Theoretical limits to the correlation between pelagic larval duration and population genetic structure

Increasing dispersal duration should result in increasing dispersal distance, facilitating higher gene flow among populations. As such, it has long been predicted that genetic structure (e.g. F(ST) ) among populations of marine species should be strongly correlated with pelagic larval duration (PLD). However, previous studies have repeatedly shown a surprisingly poor correspondence. This result has been frequently interpreted as evidence for larval behaviours or physical oceanographic processes that result in larvae failing to reach their dispersal potential, or error inherent in estimating PLD and F(ST) . This study employed a computer modelling approach to explore the impacts of various uncertainties on the correlation between measures of genetic differentiation such as F(ST) and PLD. Results indicate that variation resulting from PLD estimation error had minor impacts on the correlation between genetic structure and PLD. However, variation in effective population size between species, errors in F(ST) estimation and non-equilibrium F(ST) values all had major impacts, resulting in dramatically weaker correlations between PLD and F(ST) . These results suggest that poor correlations between PLD and F(ST) may result from variation and uncertainty in the terms associated with the calculation of F(ST) values. As such, PLD may be a much stronger determinant of realized larval dispersal than suggested by the weak-to-moderate correlations between PLD and F(ST) reported in empirical studies.     

Population structure within and between subspecies of the Mediterranean triplefin fish Tripterygion delaisi revealed by highly polymorphic microsatellite loci

Although F(ST) values are widely used to elucidate population relationships, in some cases, when employing highly polymorphic loci, they should be regarded with caution, particularly when subspecies are under consideration. Tripterygion delaisi presents two subspecies that were investigated here, using 10 microsatellite loci. A Bayesian approach allowed us to clearly identify both subspecies as two different evolutionary significant units. However, low F(ST) values were found between subspecies as a consequence of the large number of alleles per locus, while homoplasy could be disregarded as indicated by the standardized genetic distance G'(ST). Heterozygosity saturation was observed in highly polymorphic loci containing more than 15 alleles, and this threshold was used to define two loci pools. The less variable loci pool revealed higher genetic variance between subspecies, while the more variable pool showed higher genetic variance between populations. Furthermore, higher differentiation was also observed between populations using G'(ST) with the more variable loci. Nonetheless, a more reliable population structure within subspecies was obtained when all loci were included in the analyses. In T. d. xanthosoma, isolation by distance was detected between the eight analysed populations, and six genetically homogeneous clusters were inferred by Bayesian analyses that are in accordance with F(ST) values. The neighbourhood-size method also indicated rather small dispersal capabilities. In conclusion, in fish with limited adult and larval dispersal capabilities, continuous rocky habitat seems to allow contact between populations and prevent genetic differentiation, while large discontinuities of sand or deep-water channels seems to reduce gene flow.     

Genetic structure analysis of Girella laevifrons populations in central Chile

Connectivity among most marine species depends of their life cycles, and the main phase that can regulate dispersal is the larval stage of an organism. Girella laevifrons (Girellidae) is an omnivore fish inhabiting in intertidal pools as juveniles and subtidal reefs as adults from northern to central Chile. It has a pelagic larval duration for approximately 69 days. In this study, we used eight molecular markers (microsatellites) to explore the genetic structure of their populations along 400 km of the central Chilean coast.20 juveniles were sampled from four different populations. Microsatellite loci did not detect a genetic structure within a 400 km scale along the coast of Central Chile. There is evidence (F_st = ?0.0038, p = 0.8954) suggesting that populations between Coquimbo and Littoral Central behave as a single whole population. This could be an estimate that may define future management units for the species.     

Weak genetic structure indicates strong dispersal limits: a tale of two coral reef fish

The extent of dispersal by pelagic larvae in marine environments, including coral reefs, is central for understanding local population dynamics and designing sustainable marine reserves. We present here the first example of a clear stepping-stone genetic structure throughout the Caribbean basin for a common coral reef species, the French grunt (Haemulon flavolineatum). Analysis of microsatellite DNA markers indicated that French grunt population structure may be characterized by overlapping populations throughout the Caribbean, influenced by independent population dynamics but with no fixed geographical boundaries. In addition, different spatial genetic patterns were found in different oceanographic regions. A second species, the bluehead wrasse (Thalassoma bifasciatum), has a much longer pelagic larval duration than French grunts and showed no explicit spatial pattern of genetic variation. This finding is concordant with the hypothesis of a positive relationship between larval dispersal and duration in the plankton. While the magnitude of the genetic signal of population structure in French grunts was very low (F(ST) approximately 0.003), the pattern of isolation-by-distance throughout the Caribbean indicated considerable population structure with important ecological and conservation significance.     

Population structure in a common Caribbean coral-reef fish: implications for larval dispersal and early life-history traits

Genetic population structure throughout the Caribbean Basin for one of the most common and widespread reef fish species, the bicolour damselfish Stegastes partitus was examined using microsatellite DNA markers. Spatial autocorrelation analysis showed a significant positive correlation between genetic and geographic distance (isolation by distance) over distances <1000 km, suggesting that populations are connected genetically but probably not demographically, i.e. over shorter time scales. A difference in spatial patterns of populations in the eastern v. the western Caribbean also raises the probability of an important role for meso-scale oceanographic features and landscape complexity within the same species. A comparison of S. partitus population structure and life-history traits with those of two other species of Caribbean reef fish studied earlier showed the findings to be concordant with a common hypothesis that shorter pelagic larval dispersal periods are associated with smaller larval dispersal scales.     

On the spatial scale of dispersal in coral reef fishes

Marine biologists have gone through a paradigm shift, from the assumption that marine populations are largely ‘open’ owing to extensive larval dispersal to the realization that marine dispersal is ‘more restricted than previously thought’. Yet, population genetic studies often reveal low levels of genetic structure across large geographic areas. On the other side, more direct approaches such as mark‐recapture provide evidence of localized dispersal. To what extent can direct and indirect studies of marine dispersal be reconciled? One approach consists in applying genetic methods that have been validated with direct estimates of dispersal. Here, we use such an approach—genetic isolation by distance between individuals in continuous populations—to estimate the spatial scale of dispersal in five species of coral reef fish presenting low levels of genetic structure across the Caribbean. Individuals were sampled continuously along a 220‐km transect following the Mesoamerican Barrier Reef, population densities were estimated from surveys covering 17 200 m² of reef, and samples were genotyped at a total of 58 microsatellite loci. A small but positive isolation‐by‐distance slope was observed in the five species, providing mean parent‐offspring dispersal estimates ranging between 7 and 42 km (CI 1–113 km) and suggesting that there might be a correlation between minimum/maximum pelagic larval duration and dispersal in coral reef fishes. Coalescent‐based simulations indicate that these results are robust to a variety of dispersal distributions and sampling designs. We conclude that low levels of genetic structure across large geographic areas are not necessarily indicative of extensive dispersal at ecological timescales.     

Contrasting phylogeography in three endemic Hawaiian limpets (Cellana spp.) with similar life histories

The marine environment offers few obvious barriers to dispersal for broadcast-spawning species, yet population genetic structure can occur on a scale much smaller than the theoretical limits of larval dispersal. Comparative phylogeographical studies of sympatric sister species can illuminate how differences in life history, behaviour, and habitat affinity influence population partitioning. Here we use a mitochondrial DNA marker (612 bp of cytochrome c oxidase subunit I) to investigate population structure of three endemic Hawaiian broadcast-spawning limpets (Cellana spp.) with planktonic larvae that are competent to settle within 4 days. All three species exhibit significant population structure and isolation by distance, but the spatial scales of partitioning differ among the species. Cellana talcosa (n = 105) exhibits strong population structure between Kauai and the other main Hawaiian Islands (MHI) where the maximum channel width is 117 km, and no shared haplotypes were observed (Phi(CT) = 0.30, P < 0.001). In contrast, populations of Cellana exarata (n = 149) and Cellana sandwicensis (n = 109) exhibit weaker population structure within the MHI (Phi(ST) = 0.03-0.04, P < 0.05), and between the MHI and the Northwestern Hawaiian Islands (Phi(ST) = 0.03-0.09, P < 0.01), where the maximum channel width is 260 km. Biogeographical range and microhabitat use were correlated with estimates of dispersal, while phylogenetic affiliation and minimum pelagic larval duration were poor predictors of population partitioning. Despite similar life histories, these closely related limpets have contrasting patterns of population structure, illustrating the danger of relying on model species in management initiatives to predict population structure and dispersal in the context of marine protected area delineation.     

Genetic population structure of the endemic fourline wrasse (Larabicus quadrilineatus) suggests limited larval dispersal distances in the Red Sea

The connectivity among marine populations is determined by the dispersal capabilities of adults as well as their eggs and larvae. Dispersal distances and directions have a profound effect on gene flow and genetic differentiation within species. Genetic homogeneity over large areas is a common feature of coral reef fishes and can reflect high dispersal capability resulting in high levels of gene flow. If fish larvae return to their parental reef, gene flow would be restricted and genetic differentiation could occur. Larabicus quadrilineatus (Labridae) is considered as an endemic fish species of the Red Sea and Gulf of Aden. The juveniles of this species are cleaner fish that feed on ectoparasites of other fishes. Here, we investigated the genetic population structure and gene flow in L. quadrilineatus among five locations in the Red Sea to infer connectivity among them. To estimate genetic diversity, we analysed 369 bp of 237 mitochondrial DNA control region sequences. Haplotype and nucleotide diversities were higher in the southern than in the northern Red Sea. Analysis of molecular variance (amova) detected the highest significant genetic variation between northern and central/southern populations (Phi(CT) = 0.01; P < 0.001). Migration analysis revealed a several fold higher northward than southward migration, which could be explained by oceanographic conditions and spawning season. Even though the Phi(ST) value of 0.01 is rather low and implies a long larval dispersal distance, estimates based on the isolation-by-distance model show a very low mean larval dispersal distance (0.44-5.1 km) compared to other studies. In order to enable a sustainable ornamental fishery on the fourline wrasse, the results of this study suggest that populations in the northern and southern Red Sea should be managed separately as two different stocks. The rather low larval dispersal distance of about 5 km needs to be considered in the design of marine protected areas to enable connectivity and self-seeding.     

Population structure of a parasitic plant and its perennial host

Characterization of host and parasite population genetic structure and estimation of gene flow among populations are essential for the understanding of parasite local adaptation and coevolutionary interactions between hosts and parasites. We examined two aspects of population structure in a parasitic plant, the greater dodder (Cuscuta europaea) and its host plant, the stinging nettle (Urtica dioica), using allozyme data from 12 host and eight parasite populations. First, we examined whether hosts exposed to parasitism in the past contain higher levels of genetic variation. Second, we examined whether host and parasite populations differ in terms of population structure and if their population structures are correlated. There was no evidence that host populations differed in terms of gene diversity or heterozygosity according to their history of parasitism. Host populations were genetically more differentiated (F(ST) = 0.032) than parasite populations (F(ST) = 0.009). Based on these F(ST) values, gene flow was high for both host and parasite. Such high levels of gene flow could counteract selection for local adaptation of the parasite. We found no significant correlation between geographic and genetic distance (estimated as pairwise F(ST)), either for the host or for the parasite. Furthermore, host and parasite genetic distance matrices were uncorrelated, suggesting that sites with genetically similar host populations are unlikely to have genetically similar parasite populations.     

Pronounced genetic population structure in a potentially vagile fish species (Pristipomoides multidens, Teleostei; Perciformes; Lutjanidae) from the East Indies triangle

The East Indies triangle, bordered by the Phillipines, Malay Peninsula and New Guinea, has a high level of tropical marine species biodiversity. Pristipomoides multidens is a large, long-lived, fecund snapper species that is distributed throughout the East Indies and Indo-Pacific. Samples were analysed from central and eastern Indonesia and northern Australia to test for genetic discontinuities in population structure. Fish (n = 377) were collected from the Indonesian islands of Bali, Sumbawa, Flores, West Timor, Tanimbar and Tual along with 131 fish from two northern Australian locations (Arafura and Timor Seas) from a previous study. Genetic variation in the control region of the mitochondrial genome was assayed using restriction fragment length polymorphism and direct sequencing. Haplotype diversity was high (0.67-0.82), as was intraspecific sequence divergence (range 0-5.8%). F(ST) between pairs of populations ranged from 0 to 0.2753. Genetic subdivision was apparent on a small spatial scale; F(ST) was 0.16 over 191 km (Bali/Sumbawa) and 0.17 over 491 km (Bali/Flores). Constraints to dispersal that contribute to, and maintain, the observed degree of genetic subdivision are experienced presumably by all life history stages of this tropical marine finfish. The constraints may include (1) little or no movement of eggs or larvae, (2) little or no home range or migratory movement of adults and (3) loss of larval cohorts due to transport of larvae away from suitable habitat by prevailing currents.     

Ocean circulation model predicts high genetic structure observed in a long‐lived pelagic developer

Understanding the movement of genes and individuals across marine seascapes is a long‐standing challenge in marine ecology and can inform our understanding of local adaptation, the persistence and movement of populations, and the spatial scale of effective management. Patterns of gene flow in the ocean are often inferred based on population genetic analyses coupled with knowledge of species' dispersive life histories. However, genetic structure is the result of time‐integrated processes and may not capture present‐day connectivity between populations. Here, we use a high‐resolution oceanographic circulation model to predict larval dispersal along the complex coastline of western Canada that includes the transition between two well‐studied zoogeographic provinces. We simulate dispersal in a benthic sea star with a 6–10 week pelagic larval phase and test predictions of this model against previously observed genetic structure including a strong phylogeographic break within the zoogeographical transition zone. We also test predictions with new genetic sampling in a site within the phylogeographic break. We find that the coupled genetic and circulation model predicts the high degree of genetic structure observed in this species, despite its long pelagic duration. High genetic structure on this complex coastline can thus be explained through ocean circulation patterns, which tend to retain passive larvae within 20–50 km of their parents, suggesting a necessity for close‐knit design of Marine Protected Area networks.     

G'ST and D do not replace FST

The genetic differentiation among populations is affected by mutation as well as by migration, drift and selection. For loci with high mutation rates, such as microsatellites, the amount of mutation can influence the values of indices of differentiation such as G(ST) and F(ST). For many purposes, this effect is undesirable, and as a result, new indices such as G'(ST) and D have been proposed to measure population differentiation. This paper shows that these new indices are not effective measures of the causes or consequences of population structure. Both G'(ST) and D depend heavily on mutation rate, but both are insensitive to any population genetic process when the mutation rate is high relative to the migration rate. Furthermore, D is specific to the locus being measured, and so little can be inferred about the population demography from D. However, at equilibrium, D may provide an index of whether a particular marker is more strongly affected by mutation than by migration. I argue that F(ST) is a more important summary of the effects of population structure than D and that R(ST) or other measures that explicitly account for the mutation process are much better than G(ST), G'(ST), or D for highly mutable markers. Markers with lower mutation rates will often be easier to interpret.     

Allozyme variation in pygmy wood mouse Apodemus uralensis (Rodenita, Muridae) in the Ural region

Variation of 17 allozyme loci was examined in 530 Apodemus uralensis individuals caught in the Ural region in 2005 through 2007. In the populations examined, the mean value of the population genetic differentiation index F(ST) constituted 0.169. It was demonstrated that F(ST) values for the samples obtained from the 1-km2 plot in different years, as well as for the samples trapped at a distance from 0.3 to 5 km during one year, could be remarkably higher than the mean value, pointing to their high, statistically significant differentiation. It seems likely that this differentiation was caused by spatial population subdivision, associated with the mice migrations, temporal change of the population structure, and the gene drift. In A. uralensis, inhabiting the zone of East Ural radioactive trace, no differences in the allozyme sets and their frequencies were observed.     

Genetic isolation by distance among populations of the netted dog whelk Nassarius reticulatus (L.) along the European Atlantic coastline

Estimates of the average distances by which marine larvae disperse are generally poorly described, despite the central role that larval dispersal plays in the demographic connectivity of populations across geographic space. Here, we describe the population genetic structure and average dispersal distance of the netted dog whelk Nassarius reticulatus (L.) (Mollusca, Gastropoda, Prosobranchia), a widespread member of European intertidal communities, using DNA sequence variation in a fragment of the mitochondrial gene cytochrome c oxidase subunit I (COI). An analysis of 156 individuals from 6 locations spread across approximately 1700 km of the European Atlantic coastline revealed weak and nonsignificant population structure (overall Phi(ST) = 0.00013). However, pairwise Phi(ST) values revealed a slight but significant increase in genetic isolation with geographic distance (IBD), suggesting that populations are not panmictic across the sampled geographic range. If we assume that the isolation by distance is maintained by a stable, stepping stone model of gene flow, then the slope of the IBD is consistent with an average larval dispersal distance of approximately 70 km per generation. The spatial scale of larval dispersal in N. reticulatus is consistent with the life cycle of the species (planktotrophic veliger lasting 30-60 days before competent to settle). A mismatch analysis of the COI sequences revealed a signature of an ancient demographic expansion that began 61 500-160,000 years ago, well before the most recent Pleistocene glaciation event. The greatest levels of genetic diversity occur within the middle latitudes of the whelk's geographic range, consistent with the notion that historic populations of N. reticulatus might have expanded northward and southward from the centrally located Bay of Biscay.     

Influence of habitat discontinuity, geographical distance, and oceanography on fine-scale population genetic structure of copper rockfish (Sebastes caurinus)

The copper rockfish is a benthic, nonmigratory, temperate rocky reef marine species with pelagic larvae and juveniles. A previous range-wide study of the population-genetic structure of copper rockfish revealed a pattern consistent with isolation-by-distance. This could arise from an intrinsically limited dispersal capability in the species or from regularly-spaced extrinsic barriers that restrict gene flow (offshore jets that advect larvae offshore and/or habitat patchiness). Tissue samples were collected along the West Coast of the contiguous USA between Neah Bay, WA and San Diego, CA, with dense sampling along Oregon. At the whole-coast scale (approximately 2200 km), significant population subdivision (F(ST) = 0.0042), and a significant correlation between genetic and geographical distance were observed based on 11 microsatellite DNA loci. Population divergence was also significant among Oregon collections (approximately 450 km, F(ST) = 0.001). Hierarchical amova identified a weak but significant 130-km habitat break as a possible barrier to gene flow within Oregon, across which we estimated that dispersal (N(e)m) is half that of the coast-wide average. However, individual-based Bayesian analyses failed to identify more than a single population along the Oregon coast. In addition, no correlation between pairwise population genetic and geographical distances was detected at this scale. The offshore jet at Cape Blanco was not a significant barrier to gene flow in this species. These findings are consistent with low larval dispersal distances calculated in previous studies on this species, support a mesoscale dispersal model, and highlight the importance of continuity of habitat and adult population size in maintaining gene flow.     

Lack of interpopulation genetic structure in the genus Stegastes (Perciformes) with indication of local introgression

The family Pomacentridae comprises about 326 species belonging to 28 genera. The genus Stegastes is composed of nearly 33 species, and 8 are endemic to the Brazilian Province, inhabiting the Brazilian coast (Stegastes fuscus, S. variabilis, S. leucosticus, S. uenfi, and S. pictus) or Western Atlantic oceanic islands (S. trindadensis, S. rocasensis and S. sanctipauli). Stegastes species play a major role in the reef ecosystem since they interfere significantly with the composition of benthonic organisms. Studies about population genetics and speciation of Neotropical ichthyofauna are scarce, particularly at insular areas from the Western Atlantic. Random amplified polymorphic DNA markers were used to analyze the population genetic structure of the continental species S. fuscus and S. variabilis (Northeastern Brazil) as well as the insular species S. sanctipauli (Saint Paul's Rocks). Analysis of population parameters revealed a high index of intrapopulation genetic variability in the species, except for S. sanctipauli, which showed low values. The phiST values in samples of S. fuscus and S. variabilis obtained at distinct collection sites 35 km apart from each other indicated a lack of population genetic structure. An intermediary profile of species-specific markers was detected in some individuals of S. fuscus and S. variabilis from Santa Rita, Rio Grande do Norte, suggesting a putative introgression event between the two species. The genetic profiles observed in Stegastes populations indicate a higher genetic variability along the shoreline than at oceanic sites, related to a reduced effective population size on islands. The lack of genetic differentiation among coastal populations suggests that, despite some biological features such as non-migratory behavior and territoriality, the pelagic larval phase of these species is able to promote an interpopulation homogeneity among sampled areas.     

Fine-Scale Population Genetic Structure of Zhikong Scallop (Chlamys farreri): Do Local Marine Currents Drive Geographical Differentiation?

Marine scallops, with extended planktonic larval stages which can potentially disperse over large distances when advected by marine currents, are expected to possess low geographical differentiation. However, the sessile lifestyle as adult tends to form discrete "sea beds" with unique population dynamics and structure. The narrow distribution of Zhikong scallop (Chlamys farreri), its long planktonic larval stage, and the extremely hydrographic complexity in its distribution range provide an interesting case to elucidate the impact of marine currents on geographical differentiation for marine bivalves at a fine geographical scale. In this study, we analyzed genetic variation at nine microsatellite DNA loci in six locations throughout the distribution of Zhikong scallop in the Northern China. Very high genetic diversity was present in all six populations. Two populations sampled from the same marine gyre had no detectable genetic differentiation (F (ST) = 0.0013); however, the remaining four populations collected from different marine gyres or separated by strong marine currents showed low but significant genetic differentiation (F (ST) range 0.0184-0.0602). Genetic differentiation was further analyzed using the Monmonier algorithm to identify genetic barriers and using the assignment test conducted by software GeneClass2 to ascertain population membership of individuals. The genetic barriers fitting the orientation of marine gyres/currents were clearly identified, and the individual assignment analysis indicated that 95.6% of specimens were correctly allocated to one of the six populations sampled. The results support the hypothesis that significant population structure is present in Zhikong scallop at a fine geographical scale, and marine currents can be responsible for the genetic differentiation.     

Vicariance and dispersal across Baja California in disjunct marine fish populations

Abstract.— Population disjunctions, as a first step toward complete allopatry, present an interesting situation to study incipient speciation. The geological formation of the Baja California Peninsula currently divides 19 species of fish into disjunct populations that are found on its Pacific Coast and in the northern part of the Gulf of California (also called the Sea of Cortez), but are absent from the Cape (Cabo San Lucas) region. We studied the genetic makeup of disjunct populations for 12 of these 19 fish species. Phylogeographic patterns for the 12 species can be separated into two major classes: a first group (eight species) showed reciprocal monophyly and high genetic divergence between disjunct populations. A second group (four species) displayed what appeared to be panmictic populations. Population structure between Pacific Coast populations, across the Punta Eugenia biogeographic boundary, was also evaluated. While dispersal potential (inferred by pelagic larval duration) was a poor predictor of population structure between Gulf of California and Pacific populations, we found that population genetic subdivision along the Pacific Coast at Punta Eugenia was always positively correlated with differentiation between Pacific and Gulf of California populations. Vicariant events, ongoing gene flow, and ecological characteristics played essential roles in shaping the population structures observed in this study.     

Microsatellite DNA analyses support an east-west phylogeographic split of American alligator populations

We examined the population genetic structure of American alligators (Alligator mississippiensis) sampled from 12 localities across the southeastern United States. The primary goal of this study was to determine the extent of population differentiation among alligators from four Florida lakes using eight microsatellite loci and compare the results to additional sites located at varying distances from them. Analyses of population structure revealed little differentiation (F(ST)=0.039; Rho=0.012) among the four Florida lakes, Apopka, Griffin, Orange and Woodruff, which are all located in the St. John&'s River watershed in north-central Florida. Further, there was little differentiation among these samples and samples collected from the Everglades National Park (F(ST)=0.044; Rho=0.009) and south Georgia (F(ST)=0.045; Rho=0.032). Therefore, these six samples were pooled together as a "FL/sGA group." Similarly, samples collected in the western extent of the range, Anahuac National Wildlife Refuge in Texas and Salvador Wildlife Management Area, Marsh Island Wildlife Refuge and Rockefeller Wildlife Refuge in Louisiana, also lacked population structure (F(ST)=0.024; R(ST)=0.040). These four populations were pooled into the "TX/LA group." Comparisons of these two groups with samples taken from the Santee Coastal Reserve in South Carolina and Mobile, Alabama yielded three to four times more differentiation among groups (F(ST)=0.131; Rho=0.187). These and other analyses support the hypothesis of an east-west phylogeographic split in American alligator populations and are consistent with studies of many freshwater fish and aquatic and terrestrial turtles distributed throughout this same geographic region.     

Conservation implications of complex population structure: lessons from the loggerhead turtle (Caretta caretta)

Complex population structure can result from either sex-biased gene flow or population overlap during migrations. Loggerhead turtles (Caretta caretta) have both traits, providing an instructive case history for wildlife management. Based on surveys of maternally inherited mtDNA, pelagic post-hatchlings show no population structure across the northern Atlantic (phi(ST) < 0.001, P = 0.919), subadults in coastal habitat show low structure among locations (phi(ST) = 0.01, P < 0.005), and nesting colonies along the southeastern coast of the United States have strong structure (phi(ST) = 0.42, P < 0.001). Thus the level of population structure increases through progressive life history stages. In contrast, a survey of biparentally inherited microsatellite DNA shows no significant population structure: R(ST) < 0.001; F(ST) = 0.002 (P > 0.05) across the same nesting colonies. These results indicate that loggerhead females home faithfully to their natal nesting colony, but males provide an avenue of gene flow between regional nesting colonies, probably via opportunistic mating in migratory corridors. As a result, all breeding populations in the southeastern United States have similar levels of microsatellite diversity (H(E) = 0.70-0.89), whereas mtDNA haplotype diversity varies dramatically (h = 0.00-0.66). Under a conventional interpretation of the nuclear DNA data, the entire southeastern United States would be regarded as a single management unit, yet the mtDNA data indicate multiple isolated populations. This complex population structure mandates a different management strategy at each life stage. Perturbations to pelagic juveniles will have a diffuse impact on Atlantic nesting colonies, mortality of subadults will have a more focused impact on nearby breeding populations, and disturbances to adults will have pinpoint impact on corresponding breeding populations. These findings demonstrate that surveys of multiple life stages are desirable to resolve management units in migratory marine species.