous recruitment that may be locally affected by the vent dynamics or variations in larval supply. The dis- crepancy between semi-continuous reproduction and a polymodal size-frequency distribution may be explained by: (1) a limited amount of recruits within populations, (2) rapid growth during the early benthic phase, and/or (3) an accumulation of individuals of different ages in the adult cohort. Nearly continuous production of gametes and storage of mature oocytes and/or sperm seem to be a general trend selected in hydrothermal-vent polychaete species.