Evolutionary insights into the North American Necturus beyeri complex (Amphibia: Caudata) based on molecular genetic and morphological analyses

Necturus beyeri (Caudata: Proteidae), as conceived by some, contains paedomorphic salamanders distributed from the Ochlockonee drainage of Florida to the Angelina drainage of Texas. Because these salamanders differ in color pattern and karyotype across their geographic range, we performed a phylogeographic analysis that included representatives from all major drainages as well as of all congeners. The mitochondrially encoded ND2 gene was used to infer phylogenetic relationships using Bayesian inference. Morphometrics of head shape were analyzed and included as an independent data set. Our work suggests that Necturus comprises 11 lineages. A basal split within the genus separates an ancestor of two Atlantic Coastal Plain species (Necturus lewisi and Necturus punctatus) from the ancestor of nine distinct Gulf Coastal Plain lineages. One lineage is consistent with Necturus alabamensis, a species currently recognized in the Black Warrior drainage of Alabama. Two lineages comprise Necturus maculosus, as historically recognized, and six lineages comprise N. beyeri, as recognized by some, each of which occupies a unique drainage. Both of these species are demonstrated to be paraphyletic. Head morphometrics show the same patterns as the mtDNA. Overall, lineages within Necturus exhibit an east‐to‐west progression of appearance on the phylogenetic trees. This pattern corroborates biogeographic hypotheses based on previous karyological work. Within N. beyeri, this progression separates a pattern class of two eastern lineages lacking bold spotting and possessing relatively small mean body lengths from a pattern class of four western lineages possessing bold spotting and larger mean body sizes. Thus, the two eastern lineages of N. beyeri are similar in color pattern and body size to N. punctatus either through retention of the ancestral color pattern and size for the genus or through convergent selection in eastern streams of the Gulf Coastal Plain.     

Cryptic speciation in the ectocommensal Bdelloura candida (Platyhelminthes,Tricladida, Maricola) follows habitat specialization of the American horseshoe crab,Limulus polyphemus

In strict symbiotic associations, the genetic structure of the symbiont often mirrors that of its host, with interesting implications for population dynamics and phylogeography. An unresolved case of symbiotic specificity and phylogeographic consequence is the relationship between the marine triclad Bdelloura candida and its host, the American horseshoe crab, Limulus polyphemus. A recent study by Riesgo et al. (2017, Marine Biology, 164, 111) identified a strong genetic break between populations of B. candida in the Gulf of Mexico and the Atlantic Ocean but had minimal sampling around the Florida peninsula such that the exact location of the boundary zone was not specified. To solve this, a comprehensive analysis of 16S rRNA and ITS2 genetic markers was conducted from new collections around the Florida peninsula. A clear and significant genetic break was identified between populations of supposed B. candida between Cumberland Island, Georgia, and Mosquito Lagoon, Florida. This genetic break establishes two cryptic lineages, an Atlantic population as far south as Georgia and a Floridian population inclusive of the entire peninsula and Gulf of Mexico, potentially due to niche partitioning of the unique intertidal habitats of its horseshoe crab hosts in Florida. This result directly refutes the previous hypothesis that a population break exists between the coasts of the Atlantic Ocean and Gulf of Mexico, and instead matches the genetic break of its host. Furthermore, a third cryptic lineage was identified in Key West. Overall, this work demonstrates the challenges in maintaining genetic connections between populations of both B. candida and L. polyphemus across their distributions, and poses meaningful implications for both species in the larger context of marine conservation and biodiversity.     

Cryptic lineage divergence in marine environments: genetic differentiation at multiple spatial and temporal scales in the widespread intertidal goby Gobiosoma bosc

The adaptive radiation of the seven‐spined gobies (Gobiidae: Gobiosomatini) represents a classic example of how ecological specialization and larval retention can drive speciation through local adaptation. However, geographically widespread and phenotypically uniform species also do occur within Gobiosomatini. This lack of phenotypic variation across large geographic areas could be due to recent colonization, widespread gene flow, or stabilizing selection acting across environmental gradients. We use a phylogeographic approach to test these alternative hypotheses in the naked goby Gobiosoma bosc, a widespread and phenotypically invariable intertidal fish found along the Atlantic Coast of North America. Using DNA sequence from 218 individuals sampled at 15 localities, we document marked intraspecific genetic structure in mitochondrial and nuclear genes at three main geographic scales: (i) between Gulf of Mexico and Atlantic Coast, (ii) between the west coast of the Florida peninsula and adjacent Gulf of Mexico across the Apalachicola Bay, and (iii) at local scales of a few hundred kilometers. Clades on either side of Florida diverged about 8 million years ago, whereas some populations along the East Cost show divergent phylogroups that have differentiated within the last 200,000 years. The absence of noticeable phenotypic or ecological differentiation among lineages suggests the role of stabilizing selection on ancestral phenotypes, together with isolation in allopatry due to reduced dispersal and restricted gene flow, as the most likely explanation for their divergence. Haplotype phylogenies and spatial patterns of genetic diversity reveal frequent population bottlenecks followed by rapid population growth, particularly along the Gulf of Mexico. The magnitude of the genetic divergence among intraspecific lineages suggests the existence of cryptic species within Gobiosoma and indicates that modes of speciation can vary among lineages within Gobiidae.     

Biotic interactions mediate the expansion of black mangrove (Avicennia germinans) into salt marshes under climate change

Many species are expanding their distributions to higher latitudes due to global warming. Understanding the mechanisms underlying these distribution shifts is critical for better understanding the impacts of climate changes. The climate envelope approach is widely used to model and predict species distribution shifts with changing climates. Biotic interactions between species, however, may also influence species distributions, and a better understanding of biotic interactions could improve predictions based solely on climate envelope models. Along the northern Gulf of Mexico coast, USA, subtropical black mangrove (Avicennia germinans) at the northern limit of its distribution grows sympatrically with temperate salt marsh plants in Florida, Louisiana, and Texas. In recent decades, freeze‐free winters have led to an expansion of black mangrove into salt marshes. We examined how biotic interactions between black mangrove and salt marsh vegetation along the Texas coast varied across (i) a latitudinal gradient (associated with a winter‐temperature gradient); (ii) the elevational gradient within each marsh (which creates different marsh habitats); and (iii) different life history stages of black mangroves (seedlings vs. juvenile trees). Each of these variables affected the strength or nature of biotic interactions between black mangrove and salt marsh vegetation: (i) Salt marsh vegetation facilitated black mangrove seedlings at their high‐latitude distribution limit, but inhibited black mangrove seedlings at lower latitudes; (ii) mangroves performed well at intermediate elevations, but grew and survived poorly in high‐ and low‐marsh habitats; and (iii) the effect of salt marsh vegetation on black mangroves switched from negative to neutral as black mangroves grew from seedlings into juvenile trees. These results indicate that the expansion of black mangroves is mediated by complex biotic interactions. A better understanding of the impacts of climate change on ecological communities requires incorporating context‐dependent biotic interactions into species range models.     

Selachohemecus benzi n. sp. (Digenea: Sanguinicolidae) from the blacktip shark Carcharhinus limbatus (Carcharhinidae) in the northern Gulf of Mexico

Selachohemecus benzi Bullard & Overstreet n. sp. infects the heart and kidney of the blacktip shark Carcharhinus limbatus in the northern Gulf of Mexico off Florida and Mississippi, USA. Specimens of S.␣olsoni Short, 1954, the only congener and only other named blood fluke reported from a chondrichthyan in the Gulf of Mexico, were collected from the heart of the Atlantic sharpnose shark Rhizoprionodon terraenovae from two new localities, Apalachicola Bay, Florida, and Mississippi Sound, Mississippi, USA. The new species differs from S. olsoni by having a larger body (1.4–3.8 mm long), robust tegumental body spines numbering 51–63 along each lateral body margin, a testis extending from the posterior caeca to the ovary, and a medial ovary with lobes. We amend the diagnosis of Selachohemecus Short, 1954 to accommodate it and provide a diagnostic key for all named chondrichthyan blood flukes.     

Evidence of multiple vicariance in a marine suture‐zone in the Gulf of Mexico

Aim Our aim was to assess whether single or multiple vicariance event(s) have occurred in a marine suture‐zone in the northern Gulf of Mexico. We estimated the divergence time of two genetically distinct groups of lane snapper (Lutjanus synagris), distributed on either side of this zone, and compared it to the timing of other vicariance events described previously in the area. Location The northern Gulf of Mexico from the Texas coastline to the Atlantic coast of South Florida, USA. The marine suture‐zone is proximal to Mobile Bay in Alabama. Methods A Bayesian Markov chain Monte Carlo (MCMC) approach was used to estimate mutation‐corrected divergence and genetic migration between eastern and western groups of lane snapper, based on genotypic differences at 13 nuclear‐encoded microsatellites obtained previously from 77 and 171 individual samples, respectively. A second estimate of divergence time for eastern and western groups of lane snapper was generated, based on 590 base pairs of DNA sequence from the mitochondrially encoded NADH dehydrogenase subunit 4 (ND4) obtained previously from 45 and 93 individual samples, respectively. Results Both classes of genetic markers (microsatellites and mtDNA) indicated a fairly recent divergence. Confidence intervals for microsatellite‐based estimates suggested that divergence began less than 21 thousand years ago (ka), while confidence intervals for mitochondrial DNA‐based estimates suggested divergence began less than 130 ka. These estimates were not consistent with vicariance events in the literature, which are hypothesized to have occurred well before 135 ka. Main conclusions These results indicate that the marine suture‐zone in the northern Gulf of Mexico features multiple vicariance events and may be characterized by a complex geological/environmental history. We suggest that processes during or after the Wisconsin glaciation (c. 110–18 ka) may have created a previously unrecognized barrier for lane snapper and possibly other species as well.     

INCREASED SAMPLING FOR INFERRING PHYLOGEOGRAPHIC PATTERNS IN BOSTRYCHIA RADICANS/B. MORITZIANA (RHODOMELACEAE,RHODOPHYTA) IN THE EASTERN USA1

Zuccarello and West (2003) reported on the phylogenetic diversity of algae identified as Bostrychia radicans (Montagne) Montagne and B. moritziana (Sonder ex Kützing) J. Agardh from around the world. They showed that the species complex consisted of seven distinct lineages, of which two lineages were common on the East Coast of the USA and eastern Gulf of Mexico. The distribution of haplotypes within these lineages on the East Coast of the USA showed a general north–south distribution. One haplotype of lineage 5 (B) was mostly collected in northern areas, while the other common haplotype (C) was more southerly in distribution. Samples in lineage 6 (haplotype D) were not found north of Sapelo Island, Georgia. Increased sampling from the eastern USA over 5 years later has revealed an altered pattern. Haplotype D is distributed in North Carolina and is common in some populations. Haplotype C is rare or absent in many sampled populations. Haplotype B is only observed in the northern sampled sites on both sides of the Florida peninsula. This disjunct distribution agrees with geological scenarios for a strait between the western Gulf of Mexico and southern Georgia in the Miocene/Pliocene, which closed in the late Pliocene. This paper highlights the importance of increased sampling to determine phylogeographic patterns and hypotheses of dispersal scenarios in algae.     

Genetic relationships of the marine invasive crab parasite <Emphasis Type="Italic">Loxothylacus panopaei</Emphasis>: an analysis of DNA sequence variation,host specificity,and distributional range

Host specificity is a key variable of the niche breath of parasites that can be an important determinant of a parasite’s ability to invade new areas. There is increasing evidence that many parasite species may comprise a variety of genetically variable lineages, which differ in host specificity and geographic range. In this study, we (1) explored the extent of diversity in the invasive parasitic barnacle Loxothylacus panopaei (Rhizocephala) infecting mud crabs (2) examined the geographic origin for the invasive lineage and (3) assessed if further southward spread of the parasite may be impeded. Along the US Atlantic coast, L. panopaei infects different hosts in its invaded range (Chesapeake Bay to north of Cape Canaveral) compared to one portion of the native range in Southeast Florida. This difference was reflected in genetic lineages on two independent loci, mitochondrial cytochrome oxidase I and nuclear cytochrome c. Both loci were concordant in that they showed one lineage infecting crabs of the genus Panopeus in the native range and one lineage infecting Eurypanopeus depressus and Rhithropanopeus harrisii hosts in the invaded range and in the Gulf of Mexico, thus indicating Gulf of Mexico populations as the most likely source of introduction into Chesapeake Bay. Interestingly, the nuclear marker resolved an additional lineage of parasites infecting panopeid hosts in the native range. All three parasite lineages were well supported, but a decision about species status must await further analyses. Since its introduction in the 1960s, the invasive L. panopaei lineage has expanded its range southward along the US Atlantic coast, now almost reaching the northern limit of native Panopeus-infecting lineages at Cape Canaveral, Florida. We hypothesized that parasite-free E. depressus in Southeast Florida, living in sympatry with infected panopeid populations, might be resistant to infection by the invasive lineage. Our infection experiments rejected this hypothesis, suggesting that any impediment to further southward range expansion might be expected from temperature regimes of the subtropical zoogeographic region south of Cape Canaveral.     

Phylogeographic patterns of a lower intertidal isopod in the Gulf of California and the Caribbean and comparison with other intertidal isopods

A growing body of knowledge on the diversity and evolution of intertidal isopods across different regions worldwide has enhanced our understanding on biological diversification at the poorly studied, yet vast, sea–land interface. High genetic divergences among numerous allopatric lineages have been identified within presumed single broadly distributed species. Excirolana mayana is an intertidal isopod that is commonly found in sandy beaches throughout the Gulf of California. Its distribution in the Pacific extends from this basin to Colombia and in the Atlantic from Florida to Venezuela. Despite its broad distribution and ecological importance, its evolutionary history has been largely neglected. Herein, we examined phylogeographic patterns of E. mayana in the Gulf of California and the Caribbean, based on maximum‐likelihood and Bayesian phylogenetic analyses of DNA sequences from four mitochondrial genes (16S rDNA, 12S rDNA, cytochrome oxidase I gene, and cytochrome b gene). We compared the phylogeographic patterns of E. mayana with those of the coastal isopods Ligia and Excirolana braziliensis (Gulf of California and Caribbean) and Tylos (Gulf of California). We found highly divergent lineages in both, the Gulf of California and Caribbean, suggesting the presence of multiple species. We identified two instances of Atlantic–Pacific divergences. Some geographical structuring among the major clades found in the Caribbean is observed. Haplotypes from the Gulf of California form a monophyletic group sister to a lineage found in Venezuela. Phylogeographic patterns of E. mayana in the Gulf of California differ from those observed in Ligia and Tylos in this region. Nonetheless, several clades of E. mayana have similar distributions to clades of these two other isopod taxa. The high levels of cryptic diversity detected in E. mayana also pose challenges for the conservation of this isopod and its fragile environment, the sandy shores.     

Development and application of LSU rRNA probes for Karenia brevis in the Gulf of Mexico, USA

The brevetoxin producing dinoflagellate, Karenia brevis, is the target of several monitoring and research programs in the Gulf of Mexico, where it forms extensive and frequently long-lived annual blooms that can cause human intoxication and fish kills, as well as severe economic losses to coastal communities. Rapid, reliable methods for the detection and enumeration of K. brevis cells, as well as their discrimination from morphologically similar species, are valuable tools for managers and scientists alike. Our aim was to produce a species-specific molecular probe that would serve as a tool to facilitate the efficient and reliable detection of K. brevis in the Gulf of Mexico. We sequenced a fragment of the large-subunit ribosomal RNA gene (LSU rDNA) from five K. brevis cultures isolated from the Texas Gulf coast, the Florida Gulf coast, and the Atlantic coast of Florida, and detected no differences among these isolates. A consensus sequence was thus compiled and compared to a previously published sequence from Karenia mikimotoi, the closest known phylogenetic relative to K. brevis, for the purpose of identifying unique K. brevis signature sequences. Fluorescently-labeled (FITC) oligonucleotide probes targeting these regions of the K. brevis LSU rRNA were designed to include at least two base pair differences, as compared to K. mikimotoi. Among seven probes designed, one uniquely identified all K. brevis isolates to the exclusion of all other species tested (Kbprobe-7), including a Gulf of Mexico K. mikimotoi isolate (Sarasota, FL) and several additional Gymnodinium species, as well as other dinoflagellate, diatom, and raphidophyte taxa. Importantly, K. brevis cells in samples taken during a 2001 bloom, fixed with a mixture of modified saline ethanol and 10% formalin, and stored at 4 °C for 7 months were successfully labeled with Kbprobe-7. In addition, preliminary analysis of labeled cells by flow cytometry revealed that K. brevis could be distinguished from K. mikimotoi in solution, suggesting other potential applications of this probe.     

Population Genetics of Seaside Sparrow (Ammodramus maritimus) Subspecies along the Gulf of Mexico

Seaside Sparrows (Ammodramus maritimus) along the Gulf of Mexico are currently recognized as four subspecies, including taxa in Florida (A. m. juncicola and A. m. peninsulae) and southern Texas (Ammodramus m. sennetti), plus a widespread taxon between them (A. m. fisheri). We examined population genetic structure of this “Gulf Coast” clade using microsatellite and mtDNA data. Results of Bayesian analyses (Structure, GeneLand) of microsatellite data from nine locations do not entirely align with current subspecific taxonomy. Ammodramus m. sennetti from southern Texas is significantly differentiated from all other populations, but we found evidence of an admixture zone with A. m. fisheri near Corpus Christi. The two subspecies along the northern Gulf Coast of Florida are significantly differentiated from both A. m. sennetti and A. m. fisheri, but are not distinct from each other. We found a weak signal of isolation by distance within A. m. fisheri, indicating this population is not entirely panmictic throughout its range. Although continued conservation concern is warranted for all populations along the Gulf Coast, A. m. fisheri appears to be more secure than the far smaller populations in south Texas and the northern Florida Gulf Coast. In particular, the most genetically distinct populations, those in Texas south of Corpus Christi, occupy unique habitats within a very small geographic range.     

Marine gastrotrichs from the sand beaches of the northern Gulf of Mexico: species list and distribution

In this meio-faunistic survey along the northern coast of the Gulf of Mexico, gastrotrichs were found in sand collected mostly from beaches on barrier islands. Sediment from Florida and Alabama contained the largest species number. South Texas collecting sites also hosted a very diverse gastrotrich fauna. Paucitubulate Chaetonotida, previously unreported from the area, accounted for about one half of the 45 species encountered. After comparing local specimens also with high resolution videosequences of individuals collected from distant geographic regions, the amphi-Atlantic and/or cosmopolitan distribution of 27 of these species, is confirmed.     

PHYLOGEOGRAPHY OF GRACILARIA TIKVAHIAE (GRACILARIACEAE,RHODOPHYTA): A STUDY OF GENETIC DISCONTINUITY IN A CONTINUOUSLY DISTRIBUTED SPECIES BASED ON MOLECULAR EVIDENCE1

Gracilaria tikvahiae, a highly morphologically variable red alga, is one of the most common species of Gracilariaceae inhabiting Atlantic estuarine environments and the Intracoastal Waterway of eastern North America. Populations of G. tikvahiae at the extremes of their geographic range (Canada and southern Mexico) are subjected to very different environmental regimes. In this study, we used two types of genetic markers, the chloroplast‐encoded rbcL and the nuclear internal transcribed spacer (ITS) region, to examine the genetic variability within G. tikvahiae, for inferring the taxonomic and phylogenetic relationships between geographically isolated populations, and to discuss its distributional information in a phylogeographic framework. Based on rbcL and ITS phylogenies, specimens from populations collected at the extreme distributional ranges reported for G. tikvahiae are indeed part of the same species; however, rbcL‐ but not ITS‐based phylogenies detected phylogenetic structure among the ten G. tikvahiae different haplotypes found in this study. The four distinct rbcL lineages were identified as 1) a Canadian–northeast U.S. lineage, 2) a southeast Florida lineage, 3) an eastern Gulf of Mexico lineage, and 4) a western Gulf of Mexico lineage. We found no evidence for the occurrence of G. tikvahiae in the Caribbean Sea. Observed phylogeographic patterns match patterns of genetic structures reported for marine animal taxa with continuous and quasicontinuous geographic distribution along the same geographic ranges.     

Wolbachia pipientis occurs in Aedes aegypti populations in New Mexico and Florida,USA

The mosquitoes Aedes aegypti (L.) and Ae. albopictus Skuse are the major vectors of dengue, Zika, yellow fever, and chikungunya viruses worldwide. Wolbachia, an endosymbiotic bacterium present in many insects, is being utilized in novel vector control strategies to manipulate mosquito life history and vector competence to curb virus transmission. Earlier studies have found that Wolbachia is commonly detected in Ae. albopictus but rarely detected in Ae. aegypti. In this study, we used a two‐step PCR assay to detect Wolbachia in wild‐collected samples of Ae. aegypti. The PCR products were sequenced to validate amplicons and identify Wolbachia strains. A loop‐mediated isothermal amplification (LAMP) assay was developed and used for detecting Wolbachia in selected mosquito specimens as well. We found Wolbachia in 85/148 (57.4%) wild Ae. aegypti specimens from various cities in New Mexico, and in 2/46 (4.3%) from St. Augustine, Florida. Wolbachia was not detected in 94 samples of Ae. aegypti from Deer Park, Harris County, Texas. Wolbachia detected in Ae. aegypti from both New Mexico and Florida was the wAlbB strain of Wolbachia pipientis. A Wolbachia‐positive colony of Ae. aegypti was established from pupae collected in Las Cruces, New Mexico, in 2018. The infected females of this strain transmitted Wolbachia to their progeny when crossed with males of Rockefeller strain of Ae. aegypti, which does not carry Wolbachia. In contrast, none of the progeny of Las Cruces males mated to Rockefeller females were infected with Wolbachia.     

Fine‐scale structure among mesophotic populations of the great star coral Montastraea cavernosa revealed by SNP genotyping

Mesophotic reefs (30‐150 m) have been proposed as potential refugia that facilitate the recovery of degraded shallow reefs following acute disturbances such as coral bleaching and disease. However, because of the technical difficulty of collecting samples, the connectivity of adjacent mesophotic reefs is relatively unknown compared with shallower counterparts. We used genotyping by sequencing to assess fine‐scale genetic structure of Montastraea cavernosa at two sites at Pulley Ridge, a mesophotic coral reef ecosystem in the Gulf of Mexico, and downstream sites along the Florida Reef Tract. We found differentiation between reefs at Pulley Ridge (~68 m) and corals at downstream upper mesophotic depths in the Dry Tortugas (28–36 m) and shallow reefs in the northern Florida Keys (Key Biscayne, ~5 m). The spatial endpoints of our study were distinct, with the Dry Tortugas as a genetic intermediate. Most striking were differences in population structure among northern and southern sites at Pulley Ridge that were separated by just 12km. Unique patterns of clonality and outlier loci allele frequency support these sites as different populations and suggest that the long‐distance horizontal connectivity typical of shallow‐water corals may not be typical for mesophotic systems in Florida and the Gulf of Mexico. We hypothesize that this may be due to the spawning of buoyant gametes, which commits propagules to the surface, resulting in greater dispersal and lower connectivity than typically found between nearby shallow sites. Differences in population structure over small spatial scales suggest that demographic constraints and/or environmental disturbances may be more variable in space and time on mesophotic reefs compared with their shallow‐water counterparts.     

Presence of nuclear copies of mitochondrial origin (NUMTs) in two related species of stingless bee genus Melipona (Hymenoptera: Meliponini)

Transferred copies of mitochondrial DNA (mtDNA) into the nuclear genome (numts) have been reported in several Hymenoptera species, even at a high density in the honey bee nuclear genome. The accidental amplification of numts in phylogenetic studies focused on mtDNA highlights the importance of a correct determination of numts and their related mtDNA sequences. We report here the presence of numts derived from a mitochondrial rDNA 16S gene in the genome of the stingless bee species Melipona colimana and M. fasciata (tribe Meliponini) from Western Mexico. PCR products were cloned in both species obtaining thirty paralogous numts. Numts were identified by the presence of insertions and deletions and the disruption of the 16S secondary structure. Further phylogenetic analyses including alternative mitochondrial cox1 and nuclear ITS1 genes have revealed the presence of another numt (cox1) in the nuclear genome of these two species, and place both as sister lineages within the subgenus Michmelia. This is one of the first studies reporting the presence of numts in Meliponini species, and supports previous studies suggesting frequent transfer of mtDNA to the nuclear genome in Hymenoptera.     

Spatial genetic features of eastern oysters (Crassostrea virginica Gmelin) in the Gulf of Mexico: northward movement of a secondary contact zone

The eastern oyster (Crassostrea virginica Gmelin) is an economically and ecologically valuable marine bivalve occurring in the Gulf of Mexico. This study builds upon previous research that identified two divergent populations of eastern oysters in the western Gulf of Mexico. Allelic and genotypic patterns from 11 microsatellite markers were used to assess genetic structure and migration between the previously described oyster populations in Texas. The main findings are as follows: (1) there are two distinct populations (F_ST = 0.392, P < 0.001) of oysters that overlap in the Corpus Christi/Aransas Bay estuarine complex in Texas, (2) the distribution of genotypes among individuals in the contact zone suggests limited hybridization between populations, (3) the variables of salinity, temperature, dissolved oxygen, turbidity and depth are not correlated with allele frequencies on reefs in the contact zone or when analyzed across Texas, and (4) there is little evidence of directional selection acting on the loci assayed here, although patterns at four markers suggested the influence of balancing selection based on outlier analyses. These results are consistent with long‐term historical isolation between populations, followed by secondary contact. Recent hydrological changes in the area of secondary contact may be promoting migration in areas that were previously inhospitable to eastern oysters, and observed differences in the timing of spawning may limit hybridization between populations. Comparison of these findings with the results of an earlier study of oysters in Texas suggests that the secondary contact zone has shifted approximately 27 km north, in as little as a 23‐year span.     

A bridge between oceans: overland migration of marine birds in a wind energy corridor

Located at the shortest overland route between the Gulf of Mexico and the Pacific Ocean, Mexico's Tehuantepec Isthmus is a globally important migratory corridor for many terrestrial bird species. The Pacific coast of the Isthmus also contains a significant wetland complex that supports large multi‐species aggregations of non‐breeding waterbirds during the boreal winter. In recent years, extensive wind energy development has occurred in the plains bordering these wetlands, directly along the migratory flyway. Using recent studies of movement patterns of three marine‐associated bird species – reddish egrets Egretta rufescens, brown pelicans Pelecanus occidentalis, and red knots Calidris canutus – from the northern Gulf of Mexico, we assess the use of the isthmus as a migratory corridor. Our data provide evidence that marine birds from the Gulf region regularly overwinter along the Pacific coast of Mexico and use the isthmus as a migratory corridor, creating the potential for interaction with terrestrial wind farms during non‐breeding. This study is the first to describe migration by marine‐associated bird species between the Gulf of Mexico and Pacific coast. These data contribute new information toward ongoing efforts to understand the complex migration patterns of mobile marine species, with the goal of informing integrated conservation efforts for species whose year‐round habitat needs cross ecoregional and geopolitical boundaries.     

Taxonomy and biogeography of the fiddler crabs (Ocypodidae: Genus Uca) of the Atlantic and Gulf coasts of eastern North America

Fifteen species of fiddler crabs are reported for eastern North America between Massachusetts and Quintana Roo, Mexico. Thirteen occur in the United States and 11 in Mexico, with eight in common to the two countries. Of 13 species in the Gulf of Mexico, five are endemic and a sixth is restricted largely to the peninsulas of Florida and Yucatan. The status of U. rapax in the northern Gulf remains to be resolved. Range limits of most species approximate one or the other of two sets of intersecting thermal and geological boundaries that subdivide the Gulf of Mexico along north-south and east-west axes. Species belonging to subgenus Minuca tend to replace one another at the thermally-controlled Carolinian-Caribbean marine biotic boundary across the Florida peninsula and northern Gulf. However, only U. minax of all the North American fiddler crabs exhibits the classical disjunct Carolinian distribution, and this appears basically to reflect the discontinuous distribution of temperate salt marshes that are the habitat of the species. Distributions of species belonging to subgenus Celuca adhere for the most part to the subdivision of the Gulf into western terrigenous and eastern carbonate sedimentary provinces. The northern transition occurs in the vicinity of Apalachee Bay and the southern at Laguna de Terminos. A third distributional pattern is shown by U. subcylindrica , a specialized endemic species of the hypersaline Laguna Madre system of the western Gulf. The level of endemism in the fiddler crabs is relatively high in comparison with that of other marine groups within the Gulf of Mexico. This may be a consequence of the adaptations of fiddler crabs as specialized deposit feeders to regional differences in climatic and edaphic characteristics of a marginally marine upper shore habitat. The distributional patterns of the endemics could prove useful in reconstructing palaeoecological events of evolutionary significance within the Gulf of Mexico.     

Clinal variation at aspartate aminotransferase-2 in spotted seatrout,Cynoscion nebulosus (Cuvier), inhabiting the north-western Gulf of Mexico

Clinal variation was observed at the aspartate aminotransferase-2 (AAT-2) locus in the marine fish Cynoscion nebulosus (Cuvier) inhabiting the bays and estuaries of the Texas and northern Mexico Gulf coasts. Frequency of the AAT-2(80) allele increased from 0.9% at Sabine Lake, Texas to 17.1% at Rio Soto La Marina, Mexico. A statistically significant correlation existed in the frequency of this allele with degrees north latitude and west longitude. This information, if properly incorporated into a comprehensive enhancement programme, could facilitate supplemental stocking success.