Temporal stability of gene frequencies within genetically heterogeneous populations of the queen scallop Aequipecten (Chlamys) opercularis

The widely held assumption that gene frequencies within samples of multi-aged organisms are temporally stable has rarely been tested. In order to test this assumption in the commercially exploited bivalve scallop Aequipecten (Chlamys) opercularis (L.), 12 populations were sampled from around the UK over the period 1988–1990 and each was assayed at four putative polymorphic enzyme loci. Most populations were comprised of 3 to 4 year-classes, distinguishable by annual rings laid down on the shell. Three alleles from each locus were used as discriminant-function variables. Discriminant-function analysis based on allele frequencies averaged for each inividual year-class (45 demes) revealed highly significant inter-site heterogeneity. A posteriori classification of individual year-classes based on the discriminant analysis resulted in assignment of year-classes to the correct home site in 80% of cases. If allozyme neutrality is assumed, the phenomenon of predominant local allele frequency-stability through time within populations which are genetically heterogeneous from one site to another raises the possibility that queen scallop populations may be self-recruiting. Despite a planktonic larval phase lasting up to several weeks, this possibility is not excluded by the available hydrographic data.     

Genetic differentiation between morphotypes of the marine sponge Suberites ficus (Demospongiae: Hadromerida)

Sponges of three morphotypes of Suberites ficus (Johnston, 1842) were collected during February and March 1985 off the south-west of the Isle of Man, and were compared by using spicule size distributions and genetic allele frequencies of isozyme loci. The populations did not show any significant differences of spicule size or type, but could be easily differentiated into three separate species based on isozyme patterns. Samples of pale orange S. ficus growing on gastropod shells inhabited by hermit crabs (Pagurus spp.) were reproductively isolated from the redorange and the pale yellow colour morphs encrusting the bivalve Chlamys opercularis. These latter two colour morphs were genetically similar, but significant differences were observed at two of the 19 gene loci assayed. All the sponges studied were sympatric, and therefore the genetic differences, indicating reproductive isolation, are strong evidence for separate gene pools and, hence, that they are different species. The genetic identity between the two colour morphs of S. ficus on C. opercularis shells was 0.977, whilst between each of these and S. ficus on hermit crabs it was about 0.65. In all three species genetic variability was high, with mean expected and observed heterozygosity values per locus ranging from 0.17 to 0.36.     

Distribution of encrusting bryozoans and other epifauna on the subtidal bivalveChlamys opercularis

The shells of living queen scallops,Chlamys opercularis (L.), are often completely colonised with epifauna. In this study, the distribution of the major epifaunal groups across the surfaces of left (upper) and right (lower) valves is examined using samples from two commercial queen scallop beds off the Isle of Man. The composition of the epifauna differs across and between the valves and also varies with the age of the scallop. YoungerC. opercularis are commonly encrusted by sponges, but on older shells other epifauna colonise the surfaces. The encrusting bryozoan assemblage found on livingC. opercularis is not as abundant or speciose as on some other substrata such as stones and dead shell. This may be a result of intense competition from other phyla. The distribution of different bryozoan species between the surfaces is examined. Most species are more abundant on the lower valves.     

Genetic divergence between colour morphs in populations of the common intertidal sea anemones Actinia equina and A. prasina (Anthozoa: Actiniaria) in the Isle of Man

A distinctive morph ofActinia equina (L.) is found at low frequency among populations on intertidal hard substrata on some shores on the Isle of Man. This morph is red with rows of green oval spots or elongate markings running longitudinally down the column. Starch gel electrophoresis of 21 allozyme loci was used to compare samples of this morph from two localities with other sympatricActinia spp. Collections were made in early summer 1986. For one sampling site the comparison was with redA. equina and greenA. prasina Gosse, whilst at the other site with redA. equina only. At both sites significant genetic differences in allele frequencies at several loci were found between the new morph and sympatricA. equina andA. prasina. The results also confirm the reproductive isolation ofA. prasina. As with much previous work on sea anemones, levels of mean heterozygosity per locus (H) were found to be high (H=0.157 to 0.342). A surprising feature of the results, although bearing out earlier unpublished data, is that high levels of genetic differentiation are found between populations of what appear to be the same morphs collected from shores only a few km apart. Among theActinia spp. studied, differentiation between populations of the same morph on different shores was of the same order as between sympatric, reproductively isolated populations.     

Reproductive isolation with little genetic divergence between Urticina (=Tealia) felina and U. eques (Anthozoa: Actiniaria)

There has long been doubt as to whether there are one or two British species of the sea anemone genus Urticina. In the present study, populations of both putative species, which occur sympatrically in the Isle of Man, have been compared by electrophoresis of isozymes and nematocyst analysis. The analysis of fourteen isozyme loci exhibited a high genetic identity between the two taxa (I=0.907), but four loci showed significant differences in gene frequencies; thus indicating reproductive isolation. The populations displayed highly significant differences in the sizes of the various nematocyst types. The individual analysis of data for each type of nematocyst revealed that the basitrics from the actinopharynx and from the tentacles contribute most to the observed difference. Consequently, U. felina (L.) and U. eques (Gosse) are assumed to be valid species, as suggested by Manuel (1981). The mean heterozygosities for each species (0.410 and 0.436, respectively) are the highest reported for cnidarians.     

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     

To what extent does upright sessile epifauna affect benthic biodiversity and community composition?

Small-scale habitat complexity, including that caused by biological structures, is an important factor in structuring benthic communities and also sometimes in increasing biodiversity. The aim of this study was to determine if hydroid colonies have an effect on the composition of benthic communities in the Irish Sea, and if so, which components of the fauna are affected. Forty-six seabed core samples were taken by divers from two sites off Port Erin, Isle of Man, Irish Sea. Half of these were centred on hydroid colonies, half were not. All taxa retained by a 63-m sieve from the cores were identified and counted. Community composition and diversity were compared between hydroid and non-hydroid cores using multivariate and univariate methods. Benthic communities were significantly different between the two sample groups. This was almost entirely due to the presence of sessile and mobile epifaunal taxa in the hydroid cores. The tube-building amphipod, Ericthonius punctatus, was particularly abundant attached to the hydroid stems. Infauna was not significantly different between the two groups. Upright sessile epifauna may play a particularly important role in the Irish Sea as a settlement substrate for juvenile scallops (Pecten maximus and Aequipecten opercularis), which are an important fishery resource in this area. The 11-year closure of an area to dredging has not only enhanced scallop stocks but has had the added benefit of enhancing habitat complexity and biodiversity.Communicated by J.P. Thorpe, Port Erin     

Genetic differentiation between populations of Talitrus saltator and Talorchestia deshayesii (Crustacea: Amphipoda) from coastal areas of the north-western European continent

Electrophoretic studies of gene-enzyme variation in the littoral talitrids Talitrus saltator (Montagu) and Talorchestia deshayesii (Audouin) were undertaken to estimate the amount of divergence among geographically separated populations. Samples of both species were taken from sandy beaches over a transect of approximately 3 500 km along the coast of the European continent including Baltic, North Sea and Atlantic locations. A total of 22 T. saltator and 15 T. deshayesii populations were analysed for genetic variation at various enzyme loci. Both amphipods revealed relatively low levels of polymorphism and heterozygosity. Among the loci studied, phosphoglucose isomerase (Pgi) and phosphoglucomutase (Pgm) were highly polymorphic. Patterns of micro- and macrogeographic variation in terms of distributions of allele frequencies at these particular loci are compared. Interpopulation allozymic variation was shown to be lower in T. deshayesii than in T. saltator. As demonstrated by T. saltator populations sampled in coastal sites ranging from Denmark to western France, clinal variation in frequencies of two alleles became evident at the PGI locus, exhibiting a steady increase in the level of polymorphism with decreasing latitudes. It is argued that limited gene flow and, to some extent, random genetic drift may account for the gene pool structure of the talitrid species investigated.     

DNA markers indicate that distinct spawning cohorts and aggregations of Patagonian squid,<Emphasis Type="Italic"> Loligo gahi</Emphasis>, do not represent genetically discrete subpopulations

The Patagonian squid, Loligo gahi DOrbigny, has been described as having temporally and spatially identifiable spawning aggregations. Variation at six microsatellite loci was used to assess whether seasonal and geographical spawning groups around the Falkland Islands represent distinct sub-populations. Genetic variation at these loci is high in this species (mean expected heterozygosity=0.87; mean number of alleles=14.7). No evidence of significantly different allele frequencies was found, either between samples from putative spawning cohorts or geographical areas, indicating that L.gahi around the Falkland Islands comprise a single genetically homogeneous population. Age structure analysis of samples (from statolith growth increments) indicated substantial spread in hatching dates among individuals of similar size and maturity status, suggesting the potential for extensive interbreeding between seasonal cohorts. A sample of L.gahi from the SE Pacific (Peru) displayed distinctly different gene frequencies (and allele size distribution at one locus) from SW Atlantic samples, supporting the suggestion that SE Pacific and SW Atlantic populations may represent distinct subspecies.Contributed by J.P. Thorpe, Port Erin     

Strengthening recruitment of exploited scallops Pecten maximus with ocean warming

There is evidence that ocean warming has effects on the ecology, including recruitment dynamics, of marine organisms. In association with rising mean spring temperatures in the Irish Sea, a time-series of juvenile scallop Pecten maximus density around the Isle of Man showed a significant increasing trend since 1991. Favorable conditions (warmer water and correspondingly greater food availability) during gonad development can increase scallop gamete production. We examined the possibility that ocean warming has directly increased recruitment of exploited P. maximus around the Isle of Man by enhancing gonad development. From 1991–2007, there was a significant positive correlation between scallop recruitment and mean spring (the main period of gonad development) temperature in the year of larval settlement. Detrended (i.e., accounting for a time effect) recruitment data showed a marginally non-significant correlation to temperature. Gonadal somatic index of adult scallops and temperature were positively correlated. These relationships support the hypothesis that greater gamete production associated with ocean warming may be primarily responsible for observed increases in recruitment success and CPUE in a commercially important shellfish stock.     

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 similarity between samples of the orange roughyHoplostethus atlanticus from the Tasman Sea,South-west Pacific Ocean and North-east Atlantic Ocean

Gel electrophoresis was used to measure genetic variation in the orange roughyHoplostethus atlanticus. Samples were collected on the continental slope in the Tasman Sea, South-west Pacific Ocean and North-east Atlantic Ocean, at various periods from 1982 to 1984. Twenty-two enzymatic loci were resolved in seven samples to give observed heterozygosities between 0.104±0.037 and 0.125±0.044. There was little genetic differentiation between populations separated by a distance of approximately 21 000 km. Tasman-Pacific and Atlantic samples differed significantly in allele frequency at only two loci, while at a third locus a rare allele was found only in the Atlantic Ocean.     

Population genetics of the American oyster Crassostrea virginica along the Atlantic coast and the Gulf of Mexico

An examination by protein-gel electrophoresis of 19 different geographical populations of the American oyster Crassostrea virginica (Gmelin) was conducted along the Atlantic coast and the Gulf of Mexico. Estimates were made of levels of genetic variation and similarity among the populations based on 32 structural loci. The percentage of loci polymorphic ranged from 46.9 to 65.6% along the Atlantic coast while the estimate ranged from 54.8% to 68.8% on the Gulf of Mexico. The percentage of loci heterozygous ranged from 18.6 to 23.6% along the Atlantic coast and from 20.0 to 25.4% in the Gulf of Mexico. The genetic similarities between all contiguous populations from Cape Cod, Massachusetts to Corpus Christi, Texas were estimated as 99%, while the same estimate between Corpus Christi and Brownsville, Texas was computed as 93%, indicating a major transition in genetic structure for the Brownsville population of the Laguna Madre. The study revealed that the migration of planktonic oyster larvae is predominantly in a westerly direction along the Gulf of Mexico and that gene flow appears to be disrupted in the region of the Laguna Madre. Evidence has been presented for single gene selection at the Lap-2 and Pgi loci in the form of macrogeographical clines in allele frequencies with changing environmental conditions.     

Genetic structure of populations of two species of Chthamalus (Crustacea: Cirripedia) in the north-east Atlantic and Mediterranean

Protein electrophoresis on starch gels was used to investigate population genetic structure of the barnacles Chthamalus montagui Southward and C. stellatus (Poli) over their north-east Atlantic and Mediterranean ranges. In each species, a single locus exhibited marked differentiation of allele frequencies between Atlantic and Mediterranean localities; in C. stellatus, genetic differentiation between the two basins had not previously been noted. In both species, mean heterozygosity per locus appeared higher in the Mediterranean samples than in the Atlantic, and Mediterranean populations had more alleles at the loci studied. Possible explanations for the differentiation between the Atlantic and the Mediterranean populations are discussed. Received: 30 May 1996 / Accepted: 17 September 1996     

Effects of food particle concentration on feeding current velocity in sex species of marine Bryozoa

Experiments were carried out using established methods to measure feeding current velocity in six species of marine Bryozoa collected in 1982 and 1983 around low-tide mark at Port St. Mary, Isle of Man (Flustrellidra hispida, Alcyonidium gelatinosum, A. hirsutum, Electra pilosa, Membranipora membranacea and Bowerbankia gracilis feeding on Tetraselmis suecica). It was found that there was a positive correlation between feeding current velocity and the concentration of food (phytoplankton) in the environment. It was also found that, at a fixed particle concentration, feeding current velocity was strongly correlated with lophophore height and (less strongly) with tentacle number. The competitive and evolutionary implications of these relationships are considered. It is tentatively suggested that interspecific competition for food may be of some significance in marine Bryozoa and that the removal of food supplies to competing colonies may play a role in colony overgrowth and competition for space.     

Identification of genetic populations of the Pacific blue shrimp Penaeus stylirostris of the Gulf of California, Mexico

Species are often composed of discrete breeding units (i.e. populations or stocks) which, while not reproductively isolated from other such groups, may have limited opportunities to exchange genetic material because of geographic distance, barriers to migration, or spawning asynchrony. Low levels of gene flow between stocks may result, over time, in their genetic divergence, and species that are subdivided into morphologic or genetically distinct stocks are said to be structured. The aim of our investigation was to test whether or not Penaeus stylirostris from the Gulf of California (Mexico) was structured into genetically distinct populations. Shrimp samples were collected in 1996 from six regions of the Gulf where specimens with distinct morphologic characteristics had previously been identified. Statistical analysis of 324 RAPD loci (randomly amplified polymorphic DNA), resolved through polyacrylamide gels and scored for each of 78 specimens, permitted the quantification and comparison of between-stock genetic differences. The finding that genetically discrete stocks of P. stylirostris can be found in a small portion of the geographic distribution range of the species, disagrees with the long-held perception that this resource is panmictic in nature. This new evidence is not only of interest for selective breeding programs in the shrimp aquaculture industry, but is also relevant to the management of the Mexican shrimp fishery which, at present, is perceived and managed as a single stock. Received: 10 May 1999 / Accepted: 27 July 2000     

Robustness in life history of the brown seaweed Ascophyllum nodosum (Fucales, Phaeophyceae) across large scales: effects of spatially and temporally induced variability on population growth

Understanding the demography and function of biotope-forming seaweed species is of great importance for the conservation of the target species itself, as well as its associated organisms. The brown seaweed Ascophyllum nodosum is fundamental for the functioning of coastal marine ecosystems in the North Atlantic. In this study, we use a data-based size-classified matrix model to investigate the temporal and spatial variability in demography, and the environment-specific stochastic sensitivity and elasticity, of two A. nodosum populations, one in western Sweden and one on the Isle of Man in the Irish Sea. A significant difference between the two populations was that the Swedish population had comparably low and more variable stochastic population growth rate (λ _s). This pattern was partly explained by the relatively high and varying mortality rates during extreme ice-years in Sweden, and by the lower survival of small individuals during all years. There were also fewer large individuals in Sweden due to lower transitions to the larger size-classes and higher probability of shrinkage. Sensitivities were analogous in the two populations, and showed a high selection pressure for increased individual growth. Elasticities were also similar, with the exception that survival of the smallest individuals (i.e., transition a _1,1), had a higher elasticity on the Isle of Man. Overall, the stochastic growth rate (λ _s) was most sensitive to proportional changes in loop- (i.e., survival within size-class) and, to some extent, growth-transitions in both study areas. These results show that structurally and demographically diverging A. nodosum populations may be similarly sensitive to changes in vital rates. This, in turn, indicates a plastic life history of A. nodosum that may cope with large environmental variability. The results further suggest that environmental change affecting the survival or growth of the larger, reproductive A. nodosum individuals could have severe and regional effects on the abundance and biomass of this species, with potential negative effects on the biodiversity of the associated communities.     

Separation of the two Atlantic species of Pomatoceros,P. lamarckii and P. triqueter (Annelida: Serpulidae) by means of biochemical genetics

In order to confirm the status of two proposed species of the serpulid Pomatoceros, the electrophoretic mobilities of 13 enzymes encoded at 19 loci in P. lamarckii (Quatrefages) and P. triqueter (L.) were compared using horizontal starch gel electrophoresis. The serpulids were sampled from Menai Bridge (North Wales), Plymouth (England) and the Isle of Man in the winter of 1981/1982. Six loci were monomorphic or nearly so and showed no useful or significant difference between species. At a further 7 loci, some of the allozymes common to both species were present, though usually at significantly different frquencies, while others were species-specific. The remaining 6 loci presented species-specific allozymes. In P. lamarckii, 61% of the loci were polymorphic compared with 50% in P. triqueter. The mean observed heterozygosity was 0.169 for P. lamarckii and 0.208 for P. triqueter. Calculation of genetic distance and identity gave values of 0.821 and 0.440, respectively. This study, therefore, confirms the existence of two separate species of Pomatoceros as has been previously proposed on the basis of opercular structure and supported by ecological separation where these species occur sympatrically.     

Mitochondrial DNA variation and population genetic structure of the blue crab<Emphasis Type="Italic"> Callinectes sapidus</Emphasis> in the eastern United States

The genetic population structure of red grouper, Epinephelus morio (Valenciennes), and scamp, Mycteroperca phenax Jordan and Swain, from the southeastern U.S. Atlantic coast and the Gulf of Mexico was examined using nuclear microsatellite DNA markers in order to test the null hypothesis of panmixia throughout this range. Physical and biological data indicate that relatively isolated populations of these fish exist. Genetic variation was assessed at four microsatellite loci in red grouper and six loci in scamp. The fish were collected on different dates between 1991 and 2001. The microsatellite loci were highly polymorphic, with an average expected heterozygosity of 0.75 in red grouper and 0.68 in scamp. Heterozygote deficiencies (significant deviations from Hardy–Weinberg equilibrium, HWE) were found at two of four loci in all red grouper samples except the eastern Gulf of Mexico, and for all red grouper combined. In contrast, all loci conformed to HWE in the separate scamp samples. Minimal genetic differences distinguished southeastern U.S. Atlantic or Mexican red grouper from other localities, and no indication of genetic differentiation was observed in scamp. This large-scale genetic homogeneity may be attributed to ongoing gene flow and/or historical contact between present-day populations. For management purposes, genetic homogeneity does not necessarily imply a single stock. Because larval dispersal may be sufficient to homogenize gene frequencies but not to replenish depleted stocks, other data must be considered in the management of these species.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Communicated by J.P. Grassle, New Brunswick