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- Fertilization success and recruitment of dioecious and hermaphroditic fucoid seaweeds with contrasting distributions near their southern limitPublication . Ladah, Lydia; Bermudez, R.; Pearson, G. A.; Serrão, EsterNear 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. sp iralis 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 her- maphroditic, 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 significantlylower 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 south-ern 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 exter-nally fertilizing marine organisms in high water-motion environments.
- Egg release and settlement patterns of dioecious and hermaphroditic fucoid algae during the tidal cyclePublication . Ladah, Lydia; Feddersen, F.; Pearson, G. A.; Serrão, EsterThe timing of gamete release by fucoid algae, although known to be restricted to calm days is not clearly understood within a circadian time scale. The need for externally fertilizing species to avoid gamete dilution suggests that in wave-exposed areas spawning may occur during particular tidal phases. However, this may differ between mating systems, as selfing species may be less affected by gamete dilution. In this study, two different approaches were used to determine when egg release occurs during the tidal cycle in two sister species with different mating systems. First, egg settlement of Fucus vesiculosus (dioecious) and Fucus spiralis (selfing hermaphrodite) was quantified on removable substrates (egg settlement disks) every day for 2 months and settlement patterns were used to statistically estimate the radius of a circle that would encompass 99% of each patch of settled eggs (the egg dispersal radius). Also, egg release was quantified every 2 h during the tidal cycle. A significantly larger egg dispersal radius (P < 0.02) was found for F. spiralis than F. vesiculosus, and this difference was somewhat site dependent with a greater difference between species in exposed sites. The egg dispersal radius was negatively correlated with significant wave height and positively correlated with sea surface temperature for both the species (P < 0.05), with a greater effect of both the factors for F. spiralis than for F. vesiculosus. Egg release during the tidal cycle was variable between species and experiments, with F. vesiculosus releasing more eggs, later in the day, and at a lower tide, than F. spiralis, which released fewer eggs, throughout the day and at all tides. The dioecious species, F. vesiculosus, may have developed a specific adaptation for timing the egg release to periods when emersed in exposed habitats to avoid rapid dilution of gametes that require outcrossing for fertilization. On the other hand, egg release for F. spiralis, which can self-fertilize, occurred both when emersed and immersed, suggesting this species has developed less synchrony with specific environmental factors. Site dependence also suggests local wave conditions can modulate timing of release. The data are consistent with the relaxation of the selective constraints of water motion on fertilization success in a selfing hermaphrodite, relative to an obligate outcrossing species. Results support the idea that species with different mating systems evolve different sensitivities to environmental cues for gamete release with specific implications for inbreeding and successful external fertilization in the ocean.