Fertilization success and recruitment of dioecious and hermaphroditic fucoid seaweeds with contrasting distributions near their southern limit

Near its southern limit in the Northeastern Atlantic, the dioecious brown alga Fucus vesiculosus is absent from the exposed coast yet it is abundant in estuaries and coastal lagoons. In contrast, the phylogenetically and ecologically related hermaphroditic species F spiralis occurs along the open coast, though often in low abundance. We hypothesized that the absence of F vesiculosus from exposed shores near its southern limit was due to reduced external fertilization success, as its gametes may be diluted beyond the level required for successful fertilization, in contrast with its hermaphroditic, self-compatible congener. To test this hypothesis, individuals of both species were transplanted to 3 exposed sites near their southern limit in the Northeastern Atlantic. Egg settlement and fertilization success (% of eggs fertilized) were evaluated daily during the main reproductive season. Recruitment was evaluated at the end of the reproductive season, and recruit mortality was evaluated using outplants of laboratory-cultured embryos. On the exposed shores near their southern limit, transplanted adults of both species survived and released eggs, and fertilization success was unexpectedly high. However, recruitment and recruit survivorship of F vesiculosus was significantly lower than F spiralis. Our results suggest that F. vesiculosus is restricted to low water-motion environments because of recruitment failure and recruit mortality on exposed bare shores near its southern limit, and not because of inability to fertilize eggs in turbulent environments. This study does not support our hypothesis of a role for dioecy/hermaphroditism in explaining the distribution of externally fertilizing marine organisms in high water-motion environments.