Spread and potential impact of the recently introduced European green crab,Carcinus maenas,in central California

Our study examines the potential impact of the European green crab Carcinus maenas on communities of coastal embayments of western North America. We document the current distribution and range expansion of this species beyond San Francisco Bay, where C. maenas first became established along this coast in 1989–1990, and we test the effect of C. maenas predation on different species and sizes of infaunal invertebrates in field and laboratory experiments. In our samples from eight coastal locations in central California collected between June 1993 and May 1994, we found no green crabs at the two closest embayments south of San Francisco Bay and found the crabs in all four embayments sampled within 120 km north of San Francisco Bay, up to and including Bodega Harbor. C. maenas was not present in samples from sites farther north. This northward range expansion is apparently the result of larval recruitment by a single cohort, corresponding to the predominant northern transport of surface waters and the approximate distance water moves during larval green crab development. At Bodega Harbor, the current northern range limit, the C. maenas population is now well established and reproducing. Females and males became sexually mature within their first year at 40 mm carapace width, molting approximately monthly from summer through fall, and females were ovigerous in late fall of their first year at 50 mm. We expect larvae from this population to recruit locally and to the north, promoting episodic range extensions as new populations are established and reproduce. Enclosure experiments conducted during the summer of 1993 at the intertidal sandflats of Bodega Harbor showed that C. maenas significantly reduced densities of the most abundant taxa, including the bivalves Transennella confusa and T. tantilla, the cumacean Cumella vulgaris, and the amphipod Corophium sp. Furthermore, Carcinus maenas selectively removed larger (>3 mm) rather than smaller (<1 mm) Transennella spp. in both field and laboratory experiments. Based on the available data from this and other studies of green crabs, and our 10 yr study of community dynamics at Bodega Harbor, we predict C. maenas will significantly alter community structure, ecological interactions, and evolutionary processes in embayments of western North America.     

Invertebrate introductions in marine habitats: two species of hydromedusae (Cnidaria) native to the Black Sea,Maeotias inexspectata and Blackfordia virginica,invade San Francisco Bay

The hydrozoans Maeotias inexspectata Ostroumoff, 1896 and Blackfordia virginica Mayer, 1910, believed to be native to the Black Sea (i.e. Sarmatic) and resident in a variety of estuarine habitats worldwide, were found as introduced species in the Petaluma River and Napa River, California, in 1992 and 1993. These rivers are mostly-estuarine tributaries that flow into north San Francisco Bay. Both species appeared to be well-established in this brackishwater habitat. Salinities at the collection sites were about 11 during the summer, rising to nearly 20 in the early autumn and falling to near 0 in the winter. Large numbers of all sizes of both species of medusae were observed and collected, indicating that the hydroid stages of the life cycles of the two are also well-established in these rivers. In the Petaluma River, populations of both species were at maximum in late July, with numbers of individuals declining through August and into September; the Napa River was sampled only in October, and at that time only B. virginica was found. Examination of full guts of M. inexspectata and B. virginica medusae revealed that both species had fed nearly exclusively on small crustaceans, principally barnacle nauplii, copepods and their eggs and nauplii, and crab zoea larvae (M. inexspectata only). All the M. inexspectata medusae were males, indicating that the population has probably developed from the introduction of perhaps only a single male polyp or polyp bud. In spite of its inability to reproduce sexually, this population appears to be maintained by the prodigious ability of the polyp to bud and reproduce asexually, and is fully capable of invading additional low-salinity habitats from its present Petaluma River site. Male and female B. virginica medusae were collected in both the Petaluma River and the Napa River, indicating that B. virginica may have been introduced by either the polyp or medusa stage (or both), but that multiple individuals (of both sexes) must have arrived from another port in one or more invasions. As indicated for M. inexspectata, the B. virginica population will also probably seed new populations in San Francisco Bay and elsewhere. Based on its cnidome as well as the morphology of both medusa and polyp, M. inexspectata has been reclassified by moving it from the family Olindiidae, Limnomedusae, to the family Moerisiidae, Anthomedusae.     

Genetic population structure and gene flow among deep-sea amphipods,Abyssorchomene spp., from six California Continental Borderland basins

Studies of geographic population variation needed to estimate gene flow are lacking in deep-sea biology. Using allozyme electrophoresis, I have studied population-level geographic variation among scavenging lysianassoid amphipod populations (Abyssorchomene spp.) inhabiting deep-water basins of the Southern California Continental Borderland. Samples were collected from November 1987 to November 1990, using baited traps, from six basins whose bottom depths ranged from ca. 1000 to 2100 m. Five basins (San Diego Trough, Santa Catalina, San Nicolas, Santa Cruz, Tanner Basins) could be grouped together as shallow-sill basins, with physical conditions distinctly different from a single deepsill basin (San Clemente Basin). Amphipods tentatively identified asAbyssorchomene sp. 1 collected from the shallow-sill basins were morphologically discriminated from those collected in the San Clemente Basin, which were identified asAbyssorchomene sp. 2. Results from eight enzyme loci revealed significant genetic differentiation [Nei's genetic distance (D)>0.155] of deep-sill basin-dwellingAbyssorchomene sp. 2 vsAbyssorchomene sp. 1 from the shallow-sill basins and low levels of gene flow (migration rate, <1). Comparisons of benthic fauna suggest the presence of an abyssal-related assemblage in the deep-sill basin isolated from the northern shallow-sill basins. Genetic distances among the five shallow-sill basin populations ofAbyssorchomene sp. 1 were very low (D < 0.003). Estimates of gene flow among these populations were very high ( > 24 to 170) and suggested weak isolation by distance.     

Population genetics of Trochus niloticus and Tectus coerulescens,topshells with short-lived larvae

Trochus niloticus L. and Tectus coerulescens Lmk., two coral reef trochid gastropods that have similar life-histories including a lecithotrophic larval stage, were sampled from reefs in the northern, central and southern sections of the Great Barrier Reef (GBR) in 1991. Significant sex-ratio biases were noted, and these varied among reefs, apparently with latitude. Demographic data suggested that highly discontinuous and localized recruitment occurs. Surveys of allozyme frequencies at 12 loci revealed no significant genetic differences among populations of Trochus niloticus at any geographical scale. High gene flow between zones of the GBR was inferred, with the number of migrants per generation (N _m ) of the order of 100. Directional selection was thought to occur at one locus (GDH ^*). Genetic variability in T. niloticus (H=0.069 to 0.110, and only three loci polymorphic) was low compared with other trochids. This was thought to be due to smaller effective population size, resulting from an unbalanced sexratio, aggregative spatial distribution of adults, high variance in reproductive success, and/or the occurrence of population extinctions and recolonisations. In contrast, Tectus coerulescens exhibited significant genetic differences between zones, indicating a much lower rate of migration between populations (N _m 1 to 10), and displayed high genetic diversity (H=0.225 to 0.279). A trend for increasing genetic diversity from the northern to southern GBR was found in both species. It is not clear whether the occurrence of two contrasted population genetic structures in species with apparently similar life-histories is due to ecological or historical factors.     

Genetic variability in the cosmopolitan deep-water ophiuran Ophiomusium lymani

A population of the cosmopolitan deep-sea ophiuran Ophiomusium lymani was studied by gel electrophoresis, and proved to be highly variable genetically; about 53% of the 15 loci studied are polymorphic, and the average individual is heterozygous at about 17% of the loci. This is approximately the same genetic variability displayed by other species, belonging to other phyla or classes, from the same deep-sea trawl. A similarly high level of genetic variation occurs in deep-sea organisms in general, and in a shallow-water tropical species. Both the deep-sea and the tropics are trophically stable environments. On the other hand, low genetic variabilities have been found in marine species from trophically unstable environments. These data suggest that any phylogenetic effects on genetic variability are secondary, and that the trophic regime may be of major importance in determining genetic strategies of adaptation.This study is no. 5 of a series of reports on the results of Expedition Quagmire.     

Modèle de détection rapide des effets sublétaux des polluants. II. Un exemple in situ: anomalie métabolique du copépode hyponeustonique Anomalocera patersoni en coïncidence avec une marée noire

An unusual correlation has been observed between the amylase and trypsin content of a natural zooplankton population and of the hyponeustonic copepod Anomalocera patersoni and the oil spill of the Amoco Cadiz in 1978. This anomaly has also been observed in Artemia sp. cultures (San Francisco Bay strain) poisoned by copper. This phenomenon could originate from a general physiological perturbation. Such a non-specific perturbation could explain the anormal correlation between parameters which are usually independent and specifically regulated. The determination of such anomalous regulation could lead to detection of the sublethal effect of pollutants in the field. The method requires no prior knowledge of the area and can be applied for detection of a new perturbation; in fact, if organisms adapt to the new conditions, the anomaly disappears. The method has enabled physiological perturbations of zooplankton to be recorded 1 yr after the Amoco Cadiz oil spill. The hyponeustonic copepod Anomalocera patersoni was particularly perturbed, even at residual hydrocarbon concentrations usually considered by chemists as approaching normal values.     

Latitudinal trends in size-dependence of bioluminescence in the midshipman fish Porichthys notatus

The bioluminescent fish Porichthys notatus (plainfin midshipman), has a discontinuous distribution along the Pacific coast of North America. The fish is present from Cape Mendocino southward to Baja California, Mexico, absent off the coast of Oregon, USA, and abundant, northward, in Puget Sound, Washington. Interestingly, the population in Puget Sound lacks the substrate (luciferin) necessary for the luminescence reaction and, despite possessing an otherwise fully functional photophore system, is nonluminescent. The California population of P. notatus is uniformly luminescent south of Monterey Bay, but 15% of the speciments tested from San Francisco Bay and the Gulf of the Farallons have been reported to be nonluminescent. Explanations for nonluminescent midshipman in both Puget Sound and the San Francisco Bay area have focussed on a dietary requirement for luciferin. To gain further insight into reasons for nonluminescence in the San Francisco Bay region, the distribution of bioluminescence in P. notatus was studied from Monterey Bay to Cape Mendocino during 1985. A complex pattern of bioluminescence was found, in which nonluminescent individuals reflected neither a local anomaly in the San Francisco Bay region nor a simple gradient of decreasing luminescence towards the northern end of the range of the California population. Instead, a distinct size-dependent component in luminescence capability of the fish was observed. Aspects of the life history of P. notatus and related factors which might influence the bioluminescence characteristics of this population are discussed.Please address all correspondence to Dr. F.I. Tsuji at the Osaka Bioscience Institute     

Male bird song attracts females — a field experiment

Summary The initial stage of pair formation of the pied and the collared flycatcher (Ficedula hypoleuca and F. albicollis) was simulated in a field experiment. Male dummies positioned near nest boxes and singing by means of tape-recorded song from loudspeakers offered prospecting females a nest box combined with an automatic trap. An unequivocal demonstration of female arrival at the male's territory was obtained by the trapping of the female. Control nest-box traps were provided with silent dummies. Factors other than male song causing female attraction were controlled by a daily switch of the position of singing and silent dummies. Nine out of ten females were caught in nest-box traps with singing dummies (one-tailed binomial test, P=0.011). The result supports the hypothesis that male song functions in mate attraction. This hypothesis has never previously been tested in field experiments although circumstantial evidence for it has been available.     

Genetic differentiation of populations of the planktonic copepod Labidocera aestiva

Planktonic populations of the calanoid copepod Labidocera aestiva show significant biochemical genetic heterogeneity along the Atlantic coast of the USA. In summer, 1981, copepods were collected by surface tows at Beaufort Inlet, North Carolina; Fort Pierce Inlet, Florida; and Vineyard Sound, Massachusetts. Genetic variation within each population and genetic differentiation among the three populations were studied by micro-acrylamide gel electrophoresis of six loci encoding four enzymes. All six enzyme loci were polymorphic when all populations were considered together, but the North Carolina population was monomorphic at two loci. High genetic variability was indicated by the following: (1) the number of alleles per locus averaged over all loci was 2.57±0.26 SD; (2) the average proportion of loci for which the frequency of the most common allele was not greater than 0.95 was 0.78±0.10; (3) the frequency of heterozygous individuals was 0.25±0.07. Genetic differentiation among population samples in the three regions was demonstrated in several ways: allele frequencies at one aminopeptidase-I locus, Lap-1, differed significantly among samples of the three populations, and there were unique alleles of high frequency at this locus in two population samples. Values of the statistic of genetic distance (D) averaged 0.20±0.08 for pairwise comparisons between all samples. Compared to expected heterozygosity if individuals across the entire range sampled mated at random, there were highly significant heterozygote deficiencies at five of the six loci. Genetic differentiation of populations of L. aestiva may result from (1) differential selection on populations in the three regions, or (2) restricted gene flow between the populations. Gene flow may be limited by geographic separation or differences in life history, such as seasonal presence in the plankton and diapause egg production.Contribution No. 5810 of Woods Hole Oceanographic Institution     

Genetic affinities of the bivalve Macoma balthica from the Pacific coast of North America: evidence for recent introduction and historical distribution

The tellinid bivalve Macoma balthica (L.) has an extensive geographic range that reaches from temperate to arctic coastal waters in the North Atlantic and North Pacific oceans. Recent studies have indicated that eastern and western North Atlantic populations are morphologically and genetically different from one another, and that they may have diverged as sibling species. To determine the genetic relationship between M. balthica from the Pacific and Atlantic coasts of North America, populations from each coast were examined at 11 enzyme loci using standard starch gel electrophoresis. Allele frequency data indicate that M. balthica populations from San Francisco Bay, California appear more closely related to western North Atlantic populations than to populations from Oregon. We suggest that San Francisco Bay populations were introduced relatively recently from western North Atlantic populations. The Oregon populations are probably a natural extension of northern populations that occur along Northern Asia and in the eastern North Atlantic.     

Glucosephosphate isomerase genotype frequencies,homozygous excess and size relationships in the sand flounder Rhombosolea plebeia

Genetic variation was studied at two polymorphic loci for 3 yr (November 1978–February 1982) in juvenile sand flounder (Rhombosolea plebeia) in Wellington Harbour, New Zealand. The Gpi-1 locus showed a heterogeneity in genotype frequencies, but this was not clearly related to season or to fish length. At the Gpi-2 locus, genotype frequencies were dependent upon length and showed a significant excess of homozygotes in juveniles but not in adults. This latter phenomenon has been reported previously in several species of marine molluscs, and so may be common to marine organisms producing large numbers of pelagic larvae. Reasons for the excess of homozygotes are discussed, and it is suggested that assortative mating is the most likely explanation and that this is an area of marine population genetic research which has been over-looked.     

Genetic evidence that the “Padstow mussel” is Mytilus galloprovincialis

Electrophoretic analysis of loci controlling a variety of enzymes has been applied to samples of the Padstow mussel and typical Mytilus edulis L. living strictly sympatrically at Rock, Cornwall, England, in order to resolve the disputed status of the Padstow mussel. Small samples indicated similar monomorphic states at the GPDH, TO, MDH-2 loci and weak polymorphism at the 6 PGD locus in both types of mussel. The MDH-1 locus may be weakly polymorphic in the Padstow mussel and monomorphic in M. edulis. Large samples assayed for AP, LAP, PGM, and PHI produced data showing very large and highly significant differences in allele frequencies at three of these loci between the two groups of mussels separated on anatomical characters. At the LAP locus, significant deficiencies of heterozygotes were observed in both groups of mussels. A small percentage of mussels from Rock are difficult to assign with certainty to one or other group on anatomical and morphological criteria, but the genetic evidence indicates that most, if not all, such specimens are M. edulis. These genetic differences make it highly improbable that any significant degree of genetic exchange occurs between the two groups in nature and, taken together with the evidence of genetic resemblance of the Padstow mussel to M. galloprovincialis from Venice and that of Rock M. edulis to M. edulis, from the Gower peninsula of Wales we conclude that the Padstow mussel is indeed M. galloprovincialis.     

Genetic studies in marine fishes

Nine polymorphic loci were found among 42 presumptive protein-coding gene loci surveyed among 474 red drum (Sciaenops ocellatus) sampled in 1987 from 13 nearshore and 1 offshore localities from the Atlantic coast of the southeastern USA and the northern Gulf of Mexico. The mean number of alleles over the polymorphic loci was 3.8, and the average heterozygosity over all loci examined was estimated as 0.047. These data indicate that red drum have normal levels of genetic variability. Wright'sF1 _ST values (the standardized variance of allele frequencies between samples) over all polymorphic loci ranged from 0.009 to 0.027 (meanF1 _ST =0.019), and estimates of the effective number of migrants (N _e m) per generation using Wright's island model ranged from 9.0 to 27.5. High levels of gene flow among the red drum samples were also indicated by Slatkin's qualitative analysis using conditional average allele frequencies. Nei's estimates of genetic distance between pairs of samples ranged from 0.000 to 0.009, indicating a high degree of nuclear gene similarity among all samples. Highly significant heterogeneity in allele frequencies at the locus for adenosine deaminase was detected between red drum sampled from the Atlantic and those sampled from the Gulf and among red drum sampled from the Gulf.     

Introduction,dispersal and potential impacts of the green crab Carcinus maenas in San Francisco Bay,California

The North Atlantic portunid crab Carcinus maenas (Linnaeus, 1758) has invaded the North Pacific Ocean following more than two centuries of global dispersal due to human activities. C. maenas was first collected in San Francisco Bay, California, in 1989–1990, where its distribution and prey selectivity were investigated in 1992–1994. It has become abundant in shallow, warm lagoons (which as favorable and retentive microhabitats may have served as invasion incubators) and spread throughout the north, central and south bays. It may have arrived in ballast water, on fouled ships, amongst algae with imported live bait or lobsters, or by intentional release; genetic comparisons of the Bay population with possible source populations may aid in defining the transport mechanism. C. maenas' eurytopic nature, its high breeding potential, and its diet and feeding behavior suggest the potential for extensive ecosystem alterations through predator-prey interactions, competition, disturbance, and indirect effects. Although both negative economic impacts through reduction or disruption of fisheries and positive impacts of providing bait and human-food fisheries have been documented in a few regions, the potential economic impacts in San Francisco Bay remain largely unknown.     

Allozyme and colour differences between two sibling species of the heteronemertean Lineus torquatus from the Sea of Japan

Brown and cherry colour morphs of the heteronemertean Lineus torquatus Coe, 1901 from the Vostok Bay of the Sea of Japan were compared using allele frequencies at 18 isozyme loci. These morphs shared no common alleles at 9 of the 18 loci. The observed genetic differences are strong evidence that the brown and cherry colour morphs, which occur sympatrically, are different species. Nei's genetic distance between these species (D=1.135) is considerably higher than typical D values for congeneric animal species. As with much previous work on nemerteans, intraspecific allozymic variability in both species was high, with mean expected heterozygosity values 0.286±0.049 and 0.159±0.051 for L. torquatus (brown) and L. torquatus (cherry) species, respectively.     

Relationship between lead concentrations in seawater and in the mussel Mytilus edulis: a water-quality criterion

Regression analysis on data collected from Port Phillip Bay and Western Port, Australia in 1979 shows that there is a significant equilibrium relationship between total recoverable lead in seawater and its concentrations in the mussel Mytilus edulis (P<0.001). The concentration of lead in seawater should not exceed 1.27 g l^-1 if the mussel is not to reach a lead concentration of 2.5 mg kg^-1 wet weight, a value frequently used as a food standard for human consumption. When a lead value of 2.5 mg kg^-1 wet weight is reached, the concentration factor by mussels for lead from seawater is 1969. The critical value of 1.27 g l^-1 could be used as a marine water-quality criterion for lead in waters where mussels are harvested.     

Owner and floater red-winged blackbirds: determinants of status

Summary We used sequential removal experiments to test whether the resource-holding potential (RHP) of territory owner red-winged blackbirds, Agelaius phoeniceus, was superior to that of their first replacements (shallow floaters) and subsequent replacements (deep floaters). Among the removals were secondyear males, which were morphologically inferior to adults and which also tended to be competitively inferior in aviary contests. The highest proportion of secondyear males occurred in the deep floater class. Thus, the RHP of some deep floaters was inferior to that of owners and shallow floaters. However, among adults, owners, shallow floaters, and deep floaters had equivalent morphological and competitive RHP. Furthermore, replacement males that had defended territories for many days were neither morphologically nor competitively superior to males that had defended territories for only a few days. Our results suggest that RHP distinguishes adults from second-year males, but does not separate owners from floaters. The only hypothesis that is potentially supported by our observations is that owners have a greater expected payoff from their territory than intruders, and in this way owners are able to maintain site dominance. The nature of that payoff remains to be determined. Offprint requests to: P.J. Weatherhead     

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 g g^-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.     

Genetic subdivision within the eastern Australian population of the sea anemoneActinia tenebrosa

An electrophoretic survey of allozyme variation revealed substantial genetic differentiation within the eastern Australian population ofActinia tenebrosa. This differentiation appears to reflect the effects of both asexual reproduction and limited gene flow among local populations separated by up to 1050 km. Variation was assessed within groups of 27 to 55 adults sampled between September 1985 and December 1988 collected from small areas of shore within each of 24 local populations. All individuals were collected from stable rock platforms, with the exception of Boulder Bay, where some sea anemones were removed from small mobile boulders. High levels of variability were detected for each of seven enzyme-encoding loci. The patterns of genotypic variation detected imply that local populations are maintained by predominantly asexually generated recruitment. Levels of multi-locus genotypic diversity within samples were consistently less than 50% of the level expected for sexual reproduction with free recombination. This was reflected by the detection of relatively low numbers of multi-locus genotypes and significant departures from expectations for single-locus Hardy-Weinberg equilibria within 17 of the 24 local populations. Standardised genetic variances (F _ST ), calculated from the genotypes of all individual adults were typically much greater than those expected for marine organisms with widely dispersed larvae. The former values were reduced, but were still extremely large when clonal genotype frequencies were substituted into the calculation. These data imply that although widely dispersed larvae may be an important source of initial colonists, levels of gene flow among established local populations are low. Furthermore, cluster analysis revealed a clear subdivision of the population into northern and southern groups. However, this subdivision was largely explained by strong clinal variation at a GPI-encoding locus. For this locus, allele frequencies ranged from fixation of the A allele in samples from the 12 most northern sites to near fixation of the alternative B allele in southern samples. Subdivision of the eastern Australian population is consistent with the predicted off-shore movement of the Eastern Australian Current close to the border between Victoria and New South Wales. However, the split into northern and southern regions, as evidenced by the variation forGpi, could reflect patterns of gene flow and/or other factors such as natural selection or the recent patterns of colonisation.Contribution No. 78 from the Ecology and Genetics Group of the University of Wollongong     

Nuclear and mitochondrial DNA markers indicate unidirectional gene flow of Indo-Pacific to Atlantic bigeye tuna (<Emphasis Type="Italic">Thunnus obesus</Emphasis>) populations,and their admixture off southern Africa

A sharp genetic break separates Atlantic from Indo-Pacific bigeye tuna (Thunnus obesus) populations, as the frequencies of two major mitochondrial (mt) DNA types ( and ) found in this species are different across the tip of southern Africa. The level of nucleotide divergence between mtDNA types and is of the same order as that between reproductively isolated taxa. To further investigate the genetic structure of bigeye tuna over its distribution range and in the contact zone off southern Africa, bigeye tuna samples collected between 1992 and 2001 (including samples from a previous mtDNA survey) were characterized for four nuclear DNA loci and for mtDNA. Nuclear markers did not support the hypothesis that and mitochondria characterize sibling species. Significant allele-frequency differences at one intronic locus (GH2) and one microsatellite locus (µ208) were found between Atlantic and Indo-Pacific samples, although the level of nuclear genetic differentiation (Weir and Cockerhams =0.025 to 0.042) was much lower than in mtDNA ( =0.664 to 0.807). Probabilistic Bayesian assignment of individuals to a population confirmed that southern African bigeye tuna samples represent a simple mixture of individuals from Atlantic and Indian stocks that do not interbreed, with a higher contribution from Indian Ocean individuals (about 2/3 vs. 1/3).Communicated by M.S. Johnson, Crawley