Résumé
In many marine invertebrates, long-distance dispersal is achieved during
an extended pelagic larval phase. Although such dispersal should result in
high gene flow over broad spatial scales, fine-scale genetic structure has
often been reported, a pattern attributed to interfamilial variance in
reproductive success and limited homogenization during dispersal. To
examine this hypothesis, the genetic diversity of dispersing larvae must
be compared with the post-dispersal stages, i.e. benthic recruits and
adults. Such data remain however scarce due to the difficulty to sample
and analyze larvae of minute size. Here we carried out such an
investigation using the marine gastropod Crepidula fornicata. Field
sampling of three to four larval pools was conducted over the reproductive
season and repeated over three years. The genetic composition of larval
pools, obtained with 16 microsatellite loci, was compared with that of
recruits and adults sampled from the same site and years. Conversely to
samples of juveniles and adults, large genetic temporal variations between
larval pools produced at different times of the same reproductive season
were observed. In addition, full- and half-sibs were detected in early
larvae and post-dispersal juveniles, pointing to correlated dispersal
paths between several pairs of individuals. Inbred larvae were also
identified. Such collective larval dispersal was unexpected given the long
larval duration of the study species. Our results suggest that each larval
pool is produced by a small effective number of reproducers but that, over
a reproductive season, the whole larval pool is produced by large numbers
of reproducers across space and time.