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.     

Morphological and genetic evidence for vicariance and refugium in Atlantic and Gulf of Mexico populations of the hermit crab Pagurus longicarpus

The number and wide variety of southeastern United States marine taxa with significant differentiation between Gulf of Mexico and Atlantic Ocean populations suggests that these taxa may have experienced major vicariance events, whereby populations were subdivided by geological or ecological barriers. The present study compared variation in morphology, allozymes, and mtDNA in Gulf of Mexico and western Atlantic populations of the longwrist hermit crab Pagurus longicarpus Say collected during 1997 and 1998. Combined Atlantic populations had significantly fewer denticles on the second segment of the third maxilliped than did Gulf of Mexico populations, and the mean ratio of dactyl length to propodus length was significantly greater in the Atlantic crabs than in the Gulf of Mexico crabs. Allozyme allele frequencies at three loci showed genetic differentiation between a Gulf of Mexico population and two Atlantic populations. Analysis of mtDNA sequence data revealed a clear reciprocal monophyly between Gulf and Atlantic populations, with an estimated divergence age of ~0.6 million years ago. This estimated age of divergence is significantly more recent than an age previously estimated for its congener Pagurus pollicaris (~4 million years ago), suggesting that species with a similar genetic break between Gulf and Atlantic populations may not necessarily share an identical history. Surprisingly, there is evidence of geographic subdivision within Atlantic populations of P. longicarpus along the east coast of North America. This differentiation is especially strong between Nova Scotia and southern populations, suggesting that the Nova Scotia population may represent survivors from a northern refugium during the last glacial maximum.     

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.     

Genetic evidence of cryptic speciation within hammerhead sharks (Genus Sphyrna)

Surveys of genetic variation within cosmopolitan marine species often uncover deep divergences, indicating historical separation and potentially cryptic speciation. Based on broad geographic (coastal eastern North America, Gulf of Mexico, western Africa, Australia, and Hawaii) and temporal sampling (1991–2003), mitochondrial (control region [CR] and cytochrome oxidase I [COI]) and nuclear gene (lactate dehydrogenase A intron 6 [LDHA6]) variation among 76 individuals was used to test for cryptic speciation in the scalloped hammerhead, Sphyrna lewini (Griffith and Smith). CR and COI gene trees confirmed previous evidence of divergence between Atlantic and Indo-Pacific scalloped hammerhead populations; populations were reciprocally monophyletic. However, the between-basin divergence recorded in the mtDNA genome was not reflected in nuclear gene phylogenies; alleles for LDHA6 were shared between ocean basins, and Atlantic and Indo-Pacific populations were not reciprocally monophyletic. Unexpectedly, CR, COI, and LDHA6 gene trees recovered a deep phylogenetic partition within the Atlantic samples. For mtDNA haplotypes, which segregated by basin, average genetic distances were higher among Atlantic haplotypes (CR: D _HKY=0.036, COI: D _GTR=0.016) than among Indo-Pacific haplotypes (CR: D _HKY=0.010, COI: D _GTR=0.006) and approximated divergences between basins for CR (D _HKY=0.036 within Atlantic; D _HKY=0.042 between basins). Vertebral counts for eight specimens representing divergent lineages from the western north Atlantic were consistent with the genetic data. Coexistence of discrete lineages in the Atlantic, complete disequilibrium between nuclear and mitochondrial alleles within lineages and concordant partitions in genetic and morphological characters indicates reproductive isolation and thus the occurrence of a cryptic species of scalloped hammerhead in the western north Atlantic. Effective management of large coastal shark species should incorporate this important discovery and the inference from sampling that the cryptic scalloped hammerhead is less abundant than S. lewini, making it potentially more susceptible to fishery pressure.     

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.     

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.     

Horizontal movements of Atlantic blue marlin (<Emphasis Type="Italic">Makaira nigricans</Emphasis>) in the Gulf of Mexico

We examined movements of Atlantic blue marlin (Makaira nigricans) from the Gulf of Mexico based upon 42 pop-up archival transmitting (PAT) tags. Long deployments (including one 334-day track) revealed diverse movement patterns within the Gulf of Mexico. North–south seasonal changes in blue marlin distribution showed strong correspondence with established seasonal patterns of sea surface temperature and primary production. During the summer spawning season, blue marlin utilized outer shelf and shelf edge waters in the northern Gulf of Mexico, and longer duration tracks indicated overwintering habitats in the Bay of Campeche. Egress occurred throughout the year and was difficult to determine because some tracks ended in the Straits of Florida (n = 3) while other tracks recorded movement through it or the Yucatan Channel (n = 4). Our results indicate that Atlantic blue marlin have a more restricted geographic range of habitats than previously recognized and that the Gulf of Mexico provides spatially dynamic suitable habitat that is utilized year-round through seasonal movements.     

Phylogeographic study of the dwarf oyster, Ostreola stentina, from Morocco, Portugal and Tunisia: evidence of a geographic disjunction with the closely related taxa, Ostrea aupouria and Ostreola equestris

Despite the economic importance of oysters due to the high aquaculture production of several species, the current knowledge of oyster phylogeny and systematics is still fragmentary. In Europe, Ostrea edulis, the European flat oyster, and Ostreola stentina, the Provence oyster or dwarf oyster, are both present along the European and African, Atlantic and Mediterranean, coasts. In order to document the relationship not only between O. stentina and O. edulis, but also with the other Ostrea and Ostreola species, we performed a sequence analysis of the 16S mitochondrial fragment (16S rDNA: the large subunit rRNA-coding gene) and the COI fragment (COI: cytochrome oxidase subunit I). Oysters were sampled from populations in Portugal (two populations), Tunisia (two populations) and Morocco (one population), identified as O. stentina on the basis of shell morphological characters. Our data supported a high degree of differentiation between O. stentina and O. edulis and a close relationship between O. stentina and both Ostrea aupouria (from New Zealand) and Ostreola equestris (from Mexico Gulf/Atlantic). The status of this geographic disjunction between these closely related species is discussed. Furthermore, although identified in a separate genus Ostreola by Harry (Veliger 28:121–158, 1985), our molecular data on O. stentina, together with those available for the other two putative congeneric species, O. equestris and Ostreola conchaphila, would favour incorporation of Ostreola in Ostrea. Finally, a PCR-RFLP approach allowed the rapid identification of O. edulis and O. stentina.     

Population genetic structure of North American <Emphasis Type="Italic">Ophiactis</Emphasis> spp. brittle stars possessing hemoglobin

With the discovery of previously unreported populations of hemoglobin-possessing Ophiactis from the Texas coast in the Gulf of Mexico, an investigation into its population structure, including populations of O. simplex from the Pacific coast of California and O. rubropoda from the Atlantic coast of Florida, was undertaken using DNA sequence data from the mitochondrial COI gene. The reconstructed haplotype network suggests that California populations contain the ancestral source of mtDNA variation, and there is no evidence of recent introductions into Texas. Population genetic analyses reveal the California, Florida, and Texas Ophiactis populations to each be significantly differentiated from one another. Sequence divergence among the three areas is shallower than would be predicted given biogeographic history. Texas and Florida populations are equally genetically diverged from California populations as they are to one another, despite the greater potential for gene flow between these areas. The genetic distinctiveness of the Texas populations and the concordance of this pattern with phylogeographic patterns in other brittle star systems indicate an isolated and independent evolutionary history and we hypothesize that the three geographic regions included in this study each serve as hypotheses of population-level lineages that remain to be tested with independent sources of data.     

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.     

Evolutionary relationships of deep-sea mussels inferred by mitochondrial DNA sequences

In order to elucidate the evolutionary process of deep-sea Bathymodiolus mussels, we investigated the phylogenetic relationships of 16 species worldwide by analyzing nucleotide sequences of the mitochondrial COI and ND4 genes. Deep-sea mussels were clustered into three groups by basal trichotomous divergence. The first was composed of four species found in Japanese waters and one species from the Gulf of Mexico, which contain methanotrophic endosymbiotic bacteria. The second included nine species distributed in the West and East Pacific, Indian, and Atlantic Oceans. Members of the second group were trichotomously divided into the Indo-West Pacific, Atlantic, and East Pacific subclusters. The Indo-West Pacific subcluster was composed of three very closely related species with mutual genetic distances at the intraspecific level (av. 0.019 in COI and 0.009 in ND4 relative to av. 0.156 in COI and 0.265 in ND4 among Bathymodiolus species other than Cluster A species), suggesting some gene flow among these species. The third consisted of two West Pacific species. Species in the second and third groups contain mainly thioautotrophic endosymbionts, including some species harboring both methanotrophs and thioautotrophs.     

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.     

Biochemical genetics of the sea anemone Bunodosoma cavernata and the zoogeography of the Gulf of Mexico

Much of the inshore fauna of the Gulf of Mexico has a disjunct distribution around the tip of Florida. Some of this fauna is composed of temperate, Carolinian species whose separation is thought to be due to warming after the last glacial period. Other species with this distribution are more eurythermal, and the vicariant event affecting them is unclear. One such eurythermal species is the sea anemone Bunodosoma cavernata. Using the genetic distance between Gulf and Atlantic populations of this species as an estimator of time since divergence, it is concluded that the vicariant event was the emergence of Florida in the Pliocene. It is suggested that the present Gulf Stream system maintains the separation of these populations.     

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     

Genetics and shell morphology in a hybrid zone between the hard clams Mercenaria mercenaria and M. campechiensis

Populations of Mercenaria mercenaria (L.) from South Carolina, USA, and M. campechiensis (Gmelin) from the Gulf of Mexico, Florida, USA, were sampled in 1987. The two species differed at all of seven enzyme loci tested, as well as in the thickness of shell ridges and nacre color. The difference in lunule shape was not great, although differences in relative shell width, shell weight, and lunule size make morphometric discrimination between the species possible. Shell ridges, nacre color, and multivariate morphometrics in a sample of clams collected from the Indian River Lagoon on the Atlantic coast of Florida in 1985 do not assort independently. Individuals with thick ridges, white nacre, and/or campechiensis-like morphometrics have significantly different allele frequencies at most enzyme loci from individuals with thin ridges, purple nacre, and/or mercenaria-like morphometrics. The deviations are in the direction predicted from the analysis of the allopatric populations of M. mercenaria and M. campechiensis. M. mercenaria outnumber M. campechiensis in the Indian River sample, but the majority of the clams seem to be hybrids.Contribution No. 235 from South Carolina Marine Resources Center     

Phylogeographic patterns of the mysid Mesopodopsis slabberi (Crustacea, Mysida) in Western Europe: evidence for high molecular diversity and cryptic speciation

The phylogeographic patterns among populations of Mesopodopsis slabberi (Crustacea, Mysida), an ecological important mysid species of marine and estuarine habitats, were analysed by means of DNA sequencing of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) and the 16S ribosomal RNA genes. Samples of M. slabberi collected from five Atlantic and two Western Mediterranean populations were investigated. Very high levels of within-population molecular diversity were observed in all samples (mean h=0.807 and π=0.0083), with exception of the Mediterranean Ebro population which contained only one haplotype. Differentiation among populations was high, and a clear phylogeographic break was observed between the Atlantic and Mediterranean populations. Moreover, a strong differentiation was detected between both populations in the Western Mediterranean Sea (Alicante and Ebro delta), while two divergent lineages occurred in sympatry within the Atlantic Mondego estuary. The high congruence between both the COI and 16S rRNA sequence data, the reciprocal monophyly of the different mitochondrial clades and the levels of nucleotide divergence between them suggest the presence of a complex of cryptic species within M. slabberi. Estimations of divergence time between the different mitochondrial lineages indicate that a split occurred during the late Miocene/early Pliocene. Such a divergence could be concordant with vicariant events during sea-level drops within the Mediterranean region at that time. However, within the Mediterranean Sea, the potential of divergence through ecological diversification cannot be ruled out.     

Genome-size variation in bivalve molluscs determined by flow cytometry

Six of the nine described species of the mole crab genus Emerita are distributed in the Americas, two [E. analoga (Stimpson, 1857) and E. rathbunae Schmitt, 1935] on the west coast, and four [E. benedicti Schmitt, 1935, E. brasiliensis Schmitt, 1935, E. portoricensis Schmitt, 1935 and E. talpoida (Say, 1817)] on the east. The presence of an extended planktonic larval stage in all Emerita species suggests high dispersal potential and the possibility of extensive gene flow among conspecific populations. Two taxa were sampled to study the extent of gene flow between widely separated conspecific populations: E. analoga (California and Chile) and E. talpoida (Massachusetts, South Carolina, and the west coast of Florida), while all other taxa were characterized from a single location. Portions of two mitochondrial genes, cytochrome oxidase I (COI) and 16S ribosomal RNA (16S rRNA) were sequenced. For data analysis, approximately 500 bp (COI) and 400 bp (16S rRNA) were examined. Estimated genetic divergence of 5.41% in COI between E. talpoida populations sampled from the Gulf of Mexico and the Atlantic coast, and 3.47% between E. analoga sampled in Chile and California, indicates that in both cases there has been no recent gene flow between disjunct populations. Additional molecular and morphological studies are necessary to decide whether disjunct populations should be accorded specific status. We predict that many marine invertebrates with antitropical distributions similar to E. analoga may consist of sibling species. In contrast to relationships inferred earlier from distribution patterns, parsimony analyses of both COI and 16S rRNA data yield similar phylogenetic trees in which E. analoga is separated from a clade composed of other species in the Americas; a bootstrap value (67%) in the COI inferred tree marginally supports the separation, but the same tree topology with a higher bootstrap value (84%) is obtained with 16S rRNA sequence data. Genetic divergence among the taxa indicates that the Emerita species constitute an old group and that distribution of species has been modified by past climatic and geological events.     

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.     

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     

A comparative seasonal study of two populations of Hypnea musciformis from the East and West Coasts of Florida,USA I. Growth and chemistry

Dry weight levels of the red alga Hypnea musciformis (Wulfen) Lamouroux from Atlantic and Gulf of Mexico coastal sites in florida, USA were lowest in the late winter and early spring, increasing through the summer and highest in the fall. There was a two-month lag in the Gulf coast population's dry weight pattern, indicating differing growth patterns. Chlorophyll a, phycoerythrin and phycoerythrin/chlorophyll a ratios were highest in the winter and lowest in the summer for both populations. Total pigment levels for H. musciformis from the Atlantic coast site were significantly greater than those of the Gulf coast. Protein and carbohydrate percentages were inversely related in both populations, with carbohydrate levels highest in summer and protein levels highest in winter. The Gulf coast population contained significantly more protein than the Atlantic coast plants. Carrageenan levels were highest in spring and lowest in fall, the Atlantic coast population generally had higher levels than the Gulf coast population. The differences in seasonal patterns and levels of the chemical constituents were reflected by distinct morphological characteristics for each population. The Atlantic coast population was larger, darker, more coarse in texture and possessed more crozier branch tips than the Gulf coast plants. These distinctions represent acclimitization responses that relate to habitat differences.