Effects of isolation by distance and geographical discontinuity on genetic subdivision of Littoraria cingulata

Littoraria cingulata (Philippi, 1846) is a Western Australian, mangrove littorine snail, represented by two morphologically distinct subspecies, whose distributions are separated by >300 km. The southern subspecies, L. cingulata pristissini, is distinguished from the northern subspecies, L. cingulata cingulata, by having a thinner, keelless shell with more primary grooves, and lower and much more numerous ribs. In contrast with these striking differences, L. cingulata cingulata is morphologically very similar to another species, L. sulculosa, with which it also shares a nearly coincident geographic range. Allozyme comparisons at 22 presumptive loci confirmed a large genetic distance between L. cingulata and L. sulculosa, and the apparent conspecificity of the morphologically divergent subspecies of L. cingulata. Based on geological evidence, the geographical separation of the morphologically divergent forms of L. cingulata has developed within the past 5000 to 10 000 yr. The extensive continuous distribution of the northern subspecies, L. cingulatacingulata, and the large geographic disjunction between the northern and Shark Bay subspecies, L. cingulata pristissini, allowed a test of the genetic importance of this relatively recent disjunction. Within the continuous distribution of the two subspecies, a pattern of isolation by distance was visible up to distances of 300 km. Beyond 300 km, genetic subdivision, measured by pairwise G _ST (the proportion of genetic diversity due to differences between populations), averaged 0.028, whereas subdivision between Shark Bay and northern populations averaged 0.055 over the same range of distances. Although the relative paucity of barriers to gene flow tends to limit genetic subdivision in marine species with planktotrophic larvae, the results for L. cingulata suggest that subdivision can occur within a continuous distribution, but that special events leading to major disjunctions can substantially increase divergence, even over a relatively short period of time. Received: 16 February 1998 / Accepted: 23 April 1998     

Genetic variation and clonal structure in the scleractinian coral Pocillopora damicornis in the Ryukyu Archipelago, southern Japan

Samples of the scleractinian coral Pocillopora damicornis were collected from six sites located around four islands in the Ryukyu Archipelago, southern Japan, and subjected to allozyme electrophoresis. Seven polymorphic loci were examined for their allelic patterns. The ratio of observed to expected genotypic diversity (0.30 < G _o :G _e  < 0.64), the ratio of the observed number of genotypes to the number of individuals (0.47 < N _g :N _i  < 0.75), and deviations from Hardy–Weinberg equilibrium indicated that asexual reproduction plays a major role in the maintenance of established populations. However, populations were not completely dominated by a single or a few clones, and most clones were represented by only a few individual samples. The high frequency of typhoons in the region suggests that, in P. damicornis, fragmentation caused through occasional exposure to powerful waves is a major mode of asexual reproduction, but asexual production of planulae may also be contributing to the maintenance of populations. A significant genetic differentiation (F ST) was found between the six populations examined (0.027 < F ST < 0.092, average F ST = 0.056). The moderate gene flow is discussed according to characteristics of the larval stage of the species, and to circulation patterns in the region. Received: 7 August 1998 / Accepted: 18 May 1999     

Genetic differentiation in two sibling species of the brackish-water polychaete Hediste japonica complex (Nereididae)

Genetic divergence among ten populations of small- and large-egg forms of the brackish-water polychaete Hediste japonica complex was investigated on 14 isozyme loci by electrophoretic analysis. The two forms were distinguishable by complete allele substitutions at five loci, resulting in high genetic differentiation (Nei's D: 0.533 to 0.662). No genetic evidence of hybridization between the two forms was detected in sympatric populations in three rivers. These results indicate that the two forms are reproductively isolated, clearly showing that the two forms are distinct species. The genetic differentiation among populations was higher in the large-egg form (D: 0.005 to 0.111, G _ST: 0.435) than that in the small-egg form (D: 0.000 to 0.001, G _ST: 0.020). This genetic difference between the two forms seems to be attributable to a difference in their life histories. The average expected heterozygosity was low in populations of both the large-egg form (0.005 to 0.068) and the small-egg form (0.014 to 0.038) in comparison with other marine invertebrates. Received: 11 April 1997 / Accepted: 8 September 1997     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999     

Restricted gene flow and evolutionary divergence between geographically separated populations of the Antarctic octopus Pareledone turqueti

Samples of the Antarctic octopus Pareledone turqueti were taken from three locations on the Scotia Ridge in the Southern Ocean. The genetic homogeneity of these populations was investigated using isozyme electrophoresis. Whilst panmixia appeared to be maintained around South Georgia (F _ST = 0) gene flow between this island and Shag Rocks, an island only 150 km away but separated by great depths, was extremely limited (F _ST = 0.74). These results are examined with respect to the discontinuous distribution of P. turqueti throughout Antarctica. An estimate of effective population size was also calculated (N _e = 3600). Received: 7 March 1997 / Accepted: 27 March 1997     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Effects of contrasting modes of larval development on the genetic structures of populations of three species of prosobranch gastropods

 In south-eastern Australia, the prosobranch gastropods Morula marginalba (Blainville), Cominella lineolata (Lamarck) and Bedeva hanleyi (Angas) have similar fine-scale distributions, but appear to possess very different dispersal capabilities due to contrasting modes of larval development. M.marginalba produce planktonic larvae, whereas C. lineolata and B. hanleyi undergo direct development in benthic egg capsules and emerge as crawling juveniles. To test for possible effects of contrasting life histories on levels of genetic variation within and among populations, a survey was conducted of allozyme variation at six polymorphic loci in 8 to 9 local populations of each species. Collections of snails were made between June 1992 and November 1993. Sampling ranges spanned between 162 and 180 km of coast. Regardless of larval type, proportions of single-locus genotypes in each collection were consistent with the recruitment of offspring which had been generated through random mating. However, genotypic diversity was lower in those species that undergo direct development. Loci surveyed in C. lineolata and B. hanleyi were polymorphic (i.e. frequency of most common allele <95%) in fewer populations than those examined for M.␣marginalba (P <0.001) and, where polymorphisms occurred, also possessed significantly fewer alleles (P <0.001). Consequently, average levels of expected heterozygosity were greater in populations of M. marginalba than in those of either of the other species (P <0.001). Genetic variation among populations, expressed as the standardised variance in allele frequencies (F _ST ), was inversely related to expected larval dispersal capability. The nine collections of M. marginalba showed little overall differentiation (F _ST  = 0.017; P <0.001), reflecting the ability of planktonic larvae to interconnect local populations, and so limit divergence due to drift and natural selection. In contrast, there were high levels of allelic heterogeneity among the nine collections of C. lineolata (F _ST  = 0.523; P <0.001) and eight collections of B. hanleyi (F _ST  = 0.140; P <0.001). These data imply that for species which undergo direct development, local populations are effectively closed and evolve largely independent of one another. Received: 3 May 1996 / Accepted: 12 July 1996     

Protection of Genetic Diversity and Maintenance of Connectivity among Reef Corals within Marine Protected Areas

Abstract: High‐latitude coral reefs (HLRs) are potentially vulnerable marine ecosystems facing well‐documented threats to tropical reefs and exposure to suboptimal temperatures and insolation. In addition, because of their geographic isolation, HLRs may have poor or erratic larval connections to tropical reefs and a reduced genetic diversity and capacity to respond to environmental change. On Australia's east coast, a system of marine protected areas (MPAs) has been established with the aim of conserving HLRs in part by providing sources of colonizing larvae. To examine the effectiveness of existing MPAs as networks for dispersal, we compared genetic diversity within and among the HLRs in MPAs and between these HLRs and tropical reefs on the southern Great Barrier Reef (GBR). The 2 coral species best represented on Australian HLRs (the brooding Pocillopora damicornis and the broadcast‐spawning Goniastrea australensis) exhibited sharply contrasting patterns of diversity and connectedness. For P. damicornis, the 8‐locus genetic and genotypic diversity declined dramatically with increasing latitude (N_a= 3.6–1.2, H_e= 0.3–0.03, N_g:N = 0.87–0.06), although population structure was consistent with recruitment derived largely from sexual reproduction (G_o:G_e= 1.28–0.55). Genetic differentiation was high among the HLRs (F_ST[SD]= 0.32 [0.08], p < 0.05) and between the GBR and the HLRs (F_ST= 0.24 [0.06], p < 0.05), which indicates these temperate populations are effectively closed. In contrast for G. australensis, 9‐locus genetic diversity was more consistent across reefs (N_a= 4.2–3.9, H_e= 0.3–0.26, N_g:N = 1–0.61), and there was no differentiation among regions (F_ST= 0.00 [0.004], p > 0.05), which implies the HLRs and the southern GBR are strongly interconnected. Our results demonstrate that although the current MPAs appear to capture most of the genetic diversity present within the HLR systems for these 2 species, their sharply contrasting patterns of connectivity indicate some taxa, such as P. damicornis, will be more vulnerable than others, and this disparity will provide challenges for future management.     

High levels of gene flow in the surf bivalve Donax deltoides (Bivalvia: Donacidae) on the east coast of Australia

We surveyed patterns of allelic variation within twelve samples of the pipi Donax deltoides Lamarck from beaches separated by up to 1200 km but connected to varying degrees by the East Australian Current. We used these data to test the prediction that the irregular patterns of water movement would cause genetic differentiation in pipis, so that there would be more genetic variation within and among the more southern regions than the northern regions. We found that six loci were at least moderately variable within all samples, and there were no clear geographic patterns in allelic frequencies. In general, genotype frequencies within samples were consistent with predictions for an outcrossed, sexually reproducing species, and we detected no evidence of population subdivision. Within samples, with the exception of the peptidase loci, single-locus genotype frequencies were in close agreement with expectations for Hardy–Weinberg equilibrium. We observed no significant linkage disequilibrium for any pairwise comparison of loci in any sample. Our hierarchical analysis of genetic variation revealed little variation among all samples (F st = 0.009). Loci showed consistently low levels of subdivision (F st from 0.003 to 0.018). We found almost no variation among the four geographic regions sampled (F rt = 0.001). All variation was therefore attributable to variation among samples within regions (F sr = 0.010). These data imply that larvae are moving between regions and that levels of present or recent gene flow are high, and support the conclusions of other studies which have inferred widespread gene flow for animals dispersing via planktonic, outcrossed larvae in parts of this region. This implies that the East Australian Current is sufficient to produce strong larval connections despite its intermittent nature. If existing levels of population subdivision reflect current levels of gene flow, then these data imply that D. deltoides represents a single fishery on the east coast of Australia. Received: 16 September 1996 / Accepted: 25 September 1996     

Is Syllis gracilis (Polychaeta: Syllidae) a species complex? An allozyme perspective

The genetic relationships between morphologically indistinguishable marine and brackish populations of Syllis gracilis Grube, 1840 (Polychaeta: Syllidae) were studied by means of allozyme electrophoresis. Samples of S. gracilis from marine coastal and brackish-water habitats were examined for variation at 13 presumptive loci. In addition, a sample of the closely related species S. prolifera (Krohn, 1852) was analysed. Five loci were multiallelic in at least one population of S. gracilis and eight loci in S. prolifera. Low to moderate levels of within-population genetic variability were found, with average expected heterozygosity values ranging from H = 0.068 (±0.043 SE) to 0.187 (±0.069 SE) in the populations of S. gracilis; higher values were found in S. prolifera (H = 0.325 ± 0.076). The presence of various private alleles indicated a marked genetic divergence among populations of S. gracilis, with Nei's genetic distances ranging from D = 0.000 to 0.833 and a highly significant F _ST value. Furthermore, evidence for strong genetic heterogeneity between two sympatric marine populations was found. UPGMA cluster analysis and multidimensional scaling pointed out a clear genetic divergence between brackish and marine populations. At least two genetically divergent entities occurred in marine and brackish habitats. This could be due to local adaptation of individuals coming from marine populations to brackish habitats, but more presumably to the occurrence of a species complex within S. gracilis. Received: 6 June 1999 / Accepted: 7 February 2000     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Genetic evidence of the presence of two species of Crassostrea (Bivalvia: Ostreidae) on the coast of Brazil

 Although oysters are commercially very important in Brazil, there is still much dispute about the number of Crassostrea species occurring on the Brazilian coast. The dispute is centered around C. brasiliana, considered by some authors to be a junior synonym of C. rhizophorae. In this paper we compared, by allozyme electrophoresis, sympatric and allopatric populations of the two putative species. Of the 17 loci analysed, five were diagnostic for the two species in sympatry (gene identity = 0.46 to 0.47), clearly demonstrating that they are distinct biological species. Heterozygosity (h) levels were high for both species (h = 0.24 to 0.28), and no heterozygote deficiencies were observed in any population (local inbreeding, F _IS  = 0.141; P > 0.70). Levels of population structure in C. rhizophorae along 1300 km of coast were very low (population inbreeding, F _ST  = 0.026; P > 0.15), indicating that the planktonic, planktotrophic larvae of these species are capable of long-range dispersal. Received: 14 January 1999 / Accepted: 8 December 1999     

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.     

Historical demography and contemporary spatial genetic structure of an estuarine crab in the northeast Pacific (<Emphasis Type="Italic">Hemigrapsus oregonensis</Emphasis>)

Discrete estuary subpopulations of the mud crab Hemigrapsus oregonensis (Dana, 1851) are connected via larval dispersal. Sequence variation at the mtDNA COI locus was examined in eight populations sampled in 2001–2002 from central California through northern Oregon in the northeast Pacific (36.6–45.8°N) to infer patterns of dispersal and historical connectivity in the region. Strong evidence for persistence since the mid-Pleistocene, with no range truncation resulting from southward shifting temperature isoclines, was provided by a phylogeographic pattern of haplotypes of an older clade distributed throughout the sampled range. A recently derived clade became widespread only north of Cape Blanco after the last glacial maximum. Its clear pattern of restriction to the northern area, in the absence of similarly restricted southern clades, suggests that contemporary dispersal around Cape Blanco is rare (population F _ST = 0.192). Low pairwise differentiation within Oregon and within central California, as well as contrasts between northern and southern groups in the shape of the pairwise mismatch distribution, nucleotide diversity, and Tajima’s D suggest that these regions reflect different demographic histories. Potential mechanisms explaining this latitudinal break include contemporary coastal circulation patterns, selection, and ancient patterns of larval dispersal in the California Current.     

DNA sequence analysis identifies genetically distinguishable populations of harp seals (Pagophilus groenlandicus) in the Northwest and Northeast Atlantic

Harp seals (Pagophilus groenlandicus Erxleben, 1777) comprise three populations based upon whelping areas in the Greenland Sea, White Sea, and Northwest Atlantic. The last comprises two subpopulations, one whelping in the Gulf of St. Lawrence (“Gulf ”) and one on the pack ice of the southern Labrador/northern Newfoundland coastal shelf (“Front”). A total of 40 female seals from the four whelping areas were collected during the 1990 and 1992 whelping seasons. DNA sequence variation was examined in a 307 bp region of the mitochondrial cytochrome b gene. Eleven variable nucleotide positions defined 13 genotypes: a significant fraction of the genotypic variance (F _ST=0.12, or 0.09 as measured by Weir's coancestry coefficient θ) is attributable to differentiation between Northwest and Northeast Atlantic populations. There was no significant differentiation between the two whelping areas in the Northwest Atlantic, or between the Greenland Sea and White Sea. These findings suggest significant reproductive isolation exists between trans-Atlantic breeding populations. Received: 18 January 1999 / Accepted: 22 February 2000     

Genetic divergence and phylogenetic inferences in five species of Mugilidae (Pisces: Perciformes)

Allozyme electrophoresis was used to compare the genetic divergence of global populations of Mugil cephalus Linnaeus, 1758 and two congeneric [M.␣curema Cuvier & Valenciennes, 1836; M. gyrans (Jordan & Gilbert, 1885)] and two more distantly related [Liza ramada (Risso, 1826); Xenomugil thoburni (Jordan & Starks, 1896)] species on the basis of 18 enzyme loci. The amount of genetic divergence among the species examined is in agreement with their present systematic status, the differences being larger among non-congeneric species than among species of the same genus. Intraspecific genetic distances in M. cephalus (average Nei's D= 0.154), although higher than those reported for conspecific populations of fish, appear to be small when compared to the interspecific values among mugilid species (0.821 ≤ Nei's D ≤ 1.744). Phylogenetic trees obtained by genetic distance methods and discrete character parsimony analysis were of similar topology, except for the relationships within the genus Mugil and for the arrangement of M. cephalus populations. Received: 7 April 1997 / Accepted: 4 February 1998     

Population genetics of the Antarctic heteronemertean Parbolasia corrugatus from the South Orkney Islands

An allozyme survey, using starch-gel electrophoresis, was carried out on eight populations of the Antarctic nemertean worm Parborlasia corrugatus (McIntosh, 1876) collected from locations around the South Orkney Islands, Antarctica. These populations were separated by distances in the order of tens of kilometres. Genetic variation was estimated over 22 enzyme loci for all populations examined, giving an observed heterozygosity of 0.142. This was much lower than the expected heterozygosity (H _e  = 0.201), and it was found that there was a significant deficiency of het‐erozygotes across four enzyme loci ( p ≤ 0.01). A more detailed examination of this deficiency of heterozygotes was undertaken for the six populations and six variable enzyme loci for which the most complete data sets existed. A significant deficiency of heterozygotes was found at the enzyme locus Odh-1 for four of the six populations examined ( p ≤ 0.01). Mean F _is (0.240) indicated a significant ( p ≤ 0.01) within-population component of the heterozygote deficiency estimated for the six populations sampled, and this was mainly due to the␣Ap-1, Odh-1 and Pgm-1 loci. The mean F _st value (0.036) was also significant ( p ≤ 0.01), indicating a degree of genetic differentiation between populations. The observed levels of genetic differentiation between populations of P. corrugatus and the significant heterozygote deficiencies were unexpected, because this species has been reported to have a long-lived planktotrophic larva. It is hypothesised that recruitment of P. corrugatus in the South Orkney Islands originates from genetically distinct populations located in the Weddell Sea and to the west of the Antarctic Peninsula. Shifts in the relative position of the Weddell Sea Front, Weddell–Scotia Confluence and Scotia Front, relative to the South Orkney Islands, provide a mechanism for variation in the origin of recruits over time. Received: 24 July 1997 / Accepted: 31 October 1997     

Genetic protein variation in red mullet (Mullus barbatus) and striped red mullet (M. surmuletus) populations from the Mediterranean Sea

Starch-gel electrophoresis of allozymes was used to differentiate the two red mullet species (Mullus barbatus L. and M. surmuletus L.) in the Mediterranean Sea and, further, to investigate the genetic stock structure of M. barbatus in the eastern Mediterranean area. Twenty putative enzyme-coding loci were examined in eight M. barbatus samples caught in the Aegean and Ionian Seas (Greece) and in the Gulf of Lion (France), and two M. surmuletus samples caught in the Aegean and Gulf of Lion. A high degree of genetic polymorphism was found in both species. Species-specific electrophoretic patterns were found in PGI* and PGM*. Estimates of variance of allele frequencies among samples (F _ST) and ² analyses both revealed significant differences (P < 0.05) among the M. barbatus samples. Most of the genetic variation was among samples regardless of region. The mean value of Nei's genetic distance between the two species was 0.329. Genetic distance among M. barbatus samples was low (maximum Nei's D = 0.012), with the sample from Platania differing most from other M. barbatus samples. This is probably be due to founder effects existing at this area. These results suggest that allozyme analysis may provide important information on the genetic structure of the red mullet to ensure sustainable management of this species. Received: 7 May 1997 / Accepted: 13 October 1997     

Genetic differentiation in Hediste diversicolor (Polychaeta: Nereididae) for the North Sea and the Baltic Sea

Genetic differentiation between North Sea and Baltic Sea Hediste diversicolor (O.F. Müller, 1776) (Polychaeta: Nereididae) populations was studied by allozyme electrophoresis on starch gel. Thirteen loci were analyzed in eight populations. The level of genetic variation was very low (mean H _o = 0.000 to 0.015). Differentiation between H. diversicolor populations is quite high (F _ST = 0.892) and reflected by three enzyme loci (MDH-I*, MDH-II*, IDH-I*). The reduced gene flow (N _m<1) may be explained by the limited dispersal capacity of the species. Regardless of whether found in the North Sea or Baltic Sea, there appear to be two different genetic types which are parapatric or sympatric in some places. The two types hybridize at three localities, but no signs of hybridization have been found at one (Tallinn). Received: 26 June 1997 / Accepted: 10 September 1997     

High levels of genetic subdivision in peripherally isolated populations of the atherinid fish Craterocephalus capreoli in the Houtman Abrolhos Islands,Western Australia

The atherinid fish Craterocephalus capreoli Rendahl is abundant in the Houtman Abrolhos Islands, 70 km off the Western Australia coast and 250 km south of the southern limit of the range of the species along the mainland. Electrophoretic examination of 7 allozyme loci at 17 sites in the Houtman Abrolhos revealed a substantially lower level of polymorphism than found in an earlier study of the species in its mainland distribution, with many of the uncommon alleles and some common ones missing. There is a very high degree of genetic subdivision among the populations in the Houtman Abrolhos, measured by a mean F _ST of 0.437 over a distance of 35 km. This F _ST (standardized variance in allelic frequencies) is six times that found previously among populations along the mainland coast over distances up to 850 km. The subdivision of populations in the Houtman Abrolhos is similar within one island group on a scale up to 12 km, and between two groups that are separated by 15 km of deep water. Significant differences in allelic frequencies were found between populations from the open shore and enclosed lagoons less than 800 m apart, but the overall levels of subdivision were similar for the two types of environment. Previous work had shown high levels of genetic subdivision in the Houtman Abrolhos for a gastropod with direct development. The results for C. capreoli demonstrate that the archipelago favours subdivision even for a species with potentially much greater mobility and different life history.