Genetic variability and level of differentiation in North Sea and Baltic Sea populations of the green alga<Emphasis Type="Italic"> Cladophora rupestris</Emphasis>

Cladophora rupestris is a perennial filamentous macroalga belonging to the Chlorophyta. It is widely distributed on both sides of the northern Atlantic Ocean and penetrates into the brackish Baltic Sea down to ca. 4 psu salinity. In this paper we present evidence for genetic differentiation of a Baltic form of this marine alga. We assessed genetic structure within and among 11 populations ranging along a salinity gradient from the Norwegian coast to the northern Baltic Sea proper. Samples of 328 individuals were studied using starch-gel protein electrophoresis to evaluate genetic variability and interpopulation differentiation based on allozymes. Of 11 loci examined, only one was polymorphic. For this locus, encoding superoxide dismutase (SOD-3), a total of seven alleles were distinguished. We found two genetically differentiated groups of populations of C. rupestris, one Baltic Sea group and one North Sea group, with a distinct border in the southern Kattegat near the entrance to the Baltic Sea. The genetic differentiation for SOD-3, expressed as pairwise F_ST values between the populations, was generally higher within the Baltic Sea group (0.10-0.43) than within the North Sea group (0.05-0.10); in the latter group also fewer pairs of populations differed significantly. Pairs of populations from different groups had the highest F_ST values (0.20-0.60). Hierarchical analysis of variance showed that 29.6% of the total variation in the SOD-3 locus was explained by variation between the two groups, while only 4.2% was explained by variation among the populations within the groups. The remaining variation (66.2%) was found within the populations.     

Mitochondrial and microsatellite assessment of population structure of South American sea lion (<Emphasis Type="Italic">Otaria flavescens</Emphasis>) in the Southwestern Atlantic Ocean

Several major breeding areas have been defined for the South American sea lion (Otaria flavescens) along the Atlantic Ocean including the Uruguayan and Patagonian coasts. Together with a documented and severe reduction in population sizes caused by commercial hunting in the last century, these areas show opposite population trends. While Patagonian populations are recovering since hunting ceased, Uruguayan populations are declining. In this context, population genetic structure and genetic diversity were studied for the first time with both nuclear (microsatellites) and mitochondrial (control region) markers together. Alternative scenarios were found for both markers. While mitochondrial marker showed geographically structured populations, the nuclear loci showed a lack of geographical structure. These opposite patterns in genetic structure could be explained by female phylopatry and high male dispersion. The reduction in population size caused by commercial hunting did not leave a detectable footprint of bottleneck at the genetic level.     

Effects of abiotic factors on growth and chemical defenses in cultivated clones of <Emphasis Type="Italic">Laurencia dendroidea</Emphasis> J. Agardh (Ceramiales,Rhodophyta)

Laurencia dendroidea shows high inter- and intrapopulation variability in the amount of the sesquiterpene elatol, caused by genetic variation as well as environmental factors. To test the independent effect of physical and nutritional conditions, the growth and the levels of elatol in L. dendroidea clones were evaluated under different conditions of temperature, salinity, irradiance, and culture medium in the laboratory. Growth of L. dendroidea was clearly affected by all these factors, but elatol levels were influenced only by temperature and salinity. Better conditions for growth did not produce a similar effect on elatol production in L. dendroidea, contradicting the carbon/nutrient balance and growth/differentiation balance models. On the contrary, severe conditions of temperature and salinity promoted a decrease in elatol levels, as predicted by the environmental stress model. Our results using clones indicated that abiotic factors clearly take part in fostering chemical variations observed in natural populations, in addition to genetic factors, and can promote differential susceptibility of plant specimens to natural enemies.     

Genetic structure of natural populations of California red abalone (<Emphasis Type="Italic">Haliotis rufescens</Emphasis>) using multiple genetic markers

Mitochondrial cytochrome oxidase subunit one (COI) sequence, nuclear microsatellites, and amplified fragment length polymorphisms (AFLPs) were used to evaluate connectivity among nine red abalone (Haliotis rufescens) populations sampled between August 1998 and November 2003 along approximately 1,300 km of California coastline from Crescent City (41°46′N, 124°12′W) to San Miguel Island (34°02′N, 120°22′W). COI sequences and microsatellite genotypes did not show significant genetic divergence among nine sampled populations. A subset of five populations spanning the geographic range of the study was scored for 163 polymorphic AFLP markers. Of these, 41 loci showed significant divergence (P < 0.001) among populations. Still, no AFLP markers were diagnostic for any of the study populations, and assignment tests did not consistently assign individuals to the correct population. Although the AFLP data are the first to suggest there is significant genetic differentiation among California red abalone populations, the discordance between the different genetic markers needs further study before unambiguous conclusions can be drawn with respect to connectivity among the populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Multi-marker estimate of genetic connectivity of sole (Solea solea) in the North-East Atlantic Ocean

A thorough knowledge on the genetic connectivity of marine populations is important for fisheries management and conservation. Using a dense population sampling design and two types of neutral molecular markers (10 nuclear microsatellite loci and a mtDNA cytochrome b fragment), we inferred the genetic connectivity among the main known spawning grounds of sole (Solea solea L.) in the North-East Atlantic Ocean. The results revealed a clear genetic structure for sole in the North-East Atlantic Ocean with at least three different populations, namely the Kattegat/Skagerrak region, the North Sea and the Bay of Biscay, and with indications for a fourth population, namely the Irish/Celtic Sea. The lack of genetically meaningful differences between biological populations within the southern North Sea is likely due to a large effective population size and sufficient connection (gene flow) between populations. Nevertheless, an isolation-by-distance pattern was found based on microsatellite genotyping, while no such pattern was observed with the cytochrome b marker, indicating an historical pattern prevailing in the latter marker. Our results demonstrate the importance of a combined multi-marker approach to understand the connectivity among marine populations at region scales.     

Conservation genetics of <Emphasis Type="Italic">Lumnitzera littorea</Emphasis> (Combretaceae), an endangered mangrove,from the Indo-West Pacific

Mangrove forests, with their ecological significance and economic benefits, are vital inter-tidal wetland ecosystems. Lumnitzera littorea (Combreataceae) is a non-viviparous mangrove distributed in tropical Asia and North Australia. Due to natural and human impacts, populations of this species have been isolated, fragmented, and highly disturbed. In China, L. littorea is an endangered species, restricted to small regions of Hainan Island. The genetic composition of five populations of this species from the Indo-West Pacific (South China, Malay Peninsula, Sri Lanka, North Australia) was assessed using inter simple sequence repeat (ISSR) makers. At the species level, expected mean heterozygosity (He) was 0.240 with 75.6% of loci polymorphic (P). However, genetic variation was much lower at the population level (P = 37.1%, He = 0.118). A high coefficient of gene differentiation (Gst = 0.515) and low level of gene flow (Nm = 0.470) indicated significant genetic differentiation among populations. AMOVA also indicated that more than half the total variation (58.4%) was partitioned among populations. The high degree of differentiation observed among populations emphasizes the need for appropriate conservation measures that incorporate additional populations into protected areas, and achieve the restoration of separate, degraded populations.     

Genetic variation between geographic populations of the amphipods Gammarus zaddachi and G. salinus

Spatial and temporal patterns of gene-enzyme variation were estimated in the sibling species Gammarus zaddachi Sexton and G. salinus Spooner by starch gel electrophoresis. Twenty-one G. zaddachi and 18 G. salinus populations from coastal and estuarine areas in the Baltic Sea, North Sea and other localities of north-western Europe were surveyed. Both amphipods display similar electrophoresis patterns of the enzyme systems studied. Considerable interspecific and interpopulational differences were detected in allele frequencies at three highly polymorphic loci, phosphoglucose isomerase (PGI), glutamate oxalacetate transaminase (GOT) and arginine phosphokinase (APK). G. zaddachi exhibits a pronounced genetic heterogeneity in most areas of the sampled range. Populations from northern French and western English coasts differ significantly from the other samples in allele frequencies at the PGI or APK locus, respectively. Baltic populations are widely uniform in their genetic composition but can be distinguished from samples taken at North Sea sites in allele frequencies at the APK locus. The latter reveal a clinal variation, ranging from the Danish to the French coast. In contrast to G. zaddachi, a low degree of genetic differentiation was observed among the G. salinus populations examined. This indicates that migration and interregional mixing may be more important in maintaining the genetic structure than in G. zaddachi which, compared to G. salinus, prefers habitats of lower salinity levels. Evidently, less extensive dispersal capabilities owing to the confinement of G. zaddachi to brackish waters of dilute salt concentrations may account for a diminished gene flow and considerable genetic separation of local populations. This assumption is supported by the genetic homogeneity documented in Baltic G. zaddachi populations. In view of the low and constant salinities in wide areas of this brackish-water sea such barriers do not exist. Survey studies performed with selected populations over a 3-yr period demonstrated a general pattern of temporal constancy in the allozyme variation observed.     

New evidence for habitat-specific selection in Wadden Sea Zostera marina populations revealed by genome scanning using SNP and microsatellite markers

Eelgrass Zostera marina is an ecosystem-engineering species of outstanding importance for coastal soft sediment habitats that lives in widely diverging habitats. Our first goal was to detect divergent selection and habitat adaptation at the molecular genetic level; hence, we compared three pairs of permanently submerged versus intertidal populations using genome scans, a genetic marker-based approach. Three different statistical approaches for outlier identification revealed divergent selection at 6 loci among 46 markers (6 SNPs, 29 EST microsatellites and 11 anonymous microsatellites). These outlier loci were repeatedly detected in parallel habitat comparisons, suggesting the influence of habitat-specific selection. A second goal was to test the consistency of the general genome scan approach by doubling the number of gene-linked microsatellites and adding single nucleotide polymorphism (SNP) loci, a novel marker type for seagrasses, compared to a previous study. Reassuringly, results with respect to selection were consistent among most marker loci. Functionally interesting marker loci were linked to genes involved in osmoregulation and water balance, suggesting different osmotic stress, and reproductive processes (seed maturation), pointing to different life history strategies. The identified outlier loci are valuable candidates for further investigation into the genetic basis of natural selection.     

Genetic evidence for the existence of cryptic species in an endangered clam <Emphasis Type="Italic">Coelomactra antiquata</Emphasis>

Coelomactra antiquata is a commercially important bivalve species, but has been suffering from severe population decline due to over-exploitation and the deterioration of environmental conditions. Previous genetic survey of C. antiquata conducted with allozymes combined with morphology revealed high levels of genetic differentiation between northern and southern populations which suggests a cryptic species might exist in C. antiquata. To test this hypothesis, amplified fragment length polymorphisms (AFLPs) and 16S rRNA gene sequence were used to re-evaluate the spatial genetic structure of six populations of C. antiquata along the coast of China. Both genetic markers display a sharp genetic break between the four northern populations (northern lineage) and two southern population (southern lineage). Large numbers of private alleles (AFLP) were found within the northern or southern populations and a deep divergence of about 6.5% in 16S rRNA gene sequence between the northern and southern lineages suggests the occurrence of potential cryptic or sibling species of C. antiquata. Applying previously published rates of mutation, divergence between the two lineages is estimated to have occurred approximately 3 million years ago and may be due to allopatric isolation during the middle Pliocene times. While no genetic differentiation was found within the northern or southern populations in both AFLP and 16S mtDNA markers, the results indicate that the northern and southern lineage should be managed separately and any translocation between the two areas should be avoided.     

Population differentiation in megrim (<Emphasis Type="Italic">Lepidorhombus whiffiagonis</Emphasis>) and four spotted megrim (<Emphasis Type="Italic">Lepidorhombus boscii</Emphasis>) across Atlantic and Mediterranean waters and implications for wild stock management

Megrim, Lepidorhombus whiffiagonis, and four spot megrim, Lepidorhombus boscii, are two marine fish species of high commercial interest. Despite their quite heavy exploitation little is known on the genetic structure of their populations. The present work aimed at characterizing the first seven microsatellites markers available for the two megrim species. These new markers were in a second step employed to describe the population structure of the two species among their almost entire habitat range (Atlantic and Mediterranean samples). Our study confirmed the existence of a strong genetic difference between Atlantic and Mediterranean megrim species already described in the literature for L. whiffiagonis on the basis of variations at ribosomal genes. Additionally our analysis gave the first evidences of a strong genetic differentiation among Atlantic populations in both megrim species (within Atlantic global F_ST in L. whiffiagonis and L. boscii were respectively 0.158 and 0.145). When describing megrim population structure, the comparison between allele-frequency-based tests (F_ST comparisons) and genotype-based inferences (Bayesian approach) gave evidences of a hierarchical structure of the populations. In conclusion, our work enlighten the existence of two different stocks within the Atlantic Ocean and one in the Mediterranean Sea that will clearly need to be managed separately. As the present results do not fully support the current megrim stock boundaries they will surely help to rethink megrim management policies in the future.     

Fine-scale genetic structure in short-beaked common dolphins (<Emphasis Type="Italic">Delphinus</Emphasis><Emphasis Type="Italic">delphis</Emphasis>) along the East Australian Current

Oceanographic processes play a significant role in shaping the genetic structure of marine populations, but it is less clear whether they affect genetic differentiation of highly mobile vertebrates. We used microsatellite markers and mtDNA control region sequences to investigate the spatial genetic structure of short-beaked common dolphins (Delphinus delphis) in southeastern Australia, a region characterised by complex oceanographic conditions associated with the East Australian Current (EAC). A total of 115 biopsy samples of dolphins were collected from six localities spanning approximately 1,000 km of the New South Wales (NSW) coastline. We found evidence for contrasting genetic diversity and fine-scale genetic structure, characterised by three genetically differentiated populations with varying levels of admixture. Spatial genetic structure was not explained by a model of isolation by distance, instead it coincides with main patterns of oceanographic variation along the EAC. We propose that common dolphins along the EAC may be adapted to three water masses recently characterised in this region.     

Population genetic structure of the stalked barnacle <Emphasis Type="Italic">Pollicipes pollicipes</Emphasis> (Gmelin, 1789) in the northeastern Atlantic: influence of coastal currents and mesoscale hydrographic structures

Within its distribution range in the northeastern Atlantic, the stalked barnacle Pollicipes pollicipes shows a well-defined pattern of genetic variation, comprising (a) a subtropical/temperate northern assemblage, made up of populations distributed between 47°N and 28°N along the French, Iberian, North African and Canary Islands coastlines, and (b) a single isolated and highly divergent tropical population in the Cape Verde Islands (16°N), at the southernmost limit of the species’ distribution. However, within the northern assemblage several populations show a level of genetic differentiation that allows rejection of the hypothesis of genetic homogeneity. The congruence observed between genetic and hydrographic patterns suggests a crucial role of hydrodynamics, and of the dispersal of the planktonic larvae, in the determination of population structure. Along the southern European Atlantic coast, the Iberian Poleward Current and mesoscale hydrographic structures are, respectively, facilitating gene flow at the regional level and genetic differentiation at the local level. On the Atlantic coast of North Africa, the homogenizing equatorward flow of the Canary Current does not extend as far as the Cape Verde Islands. A demographic expansion is dated to the late Pleistocene, preceding the Eemian interglacial, and is oldest in the case of the long-standing Cape Verde population, sustained by a stable tropical habitat. The divergence between the Cape Verde population and the remaining populations is thus ancient, and suggests that oceanic current patterns may constitute a generalized physical barrier to the dispersal of marine organisms between Cape Verde and the rest of Macaronesia.     

Using environmental and geographic data to optimize ex situ collections and preserve evolutionary potential

Maintenance of biodiversity through seed banks and botanical gardens, where the wealth of species’ genetic variation may be preserved ex situ, is a major goal of conservation. However, challenges can persist in optimizing ex situ collections if trade-offs exist among cost, effort, and conserving species evolutionary potential, particularly when genetic data are not available. We evaluated the genetic consequences of population preservation informed by geographic (isolation by distance [IBD]) and environmental (isolation by environment [IBE]) distance for ex situ collections for which population provenance is available. We used 19 genetic and genomic data sets from 15 plant species to assess the proportion of population genetic differentiation explained by geographic and environmental factors and to simulate ex situ collections prioritizing source populations based on pairwise geographic distance, environmental distance, or both. Specifically, we tested the impact prioritizing sampling based on these distances may have on the capture of neutral, functional, or putatively adaptive genetic diversity and differentiation. Individually, IBD and IBE explained limited population genetic differences across all 3 genetic marker classes (IBD, 10–16%; IBE, 1–5.5%). Together, they explained a substantial proportion of population genetic differences for functional (45%) and adaptive (71%) variation. Simulated ex situ collections revealed that inclusion of IBD, IBE, or both increased allelic diversity and genetic differentiation captured among populations, particularly for loci that may be important for adaptation. Thus, prioritizing population collections based on environmental and geographic distance data can optimize genetic variation captured ex situ. For the vast majority of plant species for which there is no genetic information, these data are invaluable to conservation because they can guide preservation of genetic variation needed to maintain evolutionary potential within collections.     

Microsatellite DNA markers and morphometrics reveal a complex population structure in a merobenthic octopus species (<Emphasis Type="Italic">Octopus maorum</Emphasis>) in south-east Australia and New Zealand

Five polymorphic microsatellite loci were developed and then used to assess the population genetic structure of a commercially harvested merobenthic octopus species (Octopus maorum) in south-east Australian and New Zealand (NZ) waters. Beak and stylet morphometrics were also used to assess population differentiation in conjunction with the genetic data. Genetic variation across all loci and all sampled populations was very high (mean number alleles = 15, mean expected heterozygosity = 0.85). Microsatellites revealed significant genetic structuring (overall F _ST = 0.024, p < 0.001), which did not fit an isolation-by-distance model of population differentiation. Divergence was observed between Australian and NZ populations, between South Australia and north-east Tasmania, and between two relatively proximate Tasmanian sites. South Australian and southern Tasmanian populations were genetically homogeneous, indicating a level of connectivity on a scale of 1,500 km. Morphometric data also indicated significant differences between Australian and NZ populations. The patterns of population structuring identified can be explained largely in relation to regional oceanographic features.     

Population structure of the common sole (<Emphasis Type="Italic">Solea solea</Emphasis>) in the Northeastern Atlantic and the Mediterranean Sea: revisiting the divide with EPIC markers

Spatial and temporal population genetic structures of the common sole, Solea solea, were studied in Northeastern Atlantic and Mediterranean Sea populations, using three polymorphic exon-primed intron-crossing (EPIC) markers. Results demonstrated significant multilocus differentiation among Eastern Mediterranean and a group composed by Western Mediterranean and Atlantic populations (θ = 0.150, P < 0.001), but also suggested unrecorded genetic differentiation of the Adriatic Sea population. No pattern of isolation-by-distance was recorded across the range covered by sampling, from the Kattegat to the Aegean Sea. Conversely to genetically structured Mediterranean populations, Atlantic populations ranging from Denmark to Portugal could be considered as representative of the same panmictic unit (θ = 0.009, not significant). Results further demonstrated stability of multilocus genetic structure among temporarily replicated cohort samples [0+, 1+, subadults] from several coastal and estuarine locations from Bay of Biscay, excepted for the amylase locus Am2B3-2 at one location (Pertuis d’Antioche). Despite coherence of such observed patterns of multilocus differentiation with previous allozymic surveys in sole, and with patterns generally obtained for other marine fish species, single-locus results from EPICs indicated divergent coalescence schemes supporting a complex response to ecology and history of sole’s populations. Results stress the use of nuclear genes such as EPIC markers to investigate population structure, but also historical, demographic, and possibly selective processes in marine fishes.     

Molecular population structure of the kuruma shrimp <Emphasis Type="Italic">Penaeus</Emphasis><Emphasis Type="Italic"> japonicus</Emphasis> species complex in western Pacific

In a previous study on the kuruma shrimp Penaeus japonicus from the South China Sea, we detected high genetic divergence between two morphologically similar varieties (I and II) with distinct color banding patterns on the carapace, indicating the occurrence of cryptic species. In the present study, we clarify the geographical distribution of the two varieties in the western Pacific by investigating the genetic differentiation of the shrimp from ten localities. Two Mediterranean populations are also included for comparison. Based on the mitochondrial DNA sequence data, the shrimps are separated into two distinct clades representing the two varieties. Variety I comprises populations from Japan and China (including Taiwan), while variety II consists of populations from Southeast Asia (Vietnam, Singapore and the Philippines), Australia and the Mediterranean. Population differentiation is evident in variety II, as supported by restriction profiles of two mitochondrial markers and analysis of two microsatellite loci. The Australian population is genetically diverged from the others, whereas the Southeast Asian and Mediterranean populations show a close genetic relationship. Variety I does not occur in these three localities, while a small proportion of variety II is found along the northern coast of the South China Sea and Taiwan, which constitute the sympatric zone of the two varieties. The present study reveals high genetic diversity of P. japonicus. Further studies on the genetic structure of this species complex, particularly the populations in the Indian Ocean and Mediterranean, are needed not only to understand the evolutionary history of the shrimp, but also to improve the knowledge-based fishery management and aquaculture development programs of this important biological resource.     

Genetic differentiation between morphotypes in the Antarctic limpet <Emphasis Type="Italic">Nacella concinna</Emphasis> as revealed by inter-simple sequence repeat markers

The limpet Nacella concinna (Strebel 1908) was the focus of numerous studies dealing with Antarctic benthos. One of the main characteristics of the species is the presence of two distinguishable morphotypes, one inhabiting the intertidal (during summer) and the other inhabiting the subtidal. For a long time these forms were considered as an expression of phenotypic plasticity, since previous studies did not found genetic differences between them. In the present work, we performed both a morphometric and a genetic differentiation analysis (using ISSR-PCR markers) of these two forms in three stations sampled at Potter Cove, South Shetland Islands. The results confirmed the morphological differences between intertidal and subtidal forms reported in other Antarctic localities. The genetic differences detected indicate that the two forms can be considered as genetically distinct populations maintaining low levels of gene flow. The degree of reproductive isolation of the ecotypes is discussed, as well as the possible origin of the divergence. The genetic differentiation observed can also have behavioral and physiological correlates, pointing out the importance of taking into account the potential differences in the response of both populations to different conditions in future studies in this species. M. C. de Aranzamendi and R. Sahade contributed equally to this paper.     

Spatio-temporal patterns in the genetic structure of recently settled blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.) across a hybrid zone

Previous studies of a hybrid zone between the mussels Mytilus edulis Linnaeus and M. galloprovincialis Lamarck have not resolved the relative importance of the genetic composition of settling larval cohorts versus post-settlement selection in determining the distribution of the parental species and their hybrids. In the present study, recently settled mussels (spat) were collected from 20 sites in southwest England throughout the summer and fall (May–October) in 1998 and 1999. This study investigated the spatio-temporal patterns of settlement and genetics of mussel spat by genetically identifying M. edulis, M. galloprovincialis and their hybrids using the diagnostic PCR marker Glu-5. Settlement was observed earlier in populations of M. edulis than in populations of M. galloprovincialis. Settlement occurred in hybrid populations at times intermediate to and overlapping with both of the parental populations. Temporal genetic variation within years was rare at most sites, while there was some variation between the two years. Spatial genetic variation, however, was common among spat settling within the hybrid populations and matched that observed in small, sub-adults at the same sites. No consistent directional changes in allele frequency were observed over the course of several weeks after settlement. These data suggest that the observed spatial variation in the adult populations is the result of spatial variation in settling larval cohorts and not of either temporal genetic variation or of selection soon after settlement.Communicated by J.P. Grassle, New Brunswick     

Variation in the genetic composition of coral (Pocillopora damicornis and Acropora palifera) populations from different reef habitats

The genetic structure of populations of the corals Pocillopora damicornis and Acropora palifera was examined in three habitats at One Tree Island during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differences among populations of P. damicornis within each of the reef crest, lagoon and microatoll habitats. The level of differentiation among populations was similar in each of the habitats. Differences between populations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the genetic composition of A. palifera populations within or between the lagoon and microatolls, the only habitats where this species was found. Both coral species had observed:expected (G _O:G_E) genotypic diversity rations >0.80, indicating predominantly sexual reproduction. These data, the high genotype diversity and general conformance of genotype frequencies to those expected under conditions of Hardy-Weinberg, suggested panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual reproduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relative isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective means of dispersal between the habitats, suggesting that the genetic differences result from natural selection in the different environments within One Tree Lagoon and the reef crest.