Genetic structure of Atlantic and Gulf of Mexico populations of sea bass,menhaden, and sturgeon: Influence of zoogeographic factors and life-history patterns

To assess the influence of zoogeographic factors and life-history parameters (effective population size, generation length, and dispersal) on the evolutionary genetic structure of marine fishes in the southeastern USA, phylogeographic patterns of mitochondrial DNA (mtDNA) were compared between disjunct Atlantic and Gulf of Mexico populations in three coastal marine fishes whose juveniles require an estuarine or freshwater habitat for development. Black sea bass (Centropristis striata), menhaden (Brevoortia tyrannus andB. patronus) and sturgeon (Acipenser oxyrhynchus) samples were collected between 1986 and 1988. All species showed significant haplotype frequency differences between the Atlantic and Gulf, but the magnitude and distribution of mtDNA variation differed greatly among these taxa: sea bass showed little within-region mtDNA polymorphism and a clear phylogenetic distinction between the Atlantic and Gulf; menhaden showed extensive within-region polymorphism and a paraphyletic relationship between Atlantic and Gulf populations; and sturgeon exhibited very low mtDNA diversity both within regions and overall. Evolutionary effective sizes of the female populations (N _f (e)) estimated from the mtDNA data ranged fromN _f (e) = 50 (Gulf of Mexico sturgeon) toN _f (e) = 800 000 (Atlantic menhaden), and showed a strong rank-order agreement with the current-day census sizes of these species. The relationship betweenN _f (e) and the estimated times of divergence (t) among mtDNA lineages (from conventional clock calibrations) predicts the observed phylogenetic distinction between Atlantic and Gulf sea bass, as well as the paraphyletic pattern in menhaden, provided the populations have been separated by the same long-standing zoogeographic barriers thought to have influenced other coastal taxa in the southeastern USA. However, vicariant scenarios alone cannot explain other phylogenetic aspects of the menhaden (and sturgeon) mtDNA data and, for these species, recent gene flow between the Atlantic and Gulf coasts is strongly implicated. These data are relevant to management and conservation issues for these species.Please address all requests for reprints to Dr. J. C. Avise     

Phylogeography of surfclams,<Emphasis Type="Italic"> Spisula solidissima</Emphasis>, in the western North Atlantic based on mitochondrial and nuclear DNA sequences

The Atlantic surfclam, Spisula solidissima (Dillwyn), is broadly distributed in sandy sediments of the western North Atlantic between the Gulf of St. Lawrence and the Gulf of Mexico. In the United States, a substantial commercial fishery between Long Island and Cape Hatteras harvests offshore populations of one subspecies, S. s. solidissima. A smaller coastal form, S. s. similis Say (also known as S. s. raveneli Conrad), has a partially sympatric geographic distribution, but differs in several life-history characteristics. DNA sequence variation in mitochondrial cytochrome oxidase I (COI) and in introns at two nuclear calmodulin loci was examined to measure genetic divergence between the two subspecies and to test for population structure among populations of S. s. solidissima. Surfclams were collected from seven localities between 1994 and 2001. Based on both mitochondrial and nuclear DNA variation, the two subspecies of S. solidissima are reciprocally monophyletic, with a net COI divergence of 13.9%, indicating long-term reproductive isolation. The only significant differentiation among populations of S. s. solidissima (based on an AMOVA analysis of COI sequences) was between the Gulf of St. Lawrence and more southerly populations. A long internal branch in the S. s. solidissima genealogy coupled with low haplotype diversity in the northern-most population suggests that populations north and south of Nova Scotia have been isolated from each other in the past, with gene exchange more recently. Populations of S. s. similis from Atlantic and Gulf of Mexico coasts had a net COI divergence of 9.2%. Thus, diversification of Spisula spp. clams in the western North Atlantic involved an early adaptive divergence between coastal and offshore forms, with later barriers to dispersal emerging in the offshore form from north to south and in the coastal form between Atlantic and Gulf of Mexico populations.     

Phylogeography of two squid (Loligo pealei and L. plei) in the Gulf of Mexico and northwestern Atlantic Ocean

The loliginid squids Loligo pealei LeSueur and L. plei Blaineville (both recently proposed for reclassification as Doryteuthis) are commercially important, similar in appearance, and sympatric throughout much of the northwestern Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. To investigate possible cryptic speciation and population structure, we examined samples (collected from 1995 to 1997) of both species for restriction fragment length polymorphisms (RFLPs) in PCR products of the mitochondrial gene cytochrome c oxidase (subunit I). RFLP haplotypes were further characterized by direct sequencing. In North American waters, cryptic speciation was rejected by the far greater nucleotide sequence divergence between species (~14%) versus within species (<1%). Each species displayed about a dozen RFLP haplotypes, but only three of their respective haplotypes were found among 90% of L. pealei specimens (n=356) and 97% of L. plei specimens (n=431). For L. pealei, a genetic break existed between the northern Gulf of Mexico and the Atlantic Ocean; among sample units within each population, gene flow was consistent with panmixia. The phylogeography of L. pealei is likely a consequence of the eastward currents of the Florida Straits, the elevated temperatures of those surface waters, and the restriction of this species to the continental shelf. For L. plei, a genetic break existed between longitudes 88°W and 89°W, with the northwestern Gulf of Mexico and the northeastern Gulf-Atlantic Ocean comprising separate populations; among sample units within each population, gene flow fit an isolation-by-distance model. If the genetic break found for L. plei represents resident populations separated by nearshore physical parameters (e.g. effects of the Mississippi River and the sediment boundary at longitude 88°W), the lack of structure within the Gulf for L. pealei might be due to its distribution farther from shore. However, the two populations of L. plei probably represent annual recolonization from the southwestern Gulf of Mexico and from the eastern Caribbean Sea, whereas the populations of L. pealei probably are permanent residents within their respective regions.     

Speciation in western Atlantic stone crabs (genus Menippe): the role of geological processes and climatic events in the formation and distribution of species

Electrophoretically detectable variation in 38 proteins and color morphology were used to determine the evolutionary relationships of crabs of the genus Menippe (Xanthidae) in the southeastern USA. Both allele frequencies (=genotype) and color morphology (=phenotype) showed that one species, Menippe mercenaria, is probably a taxonomic supergroup composed of two taxa (semispecies). One taxon (the western Gulf form) is distributed from northwest Florida westward through Texas. The second (the peninsular Florida form) ranges through the Florida peninsula from northwest to east central Florida, and in North Carolina. The taxa appear to have hybridized in two discrete regions: in the Gulf of Mexico (northwest Florida) and in the Atlantic Ocean (east central Florida to South Carolina). The agreement of patterns of geographic variation in genotype and phenotype with the geological record and estimated times of divergence based on genetic distances suggests that the observed patterns are the product of the influence of Late Cenozoic changes in climate and geology. The Atlantic zone of hybridization was formed prior to the closure of the seaway across north Florida connecting the Gulf of Mexico and the Atlantic, and the northwest Florida zone at some time subsequent to the closure. The present distribution, and the location of zones of hybridization, between the two semispecies of M. mercenaria illustrate the importance of the interaction of historical geological and climatic events with ecological boundaries in determining the distribution and interactions of shallow water marine species.     

An investigation of genetic population structure in blue crabs,Callinectes sapidus,using nuclear gene sequences

Spatial and temporal genetic heterogeneity in the sequences of nuclear protein-coding genes were investigated in populations of the blue crab, Callinectes sapidus. Haplotype frequencies of these novel markers were determined for adult crabs collected along 300 km in the northern Gulf of Mexico in two different years (2010 and 2011), as well as for megalopal recruits collected in 2010. Tests of genetic differentiation among all locations and between locations spanning known genetic breaks in other species were conducted. In addition, samples from distant locations within the range of C. sapidus were used to assess genetic divergence on a broader geographic scale. Significant between-year differences were found for adults at one location and near significant spatial differentiation was found across northern Gulf of Mexico locations in 2010. These results suggest that although the large population sizes and meroplanktonic life history of blue crabs promote widespread gene flow on a regional scale, genetic composition can change over just one year. Substantial divergence between the northern Gulf of Mexico and Venezuela suggests the possibility that temporal shifts in haplotype frequencies could result from variation in the rate of immigration from genetically distinct source populations. The possible effects of the Deepwater Horizon Oil Spill and attendant fisheries closures during the sampling phase of this study are also considered.     

Population structure of red drum (Sciaenops ocellatus) in the northern Gulf of Mexico, as inferred from variation in nuclear-encoded microsatellites

Allelic variation at eight nuclear-encoded microsatellites was assayed among 967 red drum (Sciaenops ocellatus) sampled from four consecutive cohorts at seven geographic localities (=28 samples total) in the northern Gulf of Mexico (Gulf). Number of alleles per microsatellite ranged from 6 to 21; average direct-count heterozygosity values per sample (-SE) ranged from 0.560ǂ.018 to 0.903ǂ.009. Tests of Hardy-Weinberg equilibrium revealed significant departures from expected genotype proportions at one microsatellite, which was omitted from further analysis. Tests of genotypic equilibrium indicated that genotypes between pairs of microsatellites were randomly associated. Homogeneity tests of allele distributions across cohorts within localities were non-significant following correction for multiple tests executed simultaneously, and results from molecular analysis of variance indicated that the genetic variance component attributable to variation among cohorts did not differ significantly from zero. Homogeneity tests of allele distributions among localities (cohorts pooled) revealed significant differences both before and after correction for multiple tests. Neighbor-joining clustering of a pairwise matrix of Š values (an unbiased estimator of F_ST), spatial autocorrelations, and regression analysis revealed a pattern of isolation by distance, where genetic divergence among geographic samples increases with geographic distance between sample localities. The pattern and degree of temporal and spatial divergence in the nuclear-encoded microsatellites paralleled almost exactly those of mitochondrial (mt) DNA, as determined in a prior study. Stability of both microsatellite and mtDNA allele distributions within localities indicates that the small but significant genetic divergence among geographic samples represents true signal and that overlapping populations of red drum in the northern Gulf may be influenced by independent population dynamics. The degree of genetic divergence in microsatellites and mtDNA is virtually identical, indicating that genetic effective size of microsatellites and mtDNA in red drum are the same. This, in turn, suggests either that gene flow in red drum in the northern Gulf could be biased sexually or that red drum populations may not be in equilibrium between genetic drift and migration. If a sexual bias exists, the observation that divergence in mtDNA is considerably less than 4 times that of microsatellites could suggest female-mediated dispersal and/or male philopatry. The observed isolation-by-distance effect indicates a practical limit to dispersal. Approximate estimates of geographic neighborhood size suggest the limit is in the range 700-900 km. Although the genetic studies of red drum indicate significant genetic divergence across the northern Gulf, the genetic differences do not delimit specific populations or stocks with fixed geographic boundaries.     

Population structure of red porgy, <Emphasis Type="Italic">Pagrus pagrus</Emphasis>, in the Atlantic Ocean

The red porgy, Pagrus pagrus (L.), is a protogynous sparid associated with reefs and hard bottom habitat throughout the warm-temperate Atlantic Ocean. In this study, the degree of geographic population differentiation in Atlantic populations was examined with microsatellite and mitochondrial DNA markers (mtDNA). Six microsatellite loci were amplified and scored in 690 individuals from the eastern North Atlantic (Crete, Madeira, and Azores), western North Atlantic (North Carolina to Florida, and the eastern Gulf of Mexico), and Brazil. At two loci, fixed allelic differences were found among the three major geographic areas, while frequency differences were observed at three other loci. The DNA of 371 individuals was amplified at the mtDNA control region, and 526 bp were sequenced. Tamura–Nei’s D was used as a measure of nucleotide diversity and divergence: diversity averaged 0.011 within samples, while the corrected divergence averaged 0 between samples within the same area and 0.061 between samples from different areas. Transversion haplotype minimum spanning networks, nucleotide divergence, and F _ST values all show that the western Atlantic samples were more closely related to each other than any was to samples from the eastern North Atlantic. Within the western North Atlantic, no significant population differentiation was observed, and within the eastern North Atlantic, only the Azores sample showed detectable differences from Crete and Madeira. These data indicate general homogeneity within large areas, and deep divisions between these areas. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Genetic differentiation ofMytilus edulis in eastern North America

There is significant differentiation at five polymorphic loci ofMytilus edulis among certain geographical areas of the Atlantic coast of North America. Non-metric multidimensional numerical methods distinguished three population groups: (I) populations south of Cape Cod, (II) populations throughout the Gulf of Maine, Gulf of St. Lawrence, areas of both southern and northern Newfoundland, and southern Hudson Bay, and (III) populations in southeastern Nova Scotia, northern Newfoundland and Hudson Strait, Quebec. Each subset consists of populations that exhibit characteristic multilocus, multiple allele genotypes. Populations in Groups II and III are spatially interdigitated among each other. At least one geographical area of mixing between genetically distinct populations occurs in northeastern Newfoundland. There is no evidence for interbreeding among genetically distinct individuals in mixed populations, suggesting the possibility that populations in the Atlantic Canadian Provinces and areas of northern Canada may consist of two distinct species.     

Are meiofaunal species cosmopolitan? Morphological and molecular analysis of Xenotrichula intermedia (Gastrotricha: Chaetonotida)

Many meiofaunal species are reported to be cosmopolitan, but due to uncertainties of identification, the affiliation of specimens from geographically distant areas to the same species-taxon is problematic. In this study, we examined morphological and molecular variation in samples of Xenotrichula intermedia Remane (Gastrotricha: Chaetonotida) from the Mediterranean Sea, the northwestern Atlantic and the northern Gulf of Mexico. Univariate analysis of 16 morphological traits was unable to detect differences among populations, except for the length of the pharynx, which was significantly shorter in the Gulf of Mexico specimens. Canonical discriminant analysis separated the Gulf of Mexico specimens from the other two populations, with pharynx length contributing about half of the total discrimination. Molecular analysis based on restriction-fragment length polymorphisms (RFLPs) in a 710-base pair polymerase chain-reaction (PCR) produet representing roughly half of the mitochondrial cytochrome oxidase I (COI) gene detected four haplotypes: one each from the Mediterranean and the Gulf of Mexico populations and two coexisting within the Atlantic population. The estimated nucleotide-sequence divergence calculated for each pairwise combination of haplotypes (based on the proportion of shared fragments) ranged from 5.3 to 11.5%. The high genetic divergence and the inability to clearly separate populations based on morphology suggest that individuals characterized by different haplotypes are genetically isolated sibling species.     

Evidence of different reproductive traits in the transisthmian sister species, <Emphasis Type="Italic">Alpheus saxidomus</Emphasis> and <Emphasis Type="Italic">A. simus</Emphasis> (Decapoda,Caridea, Alpheidae): description of the first postembryonic stage

The emergence of the Isthmus of Panama, approximately 3.0-3.5 million years ago, established two very different marine systems (the Caribbean and the eastern Pacific) and separated previously continuous marine populations. The geographic isolation of transisthmian sister species provides an excellent basis for the study of divergence and speciation processes. Here we describe the morphology of the first postembryonic stage of Alpheus saxidomus and A. simus, two rock-boring alpheids; the estimated time since divergence for this transisthmian pair ranges from 4.4-6.1 to 3-9 million years. The presence of a first zoeal stage in A. saxidomus, e.g., without pleopods and a telson with 7+7 setae, indicates a prolonged developmental sequence. In contrast, hatchlings of A. simus are substantially more developed and resemble juveniles. Thus, the developmental modes of A. saxidomus and A. simus are strikingly different with abbreviated, most probably direct, development in the latter species. Reduced food availability in the Caribbean compared to the Pacific coast is likely to be a possible reason for the evolution of such important differences in life history traits of the two transisthmian sister species. It is suggested that a transition from prolonged to abbreviated development evolved gradually during the estimated time since divergence; however, such a shift may have taken place within a substantially smaller time span.     

Temporal stability and spatial divergence of mitochondrial DNA haplotype frequencies in red drum (Sciaenops ocellatus) from coastal regions of the western Atlantic Ocean and Gulf of Mexico

Restriction-site variation in mitochondrial (mt) DNA was assayed among 1675 red drum (Sciaenops ocellatus Linnaeus) sampled from 20 localities along the southeastern coast of the USA (western Atlantic) and the Gulf of Mexico (Gulf). Up to four consecutive year-classes (cohorts) were sampled at most localities. Nucleotide-sequence divergence among 170 mtDNA haplotypes identified ranged (in percentage) from 0.184 to 1.913, with a mean (±SD) of 0.887 ± 0.300. Comparisons of mtDNA haplotype frequencies across year-classes within localities were non-significant, indicating temporal stability of breeding components within localities. Significant heterogeneity in mtDNA haplotype frequencies was found across all localities, between (pooled) samples from the western Atlantic and the Gulf, and among geographically spaced, regional groupings in the Gulf. Genetic divergence between subpopulations of red drum in the western Atlantic and Gulf follows a pattern exhibited in other marine fishes, and probably stems from physical (historical environmental heterogeneity, absence of suitable habitat, and current patterns) and, perhaps, behavioral factors. Genetic differences among red drum in the Gulf appear to be due largely to an isolation-by-distance effect that is attributable to behavioral factors. The latter may include female philopatry to natal bays or estuaries, limited offshore (coastwise) movement of females relative to their natal bay or estuary, or both. Genetic divergence among red drum in the Gulf occurs despite high gene flow (estimated as the number of genetic effective migrants in an island mode). Conservation and management of red drum should be based on the premise that strategies for a given bay or estuary will impact geographically proximal bays or estuaries more than distal ones. Trajectories of correlograms in spatial autocorrelation analysis suggest a geographic neighborhood size, relative to genetic migration of red drum from a bay or estuary, of roughly 500 to 600 km. Received: 22 July 1998 / Accepted: 19 November 1998     

Population divergence in the sinistral whelks of North America,with special reference to the east Florida ecotone

This study evaluated models of species relationships among sinistral whelks in the genus Busycon in the Atlantic and Gulf of Mexico. Gene frequencies at eight polymorphic allozyme loci, shell morphology, anatomy, and partial DNA sequences for the cytochrome c oxidase I (COI) mitochondrial gene were examined in eight populations, ranging from New Jersey to the Yucatan peninsula, and from the dextrally coiled sister taxon Busycon carica (Gmelin, 1791). Whelks were collected in 1997 and 1998. The maximum COI sequence divergence recorded among 32 sinistral individuals was 1.96%, which together with the absence of any gross or qualitative morphological differences, suggested all eight populations should be considered conspecific. High levels of divergence between the allopatric western Atlantic and Gulf of Mexico populations, as revealed by fixed or nearly fixed differences at several allozyme-encoding loci were interpreted as evidence that the east Florida ecotone constitutes a significant barrier to gene flow. Size trimming also revealed several significant quantitative differences in shell and radular morphology between the three pooled Atlantic populations and five pooled Gulf populations. The Yucatan sample was the most distinctive conchologically, with heavy spines and tumid ridges, possibly related to stone crab predation. Based on the evidence all left-handed whelks of North America should be referred to the oldest available nomen, Busycon perversum (Linné, 1758), with three subspecies, B. perversum perversum along the Yucatan peninsula, B. perversum sinistrum (Hollister, 1958) in the northern and eastern Gulf of Mexico, and B. perversum laeostomum (Kent, 1982) in the Atlantic.Communicated by J.P. Grassle, New Brunswick     

Global phylogeography of the fissiparous sea-star genus <Emphasis Type="Italic">Coscinasterias</Emphasis>

The fissiparous starfish genus Coscinasterias (Verrill) is represented in shallow waters around many of the world's continents. This wide distribution could be explained by dispersal, vicariance, or translocation associated with shipping, and represents an excellent system for marine biogeographic research. We conducted a global phylogeographic analysis of 42 Coscinasterias mtDNA cytochrome oxidase 1 sequences (15 haplotypes) from 18 sites, including representatives of all four recognised species. Phylogenetic analysis yielded a robust phylogeny, with strong support for the monophyly of the genus (90% bootstrap support) and of each separate species (99-100%). Haplotypes exhibited strong phylogeographic structure, with robust mtDNA clades often associated with distinct land masses. A general lack of genetic differentiation within sites may reflect fissiparity. However, shared haplotypes over larger distances (e.g. across Japan), and the presence of related haplotypes on adjacent land masses (e.g. Tasmania, New Zealand; 1.6-1.8%) suggest that long-distance dispersal is an important biogeographical process for Coscinasterias. The 4.0-4.4% divergence between Japanese and South African sister groups may relate to transequatorial dispersal around the onset of the Pleistocene. Divergent Atlantic and Mediterranean populations of C. tenuispina (maximum 1.5%) may deserve subspecific recognition, and high divergences within Australian C. muricata (maximum 8.0%) suggest a species complex.     

DNA sequence evidence for speciation,paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods

Because the classification of extant and fossil articulate brachiopods is based largely upon shell characters observable in fossils, it identifies morphotaxa whose biological status can, in practice, best be inferred from estimates of genetic divergence. Allozyme polymorphism and restriction fragment length polymorphism of mitochondrial DNA (mtDNA RFLP) have been used to show that nuclear and mitochondrial genetic divergence between samples of the cancellothyridid brachiopods Terebratulina septentrionalis from Canada and T. retusa from Europe is compatible with biological speciation, but the genetic distances obtained were biased by methodological limitations. Here, we report estimates of divergence in 12S rDNA mitochondrial sequences within and between samples of these brachiopods. The sequence-based genetic distance between these samples (5.98ǂ.07% SE) is at least 10 times greater than within them and, since they also differ in a complex life-history trait, their species status is considered to be securely established. Divergence levels between 12S rDNA genes of three other cancellothyridids, T. unguicula from Alaska, T. crossei from near Japan, and Cancellothyris hedleyi from near Australia are higher than between the two North Atlantic species, and the mean nucleotide distance between all these cancellothyrids is similar to the mean distance between species of Littorina (Mollusca: Gastropoda). Sequences of both 12S and 16S genes from cancellothyridids and other short-looped brachiopod species show neither saturation nor lineage-specific rate differences and, when analysed with different outgroups, either separately or together, yield one unexpected, but well-supported, tree with Alaskan T. unguicula basal and C. hedleyi nested within Terebratulina, i.e. these genera are paraphyletic. A geologically dated divergence between Antarctic and New Zealand species of the short-looped brachiopod Liothyrella is used to calibrate the rate of 12S divergence at ca. 0.1% per million years (MY), and this rate is used to infer that T. septentrionalis and T. retusa have been diverging for ca. 60 MY and that they and T. unguicula have been diverging from their last common ancestor for ca. 100 MY. This indicates a Mesozoic origin for the present-day distribution of cancellothyridids and the basal position of T. unguicula suggests a possible North Pacific centre of origin, with separate Atlantic and Pacific radiations. The inclusion of Cancellothyris within Terebratulina also shows that adult shell characters such as umbo, foramen and symphytium shape, whilst probably indispensible for the practical classification of fossils, are not reliable guides to genealogy.     

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.     

Mitochondrial DNA differentiation and population structure in red drum (Sciaenops ocellatus) from the Gulf of Mexico and Atlantic Ocean

Variation in mitochondrial (mt)DNA was examined among 473 red drum (Sciaenops ocellatus) sampled in 1988 and 1989 from nearshore localities in the northern Gulf of Mexico (Gulf) and the Atlantic coast of the southeastern United States (Atlantic). Data were combined with those from a previous study to generate a total of 871 individuals sampled from 11 localities in the Gulf and 5 localities in the Atlantic. Individuals assayed were from the 1986 and 1987 year-classes. A total of 118 composite mtDNA genotypes (haplotypes) was found. The percentage nucleotide sequence divergence among the 118 haplotypes ranged from 0.184 to 1.913, with a mean (±SE) of 0.878±0.004. MtDNA nucleon diversities and intrapopulational nucleotide-sequence divergence values were similar over all Gulf and Atlantic localities, and were high relative to most fish species surveyed to date. These data indicate that the perceived decline in red drum abundance appears not to have affected the genetic variability base of the species. Significant heterogeneity in the frequencies of at least four haplotypes was detected between pooled samples from the Gulf vs pooled samples from the Atlantic. No heterogeneity was found among localities from the Gulf or localities from the Atlantic. High levels of gene flow among all localities were inferred from F _ST values (a measure of the variance in mtDNA haplotype frequencies) and from Slatkin's qualitative and quantitative analyses. Parsimony and phenetic analyses revealed no strong evidence for phylogeographic cohesion of localities, although there was weak support for cohesion of four of five localities from the Atlantic. These data indicate that the red drum population is subdivided, with weakly differentiated subpopulations (stocks) occurring in the northern Gulf and along the Atlantic coast of southeastern USA. Spatial autocor-relation analysis and heterogeneity tests of haplotype frequencies among regions within the Gulf supported the hypothesis of increased gene flow among neighboring localities; i.e., migration of individuals within the Gulf may be inversely related to geographic distance from an estuary or bay of natal origin. Estimates of evolutionary effective female-population size indicate that the red drum subpopulations may be large.     

Biogeographical,biochemical and genetic differentiation among North American saccoglossids (Hemichordata; Enteropneusta; Harrimaniidae)

Populations of two common enteropneusts, Saccoglossus bromophenolosus King et al., 1994 and S. kowalevskii (Agassiz 1873) were sampled between 1991 and 1993 from the southern and northern extremes of their respective ranges: Nova Scotia/Maine and Padilla Bay, Washington for S. bromophenolosus and Maine/South Carolina for S. kowalevskii. Though previously considered a single species, the two taxa were clearly distinct biochemically and genetically. Four of five allozyme loci were diagnostic and indicative of differentiation at the species level. Sequence divergence (27%) of a portion of the mitochondrial 16S rRNA gene suggests that the two taxa have been genetically isolated for a considerable time; hybridization was not evident in sympatric populations. Both taxa contained high concentrations of bromoorganics. The constant association of bromophenols and bromoindoles with S. bromophenolosus and bromopyrroles with S. kowalevskii when they occur sympatrically indicates that bromoorganic contents were genetically and not environmentally determined. Consistent associations between external morphology and bromoorganic contents for additional saccoglossid species support the use of bromoorganics as indices of evolutionary clades in the Enteropneusta.     

Mitochondrial gene variation in Mercenaria clam sibling species reveals a relict secondary contact zone in the western Gulf of Mexico

We investigated phylogeographic relationships among American Mercenaria taxa by assessing variation in a 444 nucleotide fragment of the mitochondrial 16S ribosomal gene in clams sampled from four representative sites in January to November 1994. Three of these sites were in the Gulf of Mexico, one was on the Atlantic coast in South Carolina. Direct sequencing of this amplified gene fragment in 85 individuals revealed 21 haplotypes. Phylogenetic analyses consistently resolved this variation into three well supported clades, and within-clade genetic divergence levels were markedly lower than among-clade values. One of the clades, A, was taxon-specific, in that it solely and exclusively contained specimens of M. mercenaria (Linnaeus, 1758) sampled in South Carolina. The other two clades, B and C, were the most divergent and both encompassed specimens of M. campechiensis (Gmelin, 1791) and of M. campechiensis texana (Dall, 1902), sampled from the three Gulf of Mexico sites. Clade B was found at high frequencies at all three Gulf sites, whereas Clade C occurred at low frequencies at two western Gulf sites. We interpret this pattern as resulting from the secondary contact and introgression of two allopatrically differentiated Mercenaria taxa in the western Gulf of Mexico. Clade C haplotypes may represent relict mitochondrial lineages from original Gulf Mercenaria spp. populations that predate massive mitochondrial introgression by M. campechiensis. We further propose that the M. campechiensis texana nuclear genome is a mosaic, heavily weighted toward M. campechiensis, but containing some relict alleles inherited from the precontact population, especially those governing shell characteristics, which may be adaptive in cohesive sediments of bays and estuaries in the northwestern Gulf of Mexico.     

Mitochondrial DNA variation in king mackerel (Scomberomorus cavalla) from the western Atlantic Ocean and Gulf of Mexico

King mackerel (Scomberomorus cavalla Cuvier) collected in 1992 and 1993 from 13 localities along the Atlantic coast of the southeastern USA and in the northern Gulf of Mexico were surveyed for variation in mitochondrial (mt)DNA and a nuclear-encoded dipeptidase locus (PEPA-2). Both polymorphic and fixed mtDNA restriction sites were identified and mapped using conventional and polymerase chain-reaction (PCR)-based methods. Heterogeneity in mtDNA haplotype frequencies was found only in comparisons of pooled haplotypes from Atlantic localities versus pooled haplotypes from Gulf localities. This finding indicates weak genetic divergence between king mackerel from the Atlantic and those from the Gulf. Frequencies of two PEPA-2 alleles essentially paralleled previous findings: one allele (PEPA-2a) was common among samples from western Gulf localities, whereas the other allele (PEPA-2b) was common among samples from Atlantic and eastern Gulf localities. There was considerable variation in PEPA-2 allele frequencies within broadly-defined regions. Variation in mtDNA haplotypes and PEPA-2 genotypes was independent, as was variation in mtDNA haplotypes with sex or age of individuals. Variation in PEPA-2 genotypes was not independent of sex or age of individuals. The latter result suggests that frequencies of PEPA-2 alleles in samples of king mackerel may stem, in part, from sex and age distributions of individuals within samples, and indicates that caution should be exercised in using allelic variation at PEPA-2 as a measure of population (stock) structure in king mackerel. The discordance in spatial patterning of mtDNA haplotypes versus PEPA-2 alleles across the Gulf (i.e. homogeneity in mtDNA haplotype frequencies versus heterogeneity in PEPA-2 allele frequencies) may be due to either female excess at several localities, sex-biased migration, or both. Observed patterns of genetic variation also are consistent with the hypothesis that king mackerel in the western Atlantic may have been subdivided during Pleistocene glaciation, and that the current distribution of PEPA-2 alleles may be a historical artefact. Received: 17 December 1996 / Accepted: 2 April 1997