Efforts to understand the population dynamics of marine species with planktonic larvae have been stymied by the fact that the larvae recruiting to a location have little chance of originating from that site. Patterns of larval movement and the spatial scale of dispersal are expected to be major forces regulating the dynamics of marine populations and communities. Unfortunately, the scale and predictability of larval dispersal and its regulation by physical circulation remains unknown due largely to the impossibility of measuring dispersal in open marine environments. Here we exploit strong genetic differentiation among marine mussel populations in southwest England to measure larval dispersal among adjoining genetic patches. This approach allows estimates of larval dispersal over relatively great distances. We combine these measurements with results from a high‐resolution model of coastal circulation to test the hypothesis that larval dispersal is regulated by physical circulation. We show that larval dispersal typically occurs over distances of ∼30 km but in some cases was at least 64 km. The circulation model accurately predicted general patterns of larval transport between genetic regions, the scale of larval dispersal, and genetic isolation created by physical barriers to circulation. We demonstrate that physical circulation models and genetic measures of larval transport can be coupled to assess the geographic scale of larval dispersal in marine environments.

The blue mussels Mytilus edulis and M. galloprovincialis hybridize in southwestern England. Within this hybrid zone environmentally based directional selection favors individuals with alleles specific to M. galloprovincialis. What forces are countering this directional selection and allowing for the maintenance of a stable hybrid population are unknown. We used both the genetics of recently settled larvae and a fine-scale model of the physical oceanography of the region to determine the patterns of larval dispersal throughout the hybrid zone and the bordering parental populations. Evidence from both the model and the genetics suggests that the hybrid zone lies between two barriers to dispersal. Start Point separates the M. edulis population from the hybrid zone and allows minimal dispersal from the hybrid zone into the M. edulis population, but none in the other direction. Likewise, the M. galloprovincialis populations along the northern coast of Cornwall regularly receive immigrating larvae from the hybrid zone, but larvae from the M. galloprovincialis population do not enter the hybrid zone. However, larvae settling at hybrid zone sites have high frequencies of alleles specific to M. edulis, suggesting that reproductive barriers, selection in the larval stage, or gene flow from an undetermined source is effectively balancing the directional selection observed in the adults.

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Cold temperatures are thought to be among the most important determining factors of geographic distribution for tropical and sub-tropical marine invertebrates. The Asian green mussel, Perna viridis, has been introduced into coastal waters of Florida where its current distribution is hypothesized to be limited by low temperatures during winter. Lethal and sub-lethal effects (heat shock protein/Hsp70 expression) of cold water and air temperatures were analyzed in two size classes of P. viridis from Florida in an effort to determine the effects of current and forecasted temperatures on the potential for range expansion. Mussels were exposed to water temperatures of 14, 10, 7 and 3°C for up to 30 days, or to air temperatures of 14, 7, 0 and –10°C for periods of 2 hr. Mortality was significantly increased at all water and air temperatures ≤ 14°C. No differences in mortality rates were observed between small (15–45 mm) and large (75–105 mm) size classes except after exposure to 7°C air, in which small mussels had higher mortality. Significant increases in Hsp70 expression were observed after a 2-hour exposure to 10°C water, but Hsp70 expression was not significantly increased at any temperatures in which mortality was not also significant. The temperature threshold for survival in this population appears to be between 10 and 14°C, suggesting that under current conditions P. viridis may already be at the northern edge of its potential range in the United States. If water temperatures increase with global climate change, northerly flowing currents may permit range expansion as temperatures allow.

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The blue mussels Mytilus edulis and M. galloprovincialis hybridize in southwestern England. Within this hybrid zone environmentally based directional selection favors individuals with alleles specific to M. galloprovincialis. What forces are countering this directional selection and allowing for the maintenance of a stable hybrid population are unknown. We used both the genetics of recently settled larvae and a fine-scale model of the physical oceanography of the region to determine the patterns of larval dispersal throughout the hybrid zone and the bordering parental populations. Evidence from both the model and the genetics suggests that the hybrid zone lies between two barriers to dispersal. Start Point separates the M. edulis population from the hybrid zone and allows minimal dispersal from the hybrid zone into the M. edulis population, but none in the other direction. Likewise, the M. galloprovincialis populations along the northern coast of Cornwall regularly receive immigrating larvae from the hybrid zone, but larvae from the M. galloprovincialis population do not enter the hybrid zone. However, larvae settling at hybrid zone sites have high frequencies of alleles specific to M. edulis, suggesting that reproductive barriers, selection in the larval stage, or gene flow from an undetermined source is effectively balancing the directional selection observed in the adults.

While long distance dispersal of planktotrophic larvae in marine organisms has traditionally been considered the norm, several recent studies have shown that local retention of larvae and hydrodynamic barriers to dispersal often exist. This study focuses on the question of whether genetically distinct populations within a blue mussel hybrid zone typically exchange larvae or if larvae are often retained within the population of origin. Larvae were tracked to their point of settlement using a genetic marker and analyzed for correlations with differential timing of reproduction and settlement among 3 genetically distinct populations (Mytilus edulis, M. galloprovincialis and hybrid). Correspondence of allele frequencies to local reproduction and settlement suggest that larvae settling within the M. edulis and the hybrid zone populations originated from those locations. On the other hand, larvae settling within the M. galloprovincialis populations often contain significant proportions of immigrants from the hybrid zone. The observed patterns are consistent with previous studies of the local hydrodynamics, suggesting that physical barriers to dispersal often exist and result in retention of larvae. KEY WORDS: Larval dispersal · Retention · Mytilus · Hybrid zone Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 351: [151][152][153][154][155][156][157][158][159][160][161] 2007 Cornwall between Start Point and the town of St. Ives (Fig. 1). The hybrid population is a true hybrid swarm with multi-locus genotypes representative of 'pure' individuals of both species as well as F1 and various multi-generational hybrids. Within the hybrid zone, frequencies of M. edulis-specific alleles at diagnostic loci differ among size classes; smaller mussels tend to have higher frequencies of M. edulis-specific alleles than do larger mussels at the same site due to selection favoring M. galloprovincialis-like genotypes. Estimates using adult demography suggest adult mussels within the hybrid zone should produce larvae with M. edulis-specific allele frequencies of approximately 0.71 ± 0.112 (Gilg & Hilbish 2003a). Mytilus edulis populations, where adult populations are fixed for M. edulis-specific alleles at diagnostic loci, occupy sites east of Start Point, whereas predominantly M. galloprovincialis populations, with frequencies of M. galloprovincialis-specific alleles > 0.9 in adult populations, reside north of St. Ives (Hilbish et al. 2002). The distinctions between each population are quite abrupt (< 5 km) which could be due to either disruption of dispersal leading to larval retention within each population, or to post-settlement selection, or both. Previous studies in this region have focused on measuring the movement of dispersing larvae using the genetic differences among the populations and a 2D model of the physical oceanography (Gilg & Hilbish 2003a,b). An observed difference in the timing of larval settlement within each of the populations now allows us to investigate ...

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