Regional philopatry of scalloped hammerhead sharks (Sphyrna lewini) to nursery areas in the Mexican Pacific

Hydrobiologia - The population genetic structure and female philopatry to nursery grounds of the scalloped hammerhead shark (Sphyrna lewini) were studied in different mangrove estuaries along the...     

Worldwide phylogeography of the blacktip shark (Carcharhinus limbatus) inferred from mitochondrial DNA reveals isolation of western Atlantic populations coupled with recent Pacific dispersal

Although many coastal shark species have widespread distributions, the genetic relatedness of worldwide populations has been examined for few species. The blacktip shark, (Carcharhinus limbatus), inhabits tropical and subtropical coastal waters throughout the world. In this study, we examined the genetic relationships of blacktip shark populations (n = 364 sharks) throughout the majority of the species' range using the entire mitochondrial control region (1067-1070 nucleotides). Two geographically distinct maternal lineages (western Atlantic, Gulf of Mexico, and Caribbean Sea clades, and eastern Atlantic, Indian, and Pacific Ocean clades) were identified and shallow population structure was detected throughout their geographic ranges. These findings indicate that a major population subdivision exists across the Atlantic Ocean, but not the Pacific Ocean. The historical dispersal of this widespread, coastal species may have been interrupted by the rise of the Isthmus of Panama. This scenario implies historical dispersal across the Pacific Ocean (supported by the recovery of the same common haplotype from the Philippines, Hawaii, and the Gulf of California reflecting recent/contemporary dispersal abilities) and an oceanic barrier to recent migration across the Atlantic. Genetic structure within the eastern Atlantic/Indo-Pacific (Phi(ST) = 0.612, P < 0.001) supports maternal philopatry throughout this area, expanding previous western Atlantic findings. Eastern Atlantic/Indo-Pacific C. limbatus control region haplotypes were paraphyletic to Carcharhinus tilstoni haplotypes in our maximum-parsimony analysis. The greater divergence of western Atlantic C. limbatus than C. tilstoni from eastern Atlantic/Indo-Pacific C. limbatus reflects the taxonomic uncertainty of western Atlantic C. limbatus.     

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.     

Sex-biased dispersal in sperm whales: contrasting mitochondrial and nuclear genetic structure of global populations

The social organization of most mammals is characterized by female philopatry and male dispersal. Such sex-biased dispersal can cause the genetic structure of populations to differ between the maternally inherited mitochondrial DNA (mtDNA) and the bi-parental nuclear genome. Here we report on the global genetic structure of oceanic populations of the sperm whale, one of the most widely distributed mammalian species. Groups of females and juveniles are mainly found at low latitudes, while males reach polar waters, returning to tropical and subtropical waters to breed. In comparisons between oceans, we did not find significant heterogeneity in allele frequencies of microsatellite loci (exact test; p = 0.23). Estimates of GST = 0.001 and RST = 0.005 also indicated negligible if any nuclear DNA differentiation. We have previously reported significant differentiation between oceans in mtDNA sequences. These contrasting patterns suggest that interoceanic movements have been more prevalent among males than among females, consistent with observations of females being the philopatric sex and having a more limited latitudinal distribution than males. Consequently, the typical mammalian dispersal pattern may have operated on a global scale in sperm whales.     

Population genetic structure and long-distance dispersal among seabird populations: implications for colony persistence

Dramatic local population decline brought about by anthropogenic-driven change is an increasingly common threat to biodiversity. Seabird life history traits make them particularly vulnerable to such change; therefore, understanding population connectivity and dispersal dynamics is vital for successful management. Our study used a 357-base pair mitochondrial control region locus sequenced for 103 individuals and 18 nuclear microsatellite loci genotyped for 245 individuals to investigate population structure in the Atlantic and Pacific populations of the pelagic seabird, Leach's storm-petrel Oceanodroma leucorhoa leucorhoa. This species is under intense predation pressure at one regionally important colony on St Kilda, Scotland, where a disparity between population decline and predation rates hints at immigration from other large colonies. AMOVA, F(ST), Φ(ST) and Bayesian cluster analyses revealed no genetic structure among Atlantic colonies (Global Φ(ST) = -0.02 P > 0.05, Global F(ST) = 0.003, P > 0.05, STRUCTURE K = 1), consistent with either contemporary gene flow or strong historical association within the ocean basin. The Pacific and Atlantic populations are genetically distinct (Global Φ(ST) = 0.32 P < 0.0001, Global F(ST) = 0.04, P < 0.0001, STRUCTURE K = 2), but evidence for interocean exchange was found with individual exclusion/assignment and population coalescent analyses. These findings highlight the importance of conserving multiple colonies at a number of different sites and suggest that management of this seabird may be best viewed at an oceanic scale. Moreover, our study provides an illustration of how long-distance movement may ameliorate the potentially deleterious impacts of localized environmental change, although direct measures of dispersal are still required to better understand this process.     

Evidence for reproductive philopatry in the bull shark Carcharhinus leucas

Reproductive philopatry in bull sharks Carcharhinus leucas was investigated by comparing mitochondrial (NADH dehydrogenase subunit 4, 797 base pairs and control region genes 837 base pairs) and nuclear (three microsatellite loci) DNA of juveniles sampled from 13 river systems across northern Australia. High mitochondrial and low microsatellite genetic diversity among juveniles sampled from different rivers (mitochondrial φ(ST) = 0·0767, P < 0·05; microsatellite F(ST) = -0·0022, P > 0·05) supported female reproductive philopatry. Genetic structure was not further influenced by geographic distance (P > 0·05) or long-shore barriers to movement (P > 0·05). Additionally, results suggest that C. leucas in northern Australia has a long-term effective population size of 11 000-13 000 females and has undergone population bottlenecks and expansions that coincide with the timing of the last ice-ages.     

Population genetic structure of Earth's largest fish, the whale shark (Rhincodon typus)

Large pelagic vertebrates pose special conservation challenges because their movements generally exceed the boundaries of any single jurisdiction. To assess the population structure of whale sharks (Rhincodon typus), we sequenced complete mitochondrial DNA control regions from individuals collected across a global distribution. We observed 51 single site polymorphisms and 8 regions with indels comprising 44 haplotypes in 70 individuals, with high haplotype (h = 0.974 +/- 0.008) and nucleotide diversity (pi = 0.011 +/- 0.006). The control region has the largest length variation yet reported for an elasmobranch (1143-1332 bp). Phylogenetic analyses reveal no geographical clustering of lineages and the most common haplotype was distributed globally. The absence of population structure across the Indian and Pacific basins indicates that oceanic expanses and land barriers in Southeast Asia are not impediments to whale shark dispersal. We did, however, find significant haplotype frequency differences (AMOVA, Phi(ST) = 0.107, P < 0.001) principally between the Atlantic and Indo-Pacific populations. In contrast to other recent surveys of globally distributed sharks, we find much less population subdivision and no evidence for cryptic evolutionary partitions. Discovery of the mating and pupping areas of whale sharks is key to further population genetic studies. The global pattern of shared haplotypes in whale sharks provides a compelling argument for development of broad international approaches for management and conservation of Earth's largest fish.     

Multiple population structure of the giant mottled eel, Anguilla marmorata

The population structure of the giant mottled eel, Anguilla marmorata, was investigated with mitochondrial and microsatellite DNA analyses using 449 specimens from 13 localities throughout the species range. Control region F-statistics indicated the North Pacific (Japan, Taiwan, Philippines, Sulawesi), South Pacific (Tahiti, Fiji, New Caledonia, Papua New Guinea), eastern Indian Ocean (Sumatra), western Indian Ocean (Réunion, Madagascar), Ambon, and Guam regions were significantly different (Phi(ST) = 0.131-0.698, P < 0.05) while only a few differences were observed between localities within the South Pacific. These regions were roughly clustered in the neighbour-joining tree, although Ambon individuals were mainly divided into North and South Pacific groups. Analysis with eight microsatellite loci showed almost identical results to those of the control region, except no genetic difference was observed between the western and eastern Indian Ocean (F(ST) = 0.009, P > 0.05). The Bayesian cluster analysis of the microsatellite data detected two genetic groups. One included four North Pacific localities, and the other included eight localities in the South Pacific, Indian Ocean, and Guam, but Ambon individuals were evenly assigned to these two groups. These results showed that A. marmorata has four genetically different populations (North Pacific, South Pacific, Indian Ocean, Guam region). The North Pacific population is fully panmictic whereas the South Pacific and Indian Ocean populations have a metapopulation structure. Interestingly, Guam was suggested to be inhabited by a reproductive population restricted to that region, and the individuals from the North and South Pacific populations co-exist in Ambon.     

Population genetics and phylogeography of freshwater mussels in North America, Elliptio dilatata and Actinonaias ligamentina (Bivalvia: Unionidae)

Extrinsic and intrinsic forces combined shape the population structure of every species differently. Freshwater mussels are obligate parasites to a host fish during a juvenile stage (glochidia). Elliptio dilatata (ED) and Actinonaias ligamentina (AL) are co-occurring freshwater mussel taxa with similar North American distribution and share some potential host fish. Using mitochondrial DNA, we determined the genotypes of 190 + individuals from collection sites in at least two tributaries in the Lake Erie and Ohio River watersheds, along with the Ouachita and Strawberry rivers in the southeast. Both species had followed a stepping-stone model of dispersal, with greater pairwise genetic structure among collection sites of ED. Also, phylogeographical analysis for ED found significant geographical structuring of haplotype diversity. Overall, within-population variation increased significantly from north to south, with low genetic diversity in the Strawberry River. We calculated significant among-population structure for both species (ED: Phi(ST) = 0.62, P < 0.001; AL: Phi(ST) = 0.16, P < 0.001). Genetic analysis identified the Ouachita River as an area of significant reproductive isolation for both species. Results for AL indicated dispersal into northern areas from two genetically distinct glacial refugia, where results for ED indicated dispersal followed by low gene flow in northern areas. The conservation strategies for mussels that co-occur in the same 'bed' could be species specific. Species such as ED have management units on the population scale, where AL has a more homogeneous genetic structure across its range.     

Possible female philopatry of the smooth hammerhead shark Sphyrna zygaena revealed by genetic structure patterns

We assessed the spatial pattern of genetic structure of smooth hammerhead shark Sphyrna zygaena in 10 localities from the Northern Mexican Pacific. A total of 35 haplotypes were identified in 129 sequences of the mtDNA control region. The results showed slight but significant genetic structure among localities (Φ_ST = 0.044, P < 0.001). In addition, the localities with highest number of juveniles were genetically different (Φ_ST = 0.058, P < 0.024), which may be representative of nursery areas. The genetic differentiation pattern can be associated to female philopatry and preference for particular birthing sites. Finally, historical demography shows that S. zygaena populations present a recent demographic expansion that occurred during glacial events in the late Pleistocene to early Holocene.     

Neither philopatric nor panmictic: microsatellite and mtDNA evidence suggests lack of natal homing but limits to dispersal in Pacific lamprey

Most species with lengthy migrations display some degree of natal homing; some (e.g. migratory birds and anadromous salmonids) show spectacular feats of homing. However, studies of the sea lamprey (Petromyzon marinus) indicate that this anadromous species locates spawning habitat based on pheromonal cues from larvae rather than through philopatry. Previous genetic studies in the anadromous Pacific lamprey (Entosphenus tridentatus) have both supported and rejected the hypothesis of natal homing. To resolve this, we used nine microsatellite loci to examine the population structure in 965 Pacific lamprey from 20 locations from central British Columbia to southern California and supplemented this analysis with mitochondrial DNA restriction fragment length polymorphism analysis on a subset of 530 lamprey. Microsatellite analysis revealed (i) relatively low but often statistically significant genetic differentiation among locations (97% pairwise F(ST) values were <0.04 but 73.7% were significant); and (ii) weak but significant isolation by distance (r(2) = 0.0565, P = 0.0450) but no geographic clustering of samples. The few moderate F(ST) values involved comparisons with sites that were geographically distant or far upstream. The mtDNA analysis--although providing less resolution among sites (only 4.7%F(ST) values were significant)--was broadly consistent with the microsatellite results: (i) the southernmost site and some sites tributary to the Salish Sea were genetically distinct; and (ii) southern sites showed higher haplotype and private haplotype richness. These results are inconsistent with philopatry, suggesting that anadromous lampreys are unusual among species with long migrations, but suggest that limited dispersal at sea precludes panmixia in this species.     

Formation of population genetic structure following the introduction and establishment of non-native American shad (<Emphasis Type="Italic">Alosa sapidissima</Emphasis>) along the Pacific Coast of North America

Biological invasions provide opportunities to examine contemporary evolutionary processes in novel environments. American shad, an anadromous fish native to the Atlantic Coast of North America, was introduced to California in 1871 and established spawning populations along the Pacific Coast that may provide insights into the dynamics of dispersal, colonization, and the establishment of philopatry. Using 13 neutral microsatellite loci we genotyped anadromous, freshwater resident and landlocked American shad from 14 locations along the US Pacific Coast to resolve population genetic structure. We observed significant differences in multilocus allele frequency distributions in nearly all (61/66; 92%) pairwise comparisons of non-native anadromous, freshwater resident and landlocked populations, and detected significant genetic differentiation for most (55/66; 83%) of these comparisons. Genetic divergence between landlocked and anadromous populations is due to genetic drift in isolation because of a physical migration barrier. However, some reproductive isolating mechanism maintains genetic differentiation between sympatric populations in the Columbia River exhibiting alternative life history strategies (i.e. anadromous vs. ‘freshwater-type’). Non-native populations possessed genetic variants that were not observed in the species’ native range and were strongly differentiated from Atlantic Coast populations (\({\text{G}}^{{\prime }}_{\text{ST}}\)?=?0.218). Our results indicate that philopatry became established shortly after dispersal and colonization along the Pacific Coast. This study contributes to our understanding of dynamic evolutionary processes during invasions.     

What influences the worldwide genetic structure of sperm whales (Physeter macrocephalus)?

The interplay of natural selection and genetic drift, influenced by geographic isolation, mating systems and population size, determines patterns of genetic diversity within species. The sperm whale provides an interesting example of a long‐lived species with few geographic barriers to dispersal. Worldwide mtDNA diversity is relatively low, but highly structured among geographic regions and social groups, attributed to female philopatry. However, it is unclear whether this female philopatry is due to geographic regions or social groups, or how this might vary on a worldwide scale. To answer these questions, we combined mtDNA information for 1091 previously published samples with 542 newly obtained DNA profiles (394‐bp mtDNA, sex, 13 microsatellites) including the previously unsampled Indian Ocean, and social group information for 541 individuals. We found low mtDNA diversity (π = 0.430%) reflecting an expansion event <80 000 years bp, but strong differentiation by ocean, among regions within some oceans, and among social groups. In comparison, microsatellite differentiation was low at all levels, presumably due to male‐mediated gene flow. A hierarchical amova showed that regions were important for explaining mtDNA variance in the Indian Ocean, but not Pacific, with social group sampling in the Atlantic too limited to include in analyses. Social groups were important in partitioning mtDNA and microsatellite variance within both oceans. Therefore, both geographic philopatry and social philopatry influence genetic structure in the sperm whale, but their relative importance differs by sex and ocean, reflecting breeding behaviour, geographic features and perhaps a more recent origin of sperm whales in the Pacific. By investigating the interplay of evolutionary forces operating at different temporal and geographic scales, we show that sperm whales are perhaps a unique example of a worldwide population expansion followed by rapid assortment due to female social organization.     

Population genetic structure of North American thinhorn sheep (Ovis dalli)

The thinhorn sheep (Ovis dalli ssp.) provides a rare example of a North American large mammal that occupies most of its native range and maintains close to ancestral population size. There are currently two recognized subspecies, Dall's sheep (O. d. dalli) and Stone's sheep (O. d. stonei), the validity of which remains uncertain. We investigated the spatial genetic structure of thinhorn sheep populations representing both subspecies by genotyping individuals (n = 919) from across the species range at 12 variable microsatellite loci. We found high levels of genetic diversity within (HE = 0.722) and significant genetic structure among the 24 sampled areas (FST = 0.160). Genetic distance measures and Bayesian clustering analyses revealed the presence of at least eight subpopulations that are delineated by mountain range topology. A strong overall pattern of isolation-by-distance is evident across the sampling range (r = 0.75, P < 0.001) suggesting limited dispersal and extensive philopatry. Partial Mantel tests of this relationship showed mountain range distinctions represent significant barriers to gene flow (P = 0.0001), supporting the Bayesian analyses. Genetic structure was more strongly pronounced in southern Yukon and Alaska than elsewhere. We also show evidence for genetic differences between the two currently recognized thinhorn subspecies.     

The life histories of endangered hammerhead sharks (Carcharhiniformes, Sphyrnidae) from the east coast of Australia

The life histories of two globally endangered hammerhead sharks, Sphyrna lewini and Sphyrna mokarran, were examined using samples collected from a range of commercial fisheries operating along the east coast of Australia. The catch of S. lewini was heavily biased towards males, and there were significant differences in von Bertalanffy growth parameters (L(∞) and k) and maturity [stretched total length (L(ST)) and age (A) at which 50% are mature, L(ST50) and A(50)] between those caught in the tropics (L(∞) = 2119 mm, k = 0·163, L(ST50) = 1471 mm, A(50) = 5·7 years) and those caught in temperate waters (L(∞) = 3199 mm, k = 0·093, L(ST50) = 2043 mm, A(50) = 8·9 years). The best-fit estimates for a three-parameter von Bertalanffy growth curve fit to both sexes were L(∞) = 3312 mm, L(0) = 584 mm and k = 0·076. Males attained a maximum age of 21 years and grew to at least 2898 mm L(ST). The longevity, maximum length and maturity of females could not be estimated as mature animals could not be sourced from any fishery. Length at birth inferred from neonates with open umbilical scars was 465-563 mm L(ST). There was no significant difference in length and age at maturity of male and female S. mokarran, which reached 50% maturity at 2279 mm L(ST) and 8·3 years. Sphyrna mokarran grew at a similar rate to S. lewini and the best-fit estimates for a two-parameter von Bertalanffy equation fit to length-at-age data for sexes combined with an assumed mean length-at-birth of 700 mm were L(∞) = 4027 mm and k = 0·079. Females attained a maximum age of 39·1 years and grew to at least 4391 mm L(ST). The oldest male S. mokarran was 31·7 years old and 3691 mm L(ST). Validation of annual growth-band deposition in S. mokarran was achieved through a mark, tag and recapture study.     

青岛近海路氏双髻鲨群体遗传学分析

研究采集了青岛近海23尾路氏双髻鲨(Sphyrna lewini), 通过线粒体DNA控制区片段对其遗传多样性进行分析。研究结果显示: 在23个个体的控制区序列上存在13个变异位点, 未检测到插入/缺失位点; 检测到7个单倍型, 其中3个为个体共享单倍型(Hap1、Hap3和Hap5), 4个为个体独有单倍型; 青岛近海路氏双髻鲨呈现中等水平的单倍型多样度和较低的核苷酸多样度; 与已报道的日照、霞浦群体间的遗传分化指数F_st值分别为–0.0571和–0.0328, 表明青岛群体与其他两个群体间不存在显著差异。以Sphyrna zygaena为外群构建NJ系统树显示本研究中7个单倍型共分成两支, 分别与来自太平洋、印度洋的单倍型类群聚类。中国近海的路氏双髻鲨作为一个具有较低遗传多样性的濒危物种, 其资源保护更应该引起足够的重视。     

High gene flow across large geographic scales reduces extinction risk for a highly specialised coral feeding butterflyfish

The vulnerability of ecologically specialised species to environmental fluctuations has been well documented. However, population genetic structure can influence vulnerability to environmental change and recent studies have indicated that specialised species may have lower genetic diversity and greater population structuring compared to their generalist counterparts. To examine whether there were differences in population genetic structure between a dietary specialist (Chaetodon trifascialis) and a dietary generalist (Chaetodon lunulatus) we compared the demographic history and levels of gene flow of two related coral-feeding butterflyfishes. Using allele frequencies of ≥11 microsatellite loci and >350 bases of mitochondrial control region sequence our analyses of C. trifascialis and C. lunulatus from five locations across the Pacific Ocean revealed contrasting demographic histories and levels of genetic structure. Heterozygosity excess tests, neutrality tests and mismatch distributions were all highly significant in the dietary specialist C. trifascialis (all P < 0.01), suggesting genetic bottlenecks have occurred in all locations. In contrast, we found little evidence of genetic bottlenecks for the dietary generalist C. lunulatus. High gene flow and low genetic structuring was detected among locations for C. trifascialis (amova: R(ST) = 0.0027, P = 0.371; Φ(ST) = 0.068, P < 0.0001). Contrary to our expectations, a greater level of genetic structuring between locations was detected for C. lunulatus (amova: R(ST) = 0.0277, Φ(ST) = 0.166, both P < 0.0001). These results suggest that dietary specialisation may affect demographic history through reductions in population size following resource declines, without affecting population structure through reductions in gene flow in the same way that habitat specialisation appears to. Although C. trifascialis is highly vulnerable to coral loss, the high gene flow detected here suggests populations will be able to recover from local declines through the migration of individuals.     

Dispersal and population structure of a New World predator, the army ant Eciton burchellii

The army ant Eciton burchellii is probably the most important arthropod predator in the Neotropics, and many animal species depend upon it. Sex-biased dispersal with winged males and permanently wingless queens may render this species especially sensitive to habitat fragmentation and natural barriers, which might have severe impacts on population structure and lead to population decline. Using nuclear microsatellite markers and mitochondrial sequences, we investigated genetic differentiation in a fragmented population in the Panama Canal area. While nuclear markers showed little differentiation between subpopulations (F(ST) = 0.017), mitochondrial differentiation was maximal in some cases (Phi(ST) = 1). This suggests that, while females are not capable of crossing barriers such as large rivers, flying males are able to promote nuclear gene flow between the studied forest patches. Consistent with this interpretation, we did not find any evidence for inbreeding or genetic deterioration on Barro Colorado Island over the last 90 years since its formation.     

Demographic processes underlying subtle patterns of population structure in the scalloped hammerhead shark, Sphyrna lewini

Genetic diversity (θ), effective population size (N(e)), and contemporary levels of gene flow are important parameters to estimate for species of conservation concern, such as the globally endangered scalloped hammerhead shark, Sphyrna lewini. Therefore, we have reconstructed the demographic history of S. lewini across its Eastern Pacific (EP) range by applying classical and coalescent population genetic methods to a combination of 15 microsatellite loci and mtDNA control region sequences. In addition to significant population genetic structure and isolation-by-distance among seven coastal sites between central Mexico and Ecuador, the analyses revealed that all populations have experienced a bottleneck and that all current values of θ are at least an order of magnitude smaller than ancestral θ, indicating large decreases in N(e) (θ = 4N(e)μ), where μ is the mutation rate. Application of the isolation-with-migration (IM) model showed modest but significant genetic connectivity between most sampled sites (point estimates of Nm = 0.1-16.7), with divergence times (t) among all populations significantly greater than zero. Using a conservative (i.e., slow) fossil-based taxon-specific phylogenetic calibration for mtDNA mutation rates, posterior probability distributions (PPDs) for the onset of the decline in N(e) predate modern fishing in this region. The cause of decline over the last several thousand years is unknown but is highly atypical as a post-glacial demographic history. Regardless of the cause, our data and analyses suggest that S. lewini was far more abundant throughout the EP in the past than at present.     

The panmixia paradigm of eastern Pacific olive ridley turtles revised: consequences for their conservation and evolutionary biology

Previous studies of the olive ridley Lepidochelys olivacea population structure in the tropical eastern Pacific have indicated the existence of a single panmictic population ranging from Costa Rica to Mexico. This information has been used to design specific management measures to conserve primary nesting beaches in Mexico. However, little is known about olive ridleys in the Baja California Peninsula, their northernmost reproductive limit, where recent observations have shown differences in nesting female behaviour and size of hatchlings relative to other continental rookeries. We used mtDNA control region sequences from 137 turtles from five continental and four peninsular nesting sites to determine whether such differences correspond to a genetic distinction of Baja California olive ridleys or to phenotypic plasticity associated with the extreme environmental nesting conditions of this region. We found that genetic diversity in peninsular turtles was significantly lower than in continental nesting colonies. Analysis of molecular variance revealed a significant population structure (Phi ST = 0.048, P = 0.006) with the inclusion of peninsular samples. Our results: (i) suggest that the observed phenotypic variation may be associated with genetic differentiation and reproductive isolation; (ii) support the recent colonization of the eastern Pacific by Lepidochelys; (iii) reveal genetic signatures of historical expansion and colonization events; and (iv) significantly challenge the notion of a single genetic and conservation unit of olive ridleys in the eastern Pacific. We conclude that conservation measures for olive ridleys in Mexico should be revised to grant peninsular beaches special attention.