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

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

A spatial gradient in the potential reproductive output of the sea mussel <Emphasis Type="Italic">Mytilus californianus</Emphasis>

Spatial variation in reproductive output from different populations within a region could have important consequences for recruitment, and cascading effects on populations and communities of marine species, but is rarely examined over meso-scales (i.e., tens to hundreds of kilometers). In this study, reproduction in the dominant mid-intertidal mussel, Mytilus californianus, was examined from sites spanning Point Conception, California over a 6-month period (March–August 2000). There was a dramatic geographic pattern in the relationship between size and potential reproductive output that was qualitatively similar across all 6 months sampled. Increases in allocation to reproductive tissue with increasing body size occurred at all sites, but the slope nearly doubled at sites south of Point Conception compared to northern sites. The spatial variation in size-specific reproductive output, coupled with additional spatial gradients in mussel density and size distributions, combined to increase total reproductive output by over eightfold at southern relative to northern sites. This study highlights the need to explicitly examine spatial patterns of reproductive output at these meso-scales, in order to better understand connectivity and source–sink dynamics in marine systems.     

Genetic heterogeneity among adult and recruit red sea urchins, Strongylocentrotus franciscanus

Allozyme electrophoresis was used to characterize genetic variation within and among natural populations of the red sea urchin Strongylocentrotus franciscanus. In 1995 to 1996, adult urchins were sampled from twelve geographically separated populations, seven from northern California and five from southern California (including Santa Rosa Island). Significant population heterogeneity in allelic frequencies was observed at five of six polymorphic loci. No geographic pattern of differentiation was evident; neighboring populations were often more genetically differentiated than distant populations. Northern and southern populations were not consistently distinguishable at any of the six loci. In order to assess within-population genetic variation and patterns of recruitment, large samples were collected from several northern California populations in 1996 and 1997, and were divided into three size classes, roughly representing large adults (>60 mm), medium-sized individuals (31 to 60 mm, “subadults”) and individuals <2 yr of age (≤30 mm test diam, referred to as “recruits”). Comparisons of allelic counts revealed significant spatial and temporal differentiation among size-stratified population samples. Recruit samples differed significantly from adult samples collected at the same locale, and showed extensive between-year variation. Genetic differentiation among recruit samples was much higher in 1997 than in 1996. Between-year differences within populations were always greater for recruits than for adults. Potential explanations for the differentiation of recruit samples include pre- and post-settlement natural selection and high interfamily variance in reproductive success or “sweepstakes” recruitment. Unless recruit differentiation can be attributed to an improbable combination of strong and spatially diverse selection, such differentiation across northern California populations indicates that the larval pool is not well mixed geographically (even on spatial scales <20 km), despite long planktonic larval duration. Received: 6 July 1999 / Accepted: 25 January 2000     

MtDNA population structure and gene flow in lingcod (<Emphasis Type="Italic">Ophiodon elongatus</Emphasis>): limited connectivity despite long-lived pelagic larvae

Lingcod, Ophiodon elongatus Girard, have a 3-month pelagic larval stage and are an important recreational and commercial species on the west coast of North America. Cytochrome-c oxidase I sequences from tissue samples were used to characterize population structure and infer patterns of gene flow from California to Alaska. No significant genetic structure was found when estimates of Wright’s F _ST (i.e., Φ_ST) were generated among all populations sampled. Nesting populations within regions, however, indicated that the inner coast of Washington State is distinct, a result corroborating previous allozyme work. Coalescent-based estimates of gene flow indicate that although migration can be high from an evolutionary perspective, nearly half of all comparisons among populations showed no gene flow in at least one direction. From an ecological perspective, moderate migration rates (Nm < 10) among most populations provide surprisingly limited connectivity at large (∼ 1,000 km) and small (∼100 km) spatial scales. Coalescent-based estimates also show that gene flow between the inner and the outer coasts is asymmetric, a result consistent with prevailing surface currents. Because the expected inter-locus variances for coalescent-based estimates of gene flow are likely large, future work will benefit from analyses of nuclear DNA markers. However, limited demographic connectivity on large spatial scales may help explain why stock recovery has been uneven, with greater recovery in the northern (87% rebuilt) than in the southern (24% rebuilt) fishery region, supporting a regional management strategy. These results suggest that despite a 3-month pelagic larval stage, some areas may be effectively closed with respect to both population dynamics and fishery management issues.     

Frequency of sublethal injury in a deepwater ophiuroid, <Emphasis Type="Italic">Ophiacantha bidentata</Emphasis>, an important component of western Atlantic <Emphasis Type="Italic">Lophelia</Emphasis> reef communities

The occurrence and relative abundance of tissue (arm) regeneration in the ophiuroid, Ophiacantha bidentata (Retzius), was examined in individuals collected primarily among colonies of the deep-water coral Lophelia pertusa off the southeastern United States. Seven deep-water coral sites (384–756 m), located between Cape Lookout, NC, and Cape Canaveral, FL, were sampled in June 2004 using a manned submersible. The presence of regenerative tissue was evaluated by visual inspection of each individual ophiuroid, and the proportion of regenerating arms per individual was examined relative to size of individual, geographic location, and depth of collection. Ophiacantha bidentata, the dominant brittle star collected, commonly displayed signs of sublethal injury with over 60% of individuals displaying some evidence of regeneration. These levels of regeneration rival those reported for shallow-water ophiuroids. Larger individuals (>6.5 mm disc size) had a higher incidence of regeneration than smaller individuals. Size of individual and percent of regeneration were negatively correlated with depth. Although O. bidentata was significantly less abundant in southern versus northern sites, ophiuroid abundance did not appear to be influenced by amount or density of coral substratum. Presence of dense aggregations of O. bidentata indicates that they are an important component of the invertebrate assemblage associated with deep-water coral habitat especially in the northern part of the study area. Assuming that observed frequencies of injury and subsequent regeneration represent predation events then dense ophiuroid aggregations in deep-water coral habitats represent an important renewable trophic resource within these communities.     

Post-glacial population history and genetic structure of the northern clingfish (Gobbiesox maeandricus), revealed from mtDNA analysis

Mitochondrial d-loop sequences were analyzed to characterize the phylogeographic and population genetic structure of the northern clingfish (Gobbiesox maeandricus Girard). Sequence analysis of 378 bp from 111 individuals sampled in 14 localities along the northeast Pacific coast and within the Strait of Georgia from 1996 to 1999 revealed marked genetic differentiation (Φ_ct=0.247) among regional population groupings. The gene genealogy distinguished two major clades of haplotypes separated by at least 1.1% sequence divergence. One clade with very low haplotype diversity (h=0.2095, n=18) occurred only within the recently unglaciated Strait of Georgia. The other clade had high haplotype diversity (h=0.8808, n=93) and was found in all populations. High haplotype diversity was found in open coastal populations, both north and south of the maximum extant of the Wisconsin ice sheet, suggesting that the clingfish range was not pushed to a southern refugium during the last glacial maximum. A nested clade analysis also did not detect a large northward expansion from a single southern refugium. The level of sequence divergence and coalescent-based analyses suggest that the observed patterns of polymorphism are the result of Pleistocene diversification within multiple refugia, followed by population expansion and asymmetrical lineage introgression. Received: 5 February 2000 / Accepted: 31 August 2000     

Anguilliform larvae collected off North Carolina

The distinctive larval stage of eels (leptocephalus) facilitates dispersal through prolonged life in the open ocean. Leptocephali are abundant and diverse off North Carolina, yet data on distributions and biology are lacking. The water column (from surface to 1,293 m) was sampled in or near the Gulf Stream off Cape Hatteras, Cape Lookout, and Cape Fear, North Carolina during summer through fall of 1999–2005, and leptocephali were collected by neuston net, plankton net, Tucker trawl, and dip net. Additional samples were collected nearly monthly from a transect across southern Onslow Bay, North Carolina (from surface to 91 m) from April 2000 to December 2001 by bongo and neuston nets, Methot frame trawl, and Tucker trawl. Overall, 584 tows were completed, and 224 of these yielded larval eels. The 1,295 eel leptocephali collected (combining all methods and areas) represented at least 63 species (nine families). Thirteen species were not known previously from the area. Dominant families for all areas were Congridae (44% of individuals, 11 species), Ophichthidae (30% of individuals, 27 species), and Muraenidae (22% of individuals, ten species). Nine taxa accounted for 70% of the overall leptocephalus catches (in order of decreasing abundance): Paraconger caudilimbatus (Poey), Gymnothorax ocellatus Agassiz complex, Ariosoma balearicum (Delaroche), Ophichthus gomesii (Castelnau), Callechelys muraena Jordan and Evermann, Letharchus aliculatus McCosker, Rhynchoconger flavus (Goode and Bean), Ophichthus cruentifer (Goode and Bean), Rhynchoconger gracilior (Ginsburg). The top three species represented 52% of the total eel larvae collected. Most leptocephali were collected at night (79%) and at depths > 45 m. Eighty percent of the eels collected in discrete depth Tucker trawls at night ranged from mean depths of 59–353 m. A substantial number (38% of discrete depth sample total) of larval eels were also collected at the surface (neuston net) at night. Daytime leptocephalus distributions were less clear partly due to low catches and lower Tucker trawl sampling effort. While net avoidance may account for some of the low daytime catches, an alternative explanation is that many species of larval eels occur during the day at depths > 350 m. Larvae of 21 taxa of typically shallow water eels were collected at depths > 350 m, but additional discrete depth diel sampling is needed to resolve leptocephalus vertical distributions. The North Carolina adult eel fauna (estuary to at least 2,000 m) consists of 51 species, 41% of which were represented in these collections. Many species of leptocephali collected are not yet known to have juveniles or adults established in the South Atlantic Bight or north of Cape Hatteras. Despite Gulf Stream transport and a prolonged larval stage, many of these eel leptocephali may not contribute to their respective populations.     

Evaluating risks associated with transport of the ghost shrimp <Emphasis Type="Italic">Neotrypaea californiensis</Emphasis> as live bait

The ghost shrimp Neotrypaea californiensis is imported into southern California from Oregon and Washington for use as live bait in recreational marine fisheries. We studied the population genetic structure of N. californiensis across much its range to assess the possibility that the transport of ghost shrimp across phylogeographic boundaries poses a risk of homogenizing existing genetic variation in the species. Analyses of two mitochondrial DNA markers showed little phylogeographic structure across the sampled range, suggesting that this risk is low. Unexpectedly, mitochondrial DNA analyses revealed that a second putative species of ghost shrimp frequently coexisted with N. californiensis in southern California intertidal habitats; almost all previous studies of soft-sediment communities in the region report the presence of N. californiensis only. We also assessed the possibility that the import of ghost shrimp might pose a risk of introduction of a parasitic castrator, the bopyrid isopod Ione cornuta, into southern California waters, where it does not appear to be native. Prevalence of living I. cornuta in samples purchased from bait shops was high (5.8%), suggesting that this is a real risk that merits further study.     

Genetic structure and phylogeography of the lined shore crab, Pachygrapsus crassipes, along the northeastern and western Pacific coasts

Marine invertebrates with high larval dispersal capacity typically exhibit low degrees of population differentiation, which reflects both contemporary and historical processes. We sampled 346 individuals from seven populations of the lined shore crab, Pachygrapsus crassipes Randall, along the northeastern Pacific Coast and Korea during summer 2003. DNA sequence analysis of 613 bp of the mitochondrial COI gene showed that overall gene diversity (h) was high (0.92±0.01), whereas overall nucleotide diversity (π) was low (0.009±0.005). A total of 154 mtDNA haplotypes were identified; however, 114 were present in only one individual. Analysis of molecular variance revealed significant genetic structuring at Point Conception, CA, USA, that is likely due to the oceanographic circulation patterns, which result in asymmetrical migration of haplotypes. However, genetic variation among eastern Pacific populations was generally low, probably because of high contemporary gene flow and recent common ancestry of haplotypes. Mismatch analysis and nested clade analysis suggested that the population history of this region is characterized by two contiguous northwards range expansions, which are congruent with Late Pleistocene glacial cycles. Highly significant genetic differentiation was detected between eastern Pacific populations and Korea, indicating transpacific gene flow is restricted. Time of divergence between the two transpacific lineages was estimated between 0.8 and 1.2 Myrs ago. The small, recently founded population of P. crassipes at Bamfield, BC, Canada, did not appear to have undergone a founder effect.     

Population structure,northern range limit,and recruitment variation in the intertidal limpet Lottia scabra

The northern range limit of the intertidal limpet Lottia scabra is Cape Arago, Oregon (43°N), where adult survival is excellent, the population is small (<300), and recruitment is low; the range limit may be set by limited recruitment. Between June 2012 and March 2013, 25 sites from the middle of the species range (33°N) to Cape Arago were sampled and population size frequency distributions, densities, and nearest neighbor distances were compared to the amount of rocky and sandy shore and kelp bed size. North and south of 37°N, the densities of new recruits averaged 22 and 86 m^?2, respectively. This shift was associated with the range limit of Macrocystis pyrifera kelp beds; we hypothesize that slower currents in M. pyrifera beds may limit larval dispersal leading to higher recruitment. North and south of 40°N, adult density averaged <1 and 458 m^?2, respectively, with the species absent from many sites to the north. This shift was associated with a sharp drop in the amount of rocky shoreline and an increase in uninhabitable sandy shore. Near the northern range limit, >80 % of the individuals were solitary and may be unable to spawn successfully. Recruitment at Cape Arago was infrequent and likely due to self-recruitment. This study suggests that the range limit was set by the absence of M. pyrifera and too little rocky shore leading to high larval wastage, low settlement, low population densities, and, due to an Allee effect, very small effective population sizes.     

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     

Geographical subdivision,demographic history and gene flow in two sympatric species of intertidal snails, <Emphasis Type="Italic">Nerita scabricosta</Emphasis> and <Emphasis Type="Italic">Nerita funiculata</Emphasis>, from the tropical eastern Pacific

The patchy distribution of rocky intertidal communities in the tropical eastern Pacific (TEP) may impose severe constraints on the genetic connectivity among populations of marine invertebrates associated with this habitat. In this study, we analyzed a portion of the mitochondrial cytochrome c oxidase subunit I (COI) gene in two sympatric species of marine snails, Nerita scabricosta and Nerita funiculata, common inhabitants of the rocky intertidal from the Gulf of California (Sea of Cortez) and outer Pacific coast of the southern Baja California (Baja) peninsula to northern South America, to assess genetic connectivity among populations of each species. One of our aims was to determine whether the morphological, behavioral, and ecological differences observed among populations of both species throughout their range in the TEP corresponded to population genetic differences. In addition, we were interested in elucidating the demographic history of both species. We found no evidence of genetic structure throughout the Gulf of California and outer coast of the Baja peninsula region for either species. Comparisons between Gulf of California/Baja and Panama populations, however, showed significant genetic differentiation for N. scabricosta, but not for N. funiculata. The genetic differences between Mexican and Panamanian populations of N. scabricosta were consistent with previously reported ecological and behavioral differences for this species between these two distant regions. However, previously reported size differences between northern and central/southern Gulf of California individuals of N. scabricosta do not correspond with our findings of genetic connectivity among these populations. Results from neutrality tests (Tajima’s D and Fu’s F _S), the mismatch distribution, and Bayesian skyline analyses suggested that both species have experienced dramatic population expansions dating to the Pleistocene.     

Temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid corals

Acroporid corals are the main reef-building corals that provide three-dimensional habitats for other reef organisms, but are decreasing on many reefs worldwide due to natural and anthropogenic disturbances. In this study, temporal patterns of larval settlement and survivorship of two broadcast-spawning acroporid coral species, Acropora muricata and A. valida, were examined through laboratory rearing experiments to better understand the potential for larval dispersal of this important coral group. Many larvae were attached (but not metamorphosed) to settlement tiles on the first examination 3–4 days after spawning (AS). The first permanent larval settlement (i.e. metamorphosed and permanently settled juvenile polyps) occurred at 5–6 days AS, and most larval settlement (85–97% of total) occurred within 9–10 days AS. Larval survivorship decreased substantially to around 50% by the first week of the experiment and to approximately 10% by the second to third week. The rates of larval attachment, settlement, and the initial drop in survivorship of larvae suggest that effective dispersal of some acroporid species may largely be completed within the first few weeks AS.     

Population genetic structure of the neon damselfish (<Emphasis Type="Italic">Pomacentrus coelestis</Emphasis>) in the northwestern Pacific Ocean

The population genetic structure of the neon damselfish (Pomacentrus coelestis) in the northwestern Pacific Ocean was revealed by the hypervariable control region of the mitochondrial gene (343 bp). In total, 170 individuals were sampled from 8 localities distributed between Taiwan and Japan, and 71 haplotypes were obtained through sequence alignment. High haplotype diversity (h = 0.956 ± 0.008) with low nucleotide diversity (π = 0.010 ± 0.006) was observed, and the results of the mismatch distribution test suggested that a historical population expansion after a period of population bottleneck might have occurred among P. coelestis populations. Based on the results of the UPGMA tree and AMOVA (Φct = 0.193, P < 0.05) analyses, fish populations from eight localities could be divided into two groups: one includes populations from localities around mainland Japan, and the other includes those from Okinawa and southern Taiwan. A genetic break was found between populations from mainland Japan and Okinawa, and this break was congruent with the pattern of phenotypic variations documented in previous studies. This evidence supports the latitudinal variation of reproductive traits among P. coelestis populations likely being genetically based. It is suggested that the changes in sea level and sea surface temperatures during past glaciations might have resulted in population bottlenecks in P. coelestis and the modern populations in the northern West Pacific are likely the results of recolonization after such events. The Kuroshio Current acts not only as a vehicle for larval transport along its pathway (between populations in southern Taiwan and Okinawa) but also as a barrier for larval dispersal across the Kuroshio axis (between populations in mainland Japan and Okinawa). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Contrasting population genetic structures of sympatric, mass-spawning Caribbean corals

Coral reef conservation management policy often focuses on larval retention and recruitment of marine fish with scant data available on important, less motile reef-building species such as corals. To evaluate the concept of population connectivity in corals, we tested whether broadcast spawning reproduction per se confers the same degree of dispersal to two sister species, Montastraea annularis (Anthozoa: Scleractinia; Ellis and Solander 1786) and M. faveolata (Ellis and Solander 1786), both dominant taxa in reefs of the northern Caribbean. Genetic analyses of ten nuclear DNA loci (seven microsatellite and three single-copy RFLP) reveal strikingly different patterns of population genetic subdivision for these closely related, sympatric species, in spite of likely identical dispersal abilities. Strong population genetic structure typified the architecture of M. annularis, whereas M. faveolata populations were principally genetically well mixed. A higher level of clonality was observed in M. annularis potentially because of a susceptibility to physical fragmentation. Clonality did not, however, significantly contribute to population genetic structure or low-level Hardy–Weinberg and linkage disequilibria observed in some populations. The lack of consistent association between reproductive mode and dispersal reinforces the perspective that population connectivity is not so much a function of predictable marine population source and sink relationships as is due to a more complex interface of oceanic currents interacting with and amplifying stochastic fluctuations in larval supply and settlement success. Our results support others promoting an overall ecosystem approach in marine protected area design.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

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     

Hybridization between the blue mussels Mytilus galloprovincialis and M. trossulus along the Pacific coast of North America: evidence for limited introgression

Two species of blue mussel, Mytilus galloprovincialis and M. trossulus, co-occur and hybridize along the Pacific coast of North America. Using a set of polymerase chain-reaction (PCR)-based genetic markers which diagnostically identify these species, we show that they are sympatric from the Cape Mendocino region to the Monterey Peninsula in northern and central California, USA. Mussels with hybrid genotypes were detected in all populations sampled in the region of sympatry, and the frequency of hybrid genotypes in individual hybrid populations ranged from 13 to 44%. Significant frequencies of first-generation backcross genotypes were detected in two individual hybrid zone populations (Berkeley and Monterey Marina) and in the hybrid zone as a whole, indicating that the potential exists for introgression between M. galloprovincialis and M. trossulus. Despite this potential, we found no evidence of advanced introgression beyond first-generation backcrosses, suggesting that gene flow between M. galloprovincialis and M. trossulus has been quite limited. The frequency of mussels with M. trossulus and hybrid genotypes declined abruptly south of Monterey Peninsula, while the frequency of mussels with M. galloprovincialis and hybrid genotypes declined precipitously north of Cape Mendocino. These abrupt genetic discontinuities indicate that this blue mussel hybrid zone is presently positioned between two prominent coastal features and there is little, if any, export of alleles from the hybrid zone into bordering parental populations. Received: 20 August 1997 / Accepted: 26 October 1998     

Movements of <Emphasis Type="Italic">Ilyanassa obsoleta</Emphasis> (Gastropoda) on an intertidal sandflat

The gastropod Ilyanassa obsoleta (Say) is native to the east coast of North America where it is locally abundant on sandflats, mudflats, and in saltmarsh creeks. The local disturbances created by snails and their movements affect soft-sediment community composition. Movements of individually marked snails were followed on an intertidal sandflat on Cape Henlopen, Delaware, U.S.A. In June 1991, 1,200 snails that had tested as trematode-uninfected were released and over 5 months 554 were sighted 971 times. Mean daily net distance moved was 1.7 m, but snails often moved 10–20 m day⁻¹ and one snail was 180 m distant after 130 days. Net dispersal of the released population was attained in ≈10 days, by which time, a typical distance from release was 15–20 m. Snails were not found crossing sandbars and most moved away from shore into a tidal gully. In June 1993, 500 snails, both uninfected and trematode-infected, were released at the same position and over 6 months, 350 snails were sighted 949 times. Sandbars were again barriers to movement, but their changed positions allowed wider dispersal. Net dispersal was complete in ≈20 days by which time a typical distance from release was 30–40 m. Mean daily net distance moved was 2.2 m, but within 10 days snails had moved 50–100 m. In both years, following initial dispersal, snails (infected or not) took up random directions from move to move. Infected and uninfected snails dispersed equal distances, but had different mean final dispersal directions. Dispersal of I. obsoleta individuals was extensive and affected by shifting sandbar positions and parasitism. Recognizing this will be important in appreciating the ecological dynamics of this gastropod and in determining its effects on soft-bottom communities.Electronic Supplementary Material Supplementary material is available for this article at     

Implications of life history for genetic structure and migration rates of southern African coastal invertebrates: planktonic,abbreviated and direct development

The amount of genetic structure in marine invertebrates is often thought to be negatively correlated with larval duration. However, larval retention may increase genetic structure in species with long-lived planktonic larvae, and rafting provides a means of dispersal for species that lack a larval dispersal phase. We compared genetic structure, demographic histories and levels of gene flow of regional lineages (in most cases defined by biogeographic region) of five southern African coastal invertebrates with three main types of larval development: (1) dispersal by long-lived planktonic larvae (mudprawn Upogebia africana and brown mussel Perna perna), (2) abbreviated larval development (crown crab Hymenosoma orbiculare) and (3) direct development (estuarine isopod Exosphaeroma hylecoetes and estuarine cumacean Iphinoe truncata). We hypothesized that H. orbiculare, having abbreviated larval development, would employ a strategy of larval retention, resulting in genetic structure comparable to that of the direct developers rather than the planktonic dispersers. However, regional population structure was significantly lower in all species with planktonic larvae, including H. orbiculare, than in the direct developers. Moreover, nested clade analysis identified demographic histories resulting from low levels of gene flow (isolation by distance and allopatric fragmentation) in the direct developers only, and migration rates were significantly higher in all three species having planktonic larvae than in the direct developers. We conclude that the amount of genetic structure within marine biogeographic regions strongly depends on the presence or absence of free-swimming larvae. Whether such larvae are primarily exported or retained, whether they have long or short larval duration, and whether or not they are capable of active dispersal seems to have little effect on connectivity among populations.     

Effects of ocean temperature on the southern range limits of two understory kelps, <Emphasis Type="Italic">Pterygophora californica</Emphasis> and <Emphasis Type="Italic">Eisenia arborea</Emphasis>, at multiple life-stages

Environmental factors have long been shown to influence species distributions, with range limits often resulting from environmental stressors exceeding organism tolerances. However, these abiotic factors may differentially affect species with multiple life-history stages. Between September 2004 and January 2006, the roles of temperature and nutrient availability in explaining the southern distributions of two understory kelps, Pterygophora californica and Eisenia arborea (Phaeophyceae, Laminariales), were investigated along the coast of California, USA and the Baja California Peninsula, Mexico, by limiting either: (a) tissue nitrogen uptake and storage by adult sporophytes during periods of elevated temperature, and/or (b) production of embryonic sporophytes by microscopic gametophytes. Results suggest that while adult sporophytes of both species are tolerant of high temperatures and low nutrients, reproduction by their microscopic stages is not. Specifically, while E. arborea produced embryonic sporophytes at both 12 and 18°C, temperatures commonly observed throughout the southern portion of its range, P. californica produced sporophytes at 12 but not at 18°C. As a result, it appears that the southern distribution of P. californica, which ends in northern Baja California, Mexico, may be limited by temperature acting on its microscopic stages. In contrast, the ability of E. arborea’s microscopic and adult stages to tolerate elevated temperatures allows it to persist in the warmer southern waters of Baja California, as well as to the north along the California coast where both species co-occur.