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Abstract. A fundamental goal in ecology is to understand distribution and abundance of species.Peripheral populations inhabiting the trailing-edge of a species' distribution may carry considerable ecological and evolutionary value yet being most threatened under predicted climate change scenarios. However, the nature of species distributional limits and the ecological and genetic implications of living at low latitude rear edges remain unclear. The assumption that population abundance declines towards range edges, where environmental selective pressure increases, is the basis of a wide range of ecological and evolutionary predictions. Empirical data have provided contrasting evidence about the consequences of living at low latitude distributional limits, raising concerns regarding their generality and highlighting the need for combined multidisciplinary tests. Here, we provide a long-term, comprehensive evaluation of latitudinal patterns in genetic diversity, demographic, morphological and life history traits in the marine macroalga Fucus guiryi.The center to edge transition was mirrored by increasing water and air temperature gradients, with sea surface temperature of coldest months being the most relevant influence on F. guiryi traits. Overall, we identified a strong correlation between ecological data, collected over one year at bimonthly intervals, and distance to the range center. We found decreasing population and individual size towards distributional margins. Similarly, reproductive capacity, threshold size for reproduction, density of reproductive population and recruitment rates showed a core to edge reduction. Temporal variability of individual reproductive effort and recruitment rates did not conform to the general pattern. In contrast, population genetic data did not show a core-edge gradient, as gene diversity and allelic richness were not significantly lower at edge populations, contradicting predictions of higher drift and bottlenecks for smaller edge populations.The contrasting support provided by genetic and ecological data highlights the need to combine multiple and cross-disciplinary evidence for a comprehensive understanding of ecological and evolutionary mechanisms linked to species ranges.

Dense stands of the invasive species Sargassum muticum (Yendo) Fensholt develop in tidal pools close to its southern distributional limit in Europe, the southwest coast of Portugal. Along this coast, sheltered tidal pools form a specific habitat in which colonization occurs. Invaded pools are originally inhabited by Cystoseira humilis Ku¨tzing. Differences in gamete release between the competing native and alien species might be important for the initial settlement and further spread of the invader. Therefore, we tested whether egg expulsion and embryo settlement in both species had the same timing with respect to lunar and tidal cycles. For more than 2 months during the reproductive season egg expulsion and embryo release were monitored daily for each species. Egg expulsion in S. muticum showed a broadly semilunar periodicity peaking around full and new moon (spring tides), when low tides take place in the morning/evening. In contrast, C. humilis egg expulsion showed an asymmetric semilunar-to-lunar periodicity peaking around waning quarter moon, when low tides occur around midday. Embryo settlement detected in pools was low for both species and less periodic. Phase differences in expulsion events between the two species with respect to the semilunar cycle suggest that cues other than the moon are involved in their timing. Our observations suggest that variations in physiological mechanisms and/or environmental conditions result in different patterns of egg expulsion between the two species. This might have consequences for fertilization success, gamete dispersal and survival. It was further found that peaks in egg expulsion and embryo release (i.e. settlement) in S. muticum were much more synchronous at a site in northern Portugal compared with a site close to the current southern distributional limit in south-west Portugal, possibly as a consequence of thermal stress experienced in the south.

Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides). Interspecific patterns of egg release were coincident at seasonal (single peak in spring to early summer) to semilunar timescales. Synthesis of available data indicated that spawning is controlled by semidiurnal tidal and daily light-dark cues, and not directly by semilunar cycles. Importantly, interspecific shifts in timing detected at the hourly scale during single tides were consistent with a partial ecological prezygotic hybridization barrier. The species displayed patterns of gamete release consistent with a power law distribution, indicating a high degree of reproductive synchrony, while the hypothesis of weaker selective constraints for synchrony in selfing versus outcrossing species was supported by observed spawning in hermaphrodites over a broader range of tidal phase than in outcrossers. Synchronous gamete release is critical to the success of external fertilization, while high-energy intertidal environments may offer only limited windows of reproductive opportunity. Within these windows, however, subtle variations in reproductive timing have evolved with the potential to form ecological barriers to hybridization.

Introduced algae form globally an increasing problem. Grazing on invaders could provide communities with resistance to algal invasions. In this study, we experimentally tested the food choice of mesoherbivores for native macroalgae versus the invader Sargassum muticum. We performed food choice experiments with common grazers (Gammarus insensibilis, Hydrobia ulvae and Stenosoma nadejda) and both the brown algal invader Sargassum muticum and its native macroalgal competitors, from both the south (Cystoseira humilis, Stypocaulon scoparium, Cladostephus spongiosus, Dictyota dichotoma, Dictyopteris polypodioides, Sargassum vulgare) and southwest (Cystoseira humilis) coast of Portugal. We tested whether (i) the invader affects the growth rates of native macro algae, and (ii) the grazers have a food preference for S. muticum compared to native macroalgae. The presence of S. muticum did not affect the growth rates of any of the native species. Grazers affected the growth rate of all seaweeds, but S. muticum had the highest growth rates with and without grazers. For the south coast set of species, Stypocaulon scoparium was the most and S. muticum was the least or among the least preferred by grazers. The grazers from the southwest coast did not show a specific preference for C. humilis or S. muticum. Contrary to our expectations the food choice of mesoherbivores may provide S. muticum with some competitive advantage on the Portuguese south coast, but this may not be applicable to the Southwest communities.