Unravelling the interactions between life-history strategies and oceanography is central to our understanding of gene flow and connectivity in the marine environment. In the present study, we investigated the population genetic structure of the shore crab in its native range in relation to oceanographic characteristics and dispersal potential. Using 10 microsatellite markers over 2 yr, we surveyed 18 locations distributed along ~4200 km within the species native range, from Sweden to Morocco, assessed the population structure by means of F ST and Bayesian clustering analysis and tested the hypothesis of isolation-by-distance (IBD) with a Mantel test. We focused particular attention along a 1200 km stretch of the Iberian Peninsula. We found no evidence of genetic structure (F ST = 0.0001, p > 0.05) along the Iberian coast, and patterns were temporally stable over 2 yr. Across the more extensive geographic spatial scale, overall genetic differentiation was low (F ST = 0.001) but statistically significant (p < 0.001). Furthermore, clustering analysis grouped the samples into 3 genetic units from (1) Sweden, (2) Wales and the Iberian Peninsula and (3) Morocco. While the correlation between genetic and geographic distances was significant, the pattern was not consistent with an IBD pattern. Results suggests that, in the absence of barriers to gene flow, shore crab populations are genetically similar across thousands of kilometres, but isolated populations still may occur within the species native range. Local oceanography and larval behaviour may have a significant influence on the structuring of the populations under study.

BackgroundPredicting the spatial and temporal patterns of marine larval dispersal and supply is a challenging task due to the small size of the larvae and the variability of oceanographic processes. Addressing this problem requires the use of novel approaches capable of capturing the inherent variability in the mechanisms involved.Methodology/Principal FindingsIn this study we test whether dispersal and connectivity patterns generated from a bio-physical model of larval dispersal of the crab Carcinus maenas, along the west coast of the Iberian Peninsula, can predict the highly variable daily pattern of wind-driven larval supply to an estuary observed during the peak reproductive season (March–June) in 2006 and 2007. Cross-correlations between observed and predicted supply were significant (p<0.05) and strong, ranging from 0.34 to 0.81 at time lags of −6 to +5 d. Importantly, the model correctly predicted observed cross-shelf distributions (Pearson r = 0.82, p<0.001, and r = 0.79, p<0.01, in 2006 and 2007) and indicated that all supply events were comprised of larvae that had been retained within the inner shelf; larvae transported to the outer shelf and beyond never recruited. Estimated average dispersal distances ranged from 57 to 198 km and were only marginally affected by mortality.Conclusions/SignificanceThe high degree of predicted demographic connectivity over relatively large geographic scales is consistent with the lack of genetic structuring in C. maenas along the Iberian Peninsula. These findings indicate that the dynamic nature of larval dispersal can be captured by mechanistic biophysical models, which can be used to provide meaningful predictions of the patterns and causes of fine-scale variability in larval supply to marine populations.

Predicting the spatial and temporal patterns of marine larval dispersal and supply is a challenging task, requiring the use of novel approaches capable of capturing the inherent variability in the mechanisms involved. Biophysical models are emerging as important tools used to understand dispersal and recruitment of marine larvae on several scales, and are used here to investigate these problems in the Northeast Atlantic Iberian Upwelling system. We used a Regional Ocean Modelling System configuration coupled with an individual-based model in order to simulate diel vertical migration and estuary-reinvasion behaviours, growth and mortality of larvae of the common shore crab Carcinus maenas. With this simulation we aimed to hindcast coast-wide connectivity patterns under the atmospheric and river flow forcings of the years 2001 to 2009. In a previous study, this model had been shown to produce time series of larval supply to a single population, at daily frequencies, that were strongly correlated with observed time series. In the current application, the model predicts that along-shore advection of larvae is variable from year to year; is correlated with the annual North Atlantic Oscillation index, which reflects the strength and persistence of equatorward winds; and is usually asymmetric, with a predominant equatorward transport. The model also predicts a source-sink cell located between the Muros-Noia and Mondego estuaries, where populations in the north consistently act as net sources of larvae supplying net sink populations to the south. This source-sink cell appears to be related to the existence of habitat gaps, coast geo metry and an oceanographic retention effect associated with upwelling dynamics.

We measured variability in daily supply levels of shore crab megalopae in an estuary on the northwest Portuguese coast, Ria de Aveiro, located in the eastern Atlantic upwelling system. The 5 yr study covered the shore crab larval season (generally February to July) in 2002 and from 2006 to 2009. We addressed the possible effects of wind-and tide-driven circulation, number of flood hours during darkness, and chlorophyll a concentration in coastal waters on larval supply variation. Megalopae supply measured over the years was an episodic phenomenon, and observations showed some predictable and haphazard patterns. In some episodes, supply was highest around spring tides and was enhanced by southerly winds, as predicted, although not all episodes fitted this pattern. The relationships between supply levels and number of flood hours during darkness or chlorophyll a concentration were ambiguous throughout the time series, although in some years increased levels of supply were positively correlated with number of flood hours during the night. The analysis of multiple years conducted in this study showed that shore crab megalopae supply patterns to Ria de Aveiro are more variable than previously assumed, suggesting the participation of several delivery mechanisms that vary within and among years. However, a proportion of supply variation exists that cannot be explained by the mechanisms we propose.

Despite the importance of larval biology in the life histories of many marine animals, relatively little information exists on the dynamics and genetic composition of larval cohorts. The supply of megalopae larvae of the shore crab, Carcinus maenas, was measured on a daily basis during 8 months spread along two larval periods (2006 and 2007) at the Ria de Aveiro estuary, on the Portuguese northwest coast. A total of 10 microsatellite DNA loci were employed to explore the genetic structure, variability and relatedness of temporally distinct megalopal events, selected from the major pulses of supply. Larval variation was also compared genetically with that of a previously studied adult crabs sample, at the same loci, collected in 2006 and 2007 along the Iberian Peninsula. Results revealed a lack of genetic differentiation and identical diversity levels among larval events over time. No evidence of reduced genetic diversity between megalopae relative to the diversity assessed from the pooled sample of adults was found. Moreover, there was no evidence of any family relatedness among larvae from temporal events. The results obtained for C. maenas contradict predictions made by the sweepstakes reproduction hypothesis, in which large variance in reproductive success is expected, which is presumably detectable as sharp genetic discontinuities among separate larval events. Data here indicate conversely a high degree of temporal genetic stability among larval supply to a given estuary under variable oceanographic conditions, consistent with the hypothesis that sampled larvae were drawn from a large number of adults that do not differ in reproductive success.