Completely disjunct mitochondrial DNA haplotype distribution without a phylogeographic break in a planktonic developing gastropod

Planktonic developing organisms are generally assumed to be good dispersers showing little genetic structuring in neutral markers. At first glance, this also applies to the planktonic developing periwinkle Tectarius striatus, an endemic gastropod from Macaronesia (i.e. Azores, Madeira, Canary Islands and Cape Verde Islands), where the only sign of genetic structuring hitherto is provided by a non-significant allozyme/RAPD heterogeneity between the Cape Verde Islands and the other archipelagos. However, partial sequences of the mitochondrial cytochrome b and cytochrome oxidase I genes now show that the Cape Verde Islands and the three other archipelagos have no haplotypes in common, whereas the latter three do share several haplotypes. Nevertheless, this highly disjunct haplotype distribution does not entail a phylogeographic break separating the haplotypes of both areas in two reciprocally monophyletic groups. This remarkable geographic and phylogenetic structuring may be explained by assuming that T. striatus colonized the Macaronesian archipelagos in periods when sea levels were lower (and/or volcanic activity was higher), so that seamounts peaked above sea level and could act as stepping-stones. Yet, after the last glacial period seamounts submerged, thus preventing further stepping-stones mediated dispersal of T. striatus between the Cape Verde Islands and the other archipelagos, while not affecting dispersal among the latter because of their closer proximity and connectivity. Hence, these contrasting patterns of neutral genetic variation in T. striatus show that genetic structuring in planktonic developing species may be far more complex than is usually assumed.     

Evidence of limited gene flow in three species of coral reef fishes in the lagoon of New Caledonia

Allozyme variation was used to investigate the genetic structure of the coral reef fishes Stegastes nigricans, Epinephelus merra and Acanthurus triostegus around New Caledonia. Each species was sampled from each of three sites in the ≃1000 km circumference of the lagoon of New Caledonia. Allelic variation was recorded for each species at 14, 13 and 17 loci, respectively, and heterozygosity diversity (H s) was 0.082, 0.065 and 0.116, respectively. Analysis of genetic differentiation between sites produced inconsistent results between species, with spatial heterogeneity in two species (S. nigricans, F st  = 0.038; A. triostegus, F st = 0.049) and homogeneity in one species (E. merra, F st = 0.000). Hydrological and climatic data from the lagoon suggest that the eastern and western sides of the lagoon are isolated, since they lie in water masses of different origin. This may explain the genetic differentiation and restricted gene flow found at a local scale for S. nigricans and A. triostegus. Homogeneity in populations of E. merra is discussed in relation to its low genetic diversity and its reproductive behaviour. Received: 23 April 1997 / Accepted: 25 September 1997     

Genetic heterogeneity among New Zealand species of hydrothermal vent mussels (Mytilidae: Bathymodiolus)

Molecular systematic studies provide evidence for three new species of Bathymodiolus-like hydrothermal vent mussels (Bivalvia: Mytilidae) from relatively shallow waters (depth less than 750 m) associated with the Kermadec Arc off northern New Zealand. Mitochondrial COI sequences from the three putative new species differed substantially from those of other known bathymodiolin species from the Pacific and Indian Oceans. Population genetic analysis of one of these species (Bathymodiolus new species NZ-1) revealed heterogeneity in allozyme gene frequencies between samples collected from two seamounts about 50 km apart. Factors that might contribute to genetic differentiation between neighbouring seamounts are discussed.Communicated by M.S. Johnson, Crawley     

Molecular phylogeography of two Italian sibling species of Calobius (Coleoptera, Hydraenidae, Ochthebiinae) inhabiting Mediterranean marine rock-pools

Marine rock-pools, commonly found along the Mediterranean coasts, are isolated patches of habitat characterised by large spatial and temporal variations. The phylogeography of Calobius quadricollis and C. urbanelliae, two sibling species of moss beetles (Coleoptera, Hydraenidae) inhabiting Italian temporary marine rock-pool ecosystems, were studied using mitochondrial cytochrome c oxidase subunit I (COI). Our data suggest a strong association between the scored genetic variability and the geographical distribution of populations sampled for both species. These analyses provided evidences of episodic species range expansion and fragmentation, recurrent “flush and crash” and bottleneck episodes probably occurring during the Glacial Cycles. The observed phylogeographical pattern is probably related to the historical and biogeographical processes of Mediterranean areas, as well as both to the ephemeral habitat tipology and limited dispersal ability of these beetles.     

Complex patterns of population structure and recruitment of <Emphasis Type="Italic">Plectropomus leopardus</Emphasis> (Pisces: Epinephelidae) in the Indo-West Pacific: implications for fisheries management

Here the population genetic structure of an ecologically and economically important coral reef fish, the coral trout Plectropomus leopardus, is investigated in the context of contemporary and historical events. Coral trout were sampled from four regions (six locations) and partial mtDNA D-loop sequences identified six populations (Fst = 0.89209, P < 0.0001): Scott Reef and the Abrolhos Islands in west Australia; the Great Barrier Reef (GBR), represented by northern and southern GBR samples; New Caledonia and Taiwan, with Taiwan containing two genetic lineages. Furthermore, this study identified source and sink populations within and among regions. Specifically, the northern population in west Australia (Scott Reef) was identified, as the source for replenishment of the Abrolhos population, whilst New Caledonia was a source for recruitment to the GBR. Based on these insights from a single mtDNA marker, this study will facilitate the development of rational management plans for the conservation of P. leopardus populations and therefore mitigate the risk of population declines from anthropogenic influences.     

Population structure and phylogeography of an acorn barnacle with induced defense and its gastropod predator in the Gulf of California

Using sequence variation in the mitochondrial cytochrome oxidase I locus, we estimated the population structure and phylogeography of the intertidal acorn barnacle, Chthamalus anisopoma, and its gastropod predator, Mexacanthina lugubris angelica. Both are endemic to the Gulf of California, being derived from taxa on the Pacific coast of the Baja peninsula, and both exhibit phenotypic plasticity for traits affecting their coevolutionary interactions. Consistent with expectations based on differences in dispersal, C. anisopoma populations generally lack geographic structure, while those of M. l. angelica are more strongly structured. However, the variable degree of differentiation in both species suggests that the extent of reciprocal selection and local adaptation in the species will vary geographically, a result consistent with the concept of a geographic mosaic of coevolution. The pattern of variation in C. anisopoma shows clear evidence of recent spatial expansion, possibly due to increased habitat availability following the last glacial maximum. Phylogeographic analyses suggest that M. l. angelica diversified into three distinct clades after the colonization of the Gulf. Overall, our results illustrate how dispersal potential, geological and climatic events, and recent population growth have impacted the pattern of sequence variation in the two species.     

Correlation between pelagic larval duration and realised dispersal: long-distance genetic connectivity between northern New Zealand and the Kermadec Islands archipelago

The extent to which marine populations are “open” (panmixia) or “closed” (self-recruitment) remains a matter of much debate, with recent reports of high levels of genetic differentiation and self-recruitment among populations of numerous species separated by short geographic. However, the interpretation of patterns of gene flow (connectivity) is often based on a stepping stone model of dispersal that can genetically homogenise even distant populations and blur genetic patterns that may better reflect realised dispersal. One way in which realised long-distance dispersal can be accurately determined is by examination of gene flow of taxa between isolated archipelagos and a mainland where there is no possible stepping stone dispersal across the open ocean. We investigated the genetic structuring of populations of the intertidal gastropod Nerita melanotragus from the subtropical Kermadec Islands and temperate New Zealand’s North Island (the mainland), separated by 750 km of open ocean and characterised by contrasting environmental conditions. Analyses of seven microsatellite markers revealed an absence of genetic structuring with low F _ST and Jost’s D values (from 0.000 to 0.007 and from 0.000 to 0.015, respectively) over large geographic distances and no evidence of isolation by distance among all populations. These results indicate that the realised dispersal of N. melanotragus is of at least 750 km, this species exhibits a very “open” form of connectivity and its larvae exhibit sufficient phenotypic plasticity to settle successfully in different environmental conditions, ranging from subtropical to cool temperate.     

Low genetic differentiation between isolated populations of the colonial ascidian Symplegma rubra Monniot, C. 1972

Pelagic larvae are highly important for maintaining the gene flow among populations of sessile marine invertebrates. Colonial ascidians consist, exclusively, of brooding species, with lecithotrophic larvae that have a limited dispersal. As a result, there is a marked differentiation among populations. In this work, we used allozyme electrophoresis to access the genetic variation in four populations of Symplegma rubra, a colonial ascidian frequently found in the intertidal zone of Southeastern Brazilian coast. High variability was found at three of the four sites sampled, the exception being Praia Grande in the State of Rio de Janeiro. At this site, there was a great preponderance of clones, which possibly reflected the enclosed nature of the location and its low water circulation that reduce the dispersal capabilities of these animals. S. rubra did not conform to expectations for random mating (Hardy–Weinberg equilibrium): there was a deficit of heterozygotes that was more related to the small population size than to inbreeding processes, since F _is analysis per locus revealed a deficiency of heterozygotes at only one locus—MDH*. The greatest variation in allele frequency was found for GPI-2*. Analyses of genetic variability revealed moderate differentiation among the populations (F _ST=0.051), which was unexpected for a species with a low dispersal capability. Rafting, a frequently underestimated means of dispersal, may be the main mode of gene flow in this species over large areas, since colonies of S. rubra are frequently seen growing on drift material and there is no evidence that the larva survive for a long time in the plankton.     

Genetic evidence for contrasting patterns of dispersal in solitary and colonial ascidians

In situ and in vitro observations indicate that brooding colonial ascidians commonly display limited larval dispersal, whilst the larvae of most solitary species are assumed to be widely dispersed. We used allozyme data to determine the population genetic consequences of reproduction and dispersal in a broadcast-spawning solitary ascidian and two brooding colonial species along the central and southern coast of New South Wales, Australia. We surveyed genetic variation at 2 to 9 variable loci for samples collected from 6 to 8 local populations of each of the stalked solitary species Pyura gibbosa gibbosa Heller, 1878; the social Stolonica australis Michaelsen, 1927 and the compound Botrylloides magnicoecum Hartmeyer, 1912. Samples from each local population displayed levels and patterns of genotypic diversity that were consistent with expectations for sexually-derived recruitment of both solitary zooids and separate colonies. However, we found clear differences in the structure of the populations of solitary and colonial species. Genotype frequencies within all nine samples of P. gibbosa gibbosa conformed to expectations for random mating (i.e. Hardy–Weinberg equilibria). Moreover, allele frequencies showed little variation among samples [mean standardised genetic variance (F _{S T} ) =0.002], which implies that local populations are strongly connected by larval dispersal. We estimate (via Wright's “island model”) that gene flow (N _e m) within this set of local populations is 125 effective migrants per generation, which is very similar to estimates obtained for other broadcast-spawning taxa in this region. In contrast, genotype frequencies within samples of both colonial species were characterised by large and statistically significant deficits of heterozygotes, consistent with expectations for highly limited dispersal of larvae or sperm. Moreover, local populations were highly differentiated (F _{S T} =0.201 and 0.202 for S. australis and B. magnicoecum, respectively) and N _e m was estimated to be ∼1.0 in each case. These values of F _{S T} and subsequent estimates of N _e m lie within the range of values reported for other New South Wales taxa with direct larval development, and imply that local populations are effectively closed to immigration. Received: 13 February 1997 / Accepted 18 July 1997     

Population connectivity in the temperate damselfish <Emphasis Type="Italic">Parma microlepis</Emphasis>: analyses of genetic structure across multiple spatial scales

This study investigated the utility of microsatellite markers for providing information on levels of population connectivity for a low dispersing reef fish in New South Wales (NSW), Australia, at scales ≤400 km. It was hypothesized that the temperate damselfish Parma microlepis, which produces benthic eggs and has limited post-settlement dispersal, would exhibit spatial genetic structure and a significant pattern of isolation-by-distance (IBD). A fully nested hierarchical sampling design incorporating three spatial scales (sites, location and regions, separated by 1–2, 10–50 and 70–80 km respectively) was used to determine genetic variability at seven microsatellite loci. Broad-scale genetic homogeneity and lack of IBD was well supported by single and multi-locus analyses. The proportion of the total genetic variation attributable to differences among regions, locations or sites was effectively zero (Φ/R-statistics ≤0.007). The geographic distribution of genetic diversity and levels of polymorphism (H _E 0.21–0.95) indicate high mutation rates, large effective population sizes, and high rates of gene flow. Significant gene flow may be driven by factors influencing pre-settlement dispersal, including the East Australian Current (EAC) and habitat continuity. Genetic connectivity may not reflect demographically important connectivity, but does imply that P. microlepis populations are well connected from an evolutionary perspective. Total observed genetic diversity was accounted for within 1–2 km of reef and could be represented within small Marine Protected Areas. Reef fishes in NSW which have life histories similar to P. microlepis (e.g. pre-settlement durations ≥2 weeks) are also likely to exhibit genetic homogeneity. Genetic markers are, therefore, most likely to provide information on demographically relevant connectivity for species with lower dispersal capabilities, small population sizes, short life spans, and whose habitats are rare, or patchily distributed along-shore. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genetic structure of camouflage grouper,<Emphasis Type="Italic"> Epinephelus polyphekadion</Emphasis> (Pisces: Serranidae), in the western central Pacific

The objective of the present study was to investigate the population genetic structure of the commercially important camouflage grouper, Epinephelus polyphekadion (Bleeker, 1849), in the western and central Pacific Ocean to improve existing management. Camouflage grouper are widely distributed in the Indo-Pacific and form brief, seasonal, spawning aggregations that are often heavily fished. The present study examined populations sampled in 1997-1998 at five sites in the western central Pacific spanning a geographic distance of ~5,000 km: New Caledonia, Great Barrier Reef, Palau, Marshall Islands, and Pohnpei (Micronesia). Primer pairs were developed to examine genetic variation at three polymorphic microsatellite loci. Cluster analysis, using genetic distance, revealed three regional groupings: (1) Palau, (2) Pohnpei and the Marshall Islands, and (3) the Great Barrier Reef and New Caledonia. Highly significant allele frequency differences were observed among sites. At Pohnpei, significant allele frequency differences in successive years were also apparent, possibly related to genetic variation among cohorts or between local spawning groups. The inter-annual differences at Pohnpei suggest that there may be further genetic structuring over relatively modest distances, a finding relative to determining management units for this commercially valuable species and suggests that future studies need to incorporate possible small-scale temporal or spatial components into study design.     

Life style and genetic variation in teleosts: the case of pelagic (<Emphasis Type="Italic">Aphia minuta</Emphasis>) and benthic (<Emphasis Type="Italic">Gobius niger</Emphasis>) gobies (Perciformes: Gobiidae)

The pattern of genetic variability of two species of Mediterranean gobiids was compared, with reference to their different life history traits (Aphia minuta paedomorphic and pelagic; Gobius niger metamorphosed and benthic). The aim was to evaluate how different life histories can affect the genetic structure in these marine teleosts. The study was carried out on populations of both species sampled in the western Mediterranean and in the Adriatic Sea. Seven restriction endonucleases were used for the RFLP analysis of a mitochondrial DNA segment comprising the NADH dehydrogenase subunits 3, 4L and 4. The results highlighted two different patterns of genetic variation, a weak genetic structure in A. minuta and population subdivision in G. niger. These observations may be explained not only in terms of the different dispersal capabilities of these species, but also considering that A. minuta is an abbreviate iteroparous spawner while G. niger is a protracted iteroparous spawner. Because abbreviate iteroparity is a reproductive strategy selected in stable environments with high resource availability, Pliocene and Pleistocene climate oscillations may have represented factors that negatively influenced the reproductive success of A. minuta, producing demographic fluctuations and bottlenecks, as suggested by the mismatch distribution analysis. The weak genetic structure of A. minuta populations seems to be therefore due to a more recent re-colonization of the Mediterranean basin after a severe population decline, rather than to the high vagility of this pelagic goby.     

Life history, ecology and the biogeography of strong genetic breaks among 15 species of Pacific rockfish, Sebastes

Strong genetic change over short spatial scales is surprising among marine species with high dispersal potential. Concordant breaks among several species signals a role for geographic barriers to dispersal. Along the coast of California, such breaks have not been seen across the biogeographic barrier of Point Conception, but other potential geographic boundaries have been surveyed less often. We tested for strong-population structure in 11 species of Sebastes sampled across two regions containing potential dispersal barriers, and conducted a meta-analysis including four additional species. We show two strong breaks north of Monterey Bay, spanning an oceanographic gradient and an upwelling jet. Moderate genetic structure is just as common in the north as it is in the south, across the biogeographic break at Point Conception. Gene flow is generally higher among deep-water species, but these conclusions are confounded by phylogeny. Species in the subgenus Sebastosomus have higher structure than those in the subgenus Pteropodus, despite having larvae with longer pelagic phases. Differences in settlement behavior in the face of ocean currents might help explain these differences. Across similar species across the same coastal environment, we document a wide variety of patterns in gene flow, suggesting that interaction of individual species traits such as settlement behavior with environmental factors such as oceanography can strongly impact population structure.     

Phylogeography and Limited Genetic Connectivity in the Endangered Boring Giant Clam across the Coral Triangle

Abstract: The Coral Triangle is the global center of marine biodiversity; however, its coral reefs are critically threatened. Because of the bipartite life history of many marine species with sedentary adults and dispersive pelagic larvae, designing effective marine protected areas requires an understanding of patterns of larval dispersal and connectivity among geographically discrete populations. We used mtDNA sequence data to examine patterns of genetic connectivity in the boring giant clam (Tridacna crocea) in an effort to guide conservation efforts within the Coral Triangle. We collected an approximately 485 base pair fragment of mtDNA cytochrome c oxidase 1 (CO1) from 414 individuals at 26 sites across Indonesia. Genetic structure was strong between regions (φ_ST=0.549, p < 0.00001) with 3 strongly supported clades: one restricted to western Sumatra, another distributed across central Indonesia, and a third limited to eastern Indonesia and Papua. Even within the single largest clade, small but significant genetic structure was documented (φ_ST=0.069, p < 0.00001), which indicates limited gene flow within and among phylogeographic regions. Significant patterns of isolation by distance indicated an average dispersal distance of only 25–50 km, which is far below dispersal predictions of 406–708 km derived from estimates of passive dispersal over 10 days via surface currents. The strong regional genetic structure we found indicates potent limits to genetic and demographic connectivity for this species throughout the Coral Triangle and provides a regional context for conservation planning. The recovery of 3 distinct evolutionarily significant units within a well‐studied taxonomic group suggests that biodiversity in this region may be significantly underestimated and that Tridacna taxa may be more endangered than currently recognized.     

Genetic differentiation in Littorina saxatilis (Gastropoda)

Littorina saxatilis, an ovoviviparous gastropod, displays considerable morphological variability, perhaps due to limited dispersal abilities. Gene and genotype frequencies of 2 polymorphic enzymatic loci were determined electrophoretically for 7 populations of L. saxatilis, to estimate degree of population differentiation. Results show that significant population differentiation may occur over distances of as little as 2 km, while widely separated populations may be nearly identical. An index of probability of genotypic identity calculated for populations of L. saxatilis shows variable and heterogeneous values. Comparison of this index for L. saxatilis with an index for 12 populations of Nassarius obsoletus —which possesses strong dispersal abilities due to long-term veliger larva and high, homogeneous indices of probability of genotypic identity between populations — indicates a strong correlation between population differentiation and dispersal ability of the organism. Possible factors influencing population differentiation in L. saxatilis are discussed.     

Megabenthic decapod crustacean assemblages on the Galician continental shelf and upper slope (north-west Spain)

The structure of megabenthic decapod crustacean assemblages on the Galician (north-west Spain) continental shelf (100 to 200 depth) and upper slope (200 to 500 m) was analyzed based on surveys carried out in autumn and spring, from 1980 to 1987. Forty species belonging to 19 families were caught. The portunid crab Polybius henslowii, a species with pelagic phases, was the most abundant species, but displayed strong spatial and temporal fluctuations. Other dominant species were the Norway lobster Nephrops norvegicus, the portunid Liocarcinus depurator, the galatheids Munida intermedia and M. sarsi, and the shrimps Solenocera membranacea, Plesionika heterocarpus, Pasiphaea sivado and Dichelopandalus bonnieri. Total abundance and biomass (average values excluding Polybius henslowii = 255 individuals and 2.06 kg/30 min tow) and species richness and diversity, H′ (6.85 species and H′ = 1.45 per tow) displayed a significant positive correlation with depth, and strong interannual fluctuations. The factors determining community organization were depth and, to a lesser extent, spatial structure. There was clear evidence of bathymetric zonation, differentiating between species characteristic of the slope (D. bonnieri and Pasiphaea sivado), shelf-slope edge (Macropipus tuberculatus, Pontophilus spinosus, Munida sarsi, S. membranacea, Processa spp.) and shelf (L. depurator, Macropodia tenuirostris, Paguridae and Chlorotocus crassicornis). The spatial zonation was related to changes in oceanography and sediment along the continental margin. Goneplax rhomboides, N. norvegicus, C. crassicornis and Alpheus glaber are benthic species which generally exhibit burrowing behaviour, and they were found mainly in the southern area where there are fine sediments due to the outwelling from the Rías Baixas. Different benthopelagic shrimps (Pontophilus spinosus, Plesionika heterocarpus,Processa spp. and Pasiphaea sivado) were typical of the zone just north of Fisterra, characterized by a convergence of water masses bringing about an increase in productivity due to upwelling. The benthic anomuran and brachyuran crabs Munida intermedia, M.␣sarsi, L.␣depurator and Macropipus tuberculatus were characteristic of the northwestern zone between Fisterra and Estaca, where the infauna reaches high biomass despite coarser sediments with a lower concentration of organic material than in the southern area. Lastly, both the Paguridae and Macropodia tenuirostris were species typically found in the waters in the northern shelf. Based on interannual changes in assemblage structure, two periods could be distinguished: between 1980 and 1984, when Polybius henslowii, D. bonnieri and Pasiphaea sivado had abundance peaks; and another period from 1985 to 1987 when L. depurator, Munida intermedia, M.␣sarsi and Macropipus tuberculatus increased in abundance. Received: 21 May 1996 / Accepted: 9 August 1996     

Geographic structure in Alaskan Pacific ocean perch (<Emphasis Type="Italic">Sebastes alutus</Emphasis>) indicates limited lifetime dispersal

Prevailing oceanographic processes, pelagic larvae, adult mobility, and large populations of many marine species often leads to the assumption of wide-ranging populations. Applying this assumption to more localized populations can lead to inappropriate conservation measures. The Pacific ocean perch (Sebastes alutus, POP) is economically and ecologically valuable, but little is known about its population structure and life history in Alaskan waters. Fourteen microsatellite loci were used to characterize geographic structure and connectivity of POP collections (1999–2005) sampled along the continental shelf break from Dixon Entrance to the Bering Sea. Despite opportunities for dispersal, there was significant, geographically related genetic structure (F _ST = 0.0123, P < 10⁻⁵). Adults appear to belong to neighborhoods at geographic scales less than 400 km, and possibly as small as 70 km, which indicates limited dispersal throughout their lives. The population structure observed has a finer geographic scale than current management, which suggests that measures for POP fisheries conservation should be revisited.     

Chaotic genetic patchiness and high relatedness of a poecilogonous polychaete in a heterogeneous estuarine landscape

The genetic structure of benthic marine invertebrates is often described as “chaotic” when genetic structure cannot be explained and barriers to dispersal and gene flow cannot be identified. Here, chaotic patterns of genetic structure for the polychaete Pygospio elegans (Claparède) sampled at 16 locations from the heterogeneous Isefjord–Roskilde Fjord estuary complex in Denmark were found. There was no isolation by distance, and the geography of the estuary complex did not seem to pose a barrier to dispersal and gene flow in this species. We investigated whether characteristics of the environment could be related to the genetic structure and possibly restrict gene flow in this species. Additionally, since P. elegans is poecilogonous, producing larvae with different pelagic developmental periods, we investigated whether observed developmental modes in the samples might clarify the genetic patterns. None of the tested factors explained the population genetic structure. However, a high degree of relatedness among individuals in almost all samples was found. Samples with a larger percentage of young individuals had more related individuals, suggesting that different cohorts could be comprised of individuals with different degrees of relatedness. Relatedness within a site could be increased by limited larval dispersal, collective dispersal of related larvae, sweepstakes reproductive success, or asexual reproduction, but distinguishing between these requires further study. Using a “seascape genetics” approach allowed us to investigate some of the numerous potential factors that could influence population genetic structure in a poecilogonous species.     

Mode of reproduction,recruitment, and genetic subdivision in the brooding sponge<Emphasis Type="Italic"> Haliclona</Emphasis> sp.

Sponges display a variety of reproductive strategies that have the potential to influence population genetic structure. Histological examination of ten reproductive individuals of the Western Australian sponge Haliclona sp. showed that this species broods embryonic larvae that are potentially limited in dispersal capabilities. Because sponges have the potential to propagate in a number of modes, allozyme electrophoresis was used to assess the relative importance of asexual and sexual reproduction to recruitment, and to quantify genetic subdivision over different spatial scales. Tissue samples from 227 sponges were collected from reefs within two areas 400 km apart: Hamelin Bay and Rottnest Island. Contrary to expectations for highly clonal populations, genotypic diversity within sites was high, no linkage disequilibrium was found, and there was no evidence of genotypic clustering within reefs. There was no genetic evidence that asexual reproduction is important for the maintenance of populations. Genetic comparisons were consistent with mixing of sexually produced recruits within reefs, on a scale up to a few hundred metres, but significant genetic subdivision between reefs (F_ST=0.069 at Hamelin Bay, 0.130 at Rottnest Island) indicated that water gaps of several hundred metres are effective at preventing dispersal. Subdivision between the two areas, separated by 400 km, was moderately greater (F_ST=0.142) than within, but the same alleles were predominant in the two areas. These genetic patterns are consistent with limited dispersal capabilities of brooded larvae.Communicated by G.F. Humphrey, Sydney