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Abundance surveys are a common and practical technique to estimate plant or animal densities. Methods used to conduct these surveys often require jointly estimating uncertainty in both counts and detection probability. The estimation of detection probability requires additional measurements that take time, potentially reducing the efficiency of the survey for high-density populations. We conducted quadrat, removal, and distance surveys of zebra mussels (Dreissena polymorpha) in three central Minnesota lakes and used an efficiency measure to help compare survey designs. Survey efficiency depended on both the survey design and the population density. The efficiency of survey designs that required estimates of detection probabilities (relative to designs that did not) decreased with density, leading to a change in the most efficient survey design at high densities. These results demonstrate that the best survey design may be context-specific, requiring some prior knowledge of the underlying population density and the cost/time needed to collect additional information for estimating detection probabilities.

The zebra mussel, Dreissena polymorpha, continues to spread from its native range in Eurasia to Europe and North America, causing billions of dollars in damage and dramatically altering invaded aquatic ecosystems. Despite these impacts, there are few genomic resources for Dreissena or related bivalves. Although the D. polymorpha genome is highly repetitive, we have used a combination of long-read sequencing and Hi-C-based scaffolding to generate a high-quality chromosome-scale genome assembly. Through comparative analysis and transcriptomics experiments, we have gained insights into processes that likely control the invasive success of zebra mussels, including shell formation, synthesis of byssal threads, and thermal tolerance. We identified multiple intact steamer-like elements, a retrotransposon that has been linked to transmissible cancer in marine clams. We also found that D. polymorpha have an unusual 67 kb mitochondrial genome containing numerous tandem repeats, making it the largest observed in Eumetazoa. Together these findings create a rich resource for invasive species research and control efforts.

Estimating the density and distribution of invasive populations is critical for management and control efforts but can be a challenge in nascent infestations when population densities are l...

Advances in sequencing and informatics and rapidly falling costs have made genome sequencing projects far more accessible to researchers in all of the life sciences, including invasion biology. A complete genome is now the most efficient first step towards identifying and characterizing candidate genes that control invasiveness. At the genomic level, fundamental problems in invasion science can be pursued with great precision and rigor. This includes reconstruction of the history of invasions, analysis of demographic dynamics within colonizing populations, and study of the rapid, adaptive evolution of invasiveness. This update documents new developments in the emerging field of invasion genomics. Our review found that of 100 of the world’s most damaging invasive species, assembled genomes are available for 27—a minority but still a considerable resource. This calls for a larger investment in genomics, but also highlights publicly available genomic resources for invasive species that remain underutilized. We examine the value of reference genomes. We conclude that while some technologies (e.g. genotyping by Next Generation Sequencing) can be applied without reference genomes or with fragmented ones, investments in high quality genome assemblies will provide considerable long-term benefits in invasion and conservation genomics research programs.

Invasion genetic studies show great promise for inferring sources, pathways and vectors of spread, but have to date focused on introductions from native ranges at continental scales and larger. Here we present genetic analyses of post-introduction (i.e. secondary) spread of zebra mussels at the scale of a US state, Minnesota (MN). We genotyped 9 microsatellite DNA loci in 2047 zebra mussels collected from 40 lakes and 4 rivers that geographically and chronologically span the MN invasion. We analyzed genetic variation, geographic differentiation, and genetic clustering of populations across water bodies, and performed invasion-scenario contrasts using Approximate Bayesian Computation. Our population genetic analyses revealed that the pattern of spread of zebra mussels to inland lakes did not conform to the hub-lake model that is often proposed for this species. Consistent with a stratified dispersal model, lake rich regions were colonized from afar, followed by spread within regions by short-distance dispersal, but high-traffic hub lakes made no contribution. For the first time, we obtained evidence about the pattern of spread of zebra mussels to inland lakes that is not based on inference from analysis of boater movements; rather it derives from direct evaluation of genetic attributes of invasive populations. Local introductions overwhelmed more distant ones in three independently invaded regions, suggesting a need to understand the lake colonization process and identify the vectors responsible.

Aquatic invasive species (AIS) are of concern in North America due to their devastating impacts on ecosystems and economies. The Great Lakes region is particularly vulnerable to AIS introduction an...

A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10-year period: May 2004 - May 2014. Daily profiles of subinertial velocity, temperature, salinity and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8 - 28.125 kg/m3 (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 x 106 m3/s ± 1.9 x 106 m3/s is obtained. Notably, a statistically-significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 x 106 m3/s at the beginning of the field program to 5.8 x 106 m3/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 x 106 m3/s to 19.1 x 106 m3/s. In contrast, no long-term trend is observed in upper-ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.

The zebra mussel,Dreissena polymorpha, continues to spread from its native range in Eurasia to Europe and North America, causing billions of dollars in damage and dramatically altering invaded aquatic ecosystems. Despite these impacts, there are few genomic resources forDreissenaor related bivalves, with nearly 450 million years of divergence between zebra mussels and its closest sequenced relative. Although theD. polymorphagenome is highly repetitive, we have used a combination of long-read sequencing and Hi-C-based scaffolding to generate the highest quality molluscan assembly to date. Through comparative analysis and transcriptomics experiments we have gained insights into processes that likely control the invasive success of zebra mussels, including shell formation, synthesis of byssal threads, and thermal tolerance. We identified multiple intact Steamer-Like Elements, a retrotransposon that has been linked to transmissible cancer in marine clams. We also found thatD. polymorphahave an unusual 67 kb mitochondrial genome containing numerous tandem repeats, making it the largest observed in Eumetazoa. Together these findings create a rich resource for invasive species research and control efforts.

The byssus is a structure unique to bivalves. Byssal threads composed of many proteins extend like tendons from muscle cells, ending in adhesive pads that attach underwater. Crucial to settlement and metamorphosis, larvae of virtually all species are byssate. By contrast, in adults, the byssus is scattered throughout bivalves, where it has had profound effects on morphological evolution and been key to adaptive radiations of epifaunal species. I compare byssus structure and proteins in blue mussels ( Mytilus ), by far the best characterized, to zebra mussels ( Dreissena polymorpha ), in which several byssal proteins have been isolated and sequenced. By mapping the adult byssus onto a recent phylogenomic tree, I confirm its independent evolution in these and other lineages, likely parallelisms with common origins in development. While the byssus is superficially similar in Dreissena and Mytilus , in finer detail it is not, and byssal proteins are dramatically different. I used the chromosome-scale D. polymorpha genome we recently assembled to search for byssal genes and found 37 byssal loci on 10 of the 16 chromosomes. Most byssal genes are in small families, with several amino acid substitutions between paralogs. Byssal proteins of zebra mussels and related quagga mussels ( D. rostriformis ) are divergent, suggesting rapid evolution typical of proteins with repetitive low complexity domains. Opportunities abound for proteomic and genomic work to further our understanding of this textbook example of a marine natural material. A priority should be invasive bivalves, given the role of byssal attachment in the spread of, and ecological and economic damage caused by zebra mussels, quagga mussels and others. This article is part of the Theo Murphy meeting issue ‘Molluscan genomics: broad insights and future directions for a neglected phylum’.

About 70% of freshwater mussel species in North America are threatened, endangered, or recently extinct. A large number of the surviving species are endemics. One hotspot for endemic diversity is the southeastern U.S.A. Lake Waccamaw in southeastern North Carolina contains two putative endemic unionids, Lampsilis fullerkati (Johnson, 1984) and Elliptio waccamawensis (Lea, 1863). We used multiple phylogenetic analyses to assess phylogenetic affinities and test the status of these named species. Mitochondrial 16S rRNA gene, cox1, and nad1 sequences were obtained from 109 individuals of the putative endemics and potentially related species from Lake Waccamaw, the Waccamaw River, and the Yadkin/Pee Dee, Little Pee Dee, and Lumber Rivers in the Pee Dee Drainage. In addition to the analysis of each individual gene region, a total evidence analysis, in which all three regions were combined, was conducted using Bayesian phylogenetic methods. All three genes were analyzed using both Bayesian and maximum ...

The Antarctic Slope Current (ASC), defined here as the region of westward flow along the continental slope off Antarctica, forms the southern limb of the subpolar gyres. It regulates the exchange of water across the shelf break, and provides a path for interbasin westward transport. Despite its significance, the ASC remains largely unobserved around most of the Antarctic continent. Here we present direct velocity observations from a 17-month current meter moored array deployed across the continental slope between the 1000 and the 4200 m isobaths, in the southeastern Indian Ocean near 113°E. The observed time-mean flow consists of a surface intensified jet associated with the Antarctic Slope Front (ASF) and a broader bottom intensified westward flow that extends out to approximately the 4000 m isobath and is strongest along the upper slope. The time-mean transport of the ASC is -29.2 Sv. Fluctuations in the transport are large, typically exceeding the mean by a factor of 2. They are mainly due to changes in the northward extent of the current over the lower slope. However, seasonal changes in the wind also drive variations in the transport of the ASF and the flow in the upper slope. Both mean and variability are largely barotropic, thus invisible to traditional geostrophic methods. This article is protected by copyright. All rights reserved.

Determination of stock structure is an important component of fisheries management; incorporation of molecular genetic data is an effective method for assessing differentiation among putative populations. We examined genetic variation in Southern Flounder Paralichthys lethostigma within and between the U.S. South Atlantic and Gulf of Mexico basins to improve our understanding of the scale of population structure in this wide-ranging species. Analysis of amplified fragment length polymorphism (AFLP) fingerprints and analysis of mitochondrial DNA (mtDNA) control region sequences found clear divergence between ocean basins. Based on mtDNA sequences, no genetic differentiation was detected within the U.S. South Atlantic at spatial scales that were broad (among states: North Carolina, South Carolina, Georgia, and Florida) or fine (among estuarine regions within North Carolina). Increased genetic resolution was observed with AFLP fingerprint data, and we found significant subdivision between nearly all ...

Aim Phylogeographical ‘breaks’ often occur at biogeographical province boundaries, suggesting common causative factors. Cape Hatteras, North Carolina, on the US Atlantic coast, separates water masses of sharply differing temperature and divides the Virginian and Carolinian provinces, but has not often been detected as a phylogeographical break. We studied the black sea bass, Centropristis striata (Linnaeus, 1758), a species with philopatric adults but long-lived planktonic larvae. An earlier study of only two Atlantic populations, and lower-resolution mitochondrial RFLP markers, showed strong separation between the Gulf of Mexico and the Atlantic, but not across Cape Hatteras. Location Over 300 adults were collected from populations from the Gulf of Mexico, the US south Atlantic (North Carolina south of Cape Hatteras, the east coast of Florida, and South Carolina), and the US mid-Atlantic (North Carolina north of Cape Hatteras, Virginia, Delaware, New Jersey and Massachusetts). Methods We sequenced the mitochondrial control region, analysed DNA sequence and haplotype frequency differentiation and haplotype networks, and performed coalescent analyses of population sizes and migration rates. Results Sequences formed three clusters – Gulf of Mexico, south Atlantic and mid-Atlantic – the Atlantic clusters being divided at Cape Hatteras. Analyses of molecular variance (AMOVA) showed that 31% of sequence variation was between populations on opposite sides of Cape Hatteras, while only 5% was between populations on the same side, yet migrate analysis detected asymmetric migration across the Cape, greater from north to south than in the opposite direction. A transitional population with roughly equal frequencies of south and mid-Atlantic haplotypes occupied a narrow stretch between Cape Hatteras and the Virginia border. Main conclusions Strong barriers to gene flow exist between the Gulf of Mexico and the Atlantic, and across Cape Hatteras. Adult migratory and spawning behaviours that separate mid-Atlantic and south Atlantic populations, limited larval exchange, and selection imposed by thermal gradients are each potentially responsible. In addition, these data indicate that fishery stocks of black sea bass north and south of Cape Hatteras are genetically distinct, supporting their separate management.

Intraspecific genetic variation is widely recognized to affect how bivalve larvae respond to variation in environmental conditions, but has been largely ignored in comparisons of larval performance between closely related sibling species. Replicates of five different full-sib families of larval blue mussels (Mytilus edulis and its more northerly congener, M. trossulus) were reared under different temperature (10, 13, and 17 °C) and food conditions in a full factorial design. Growth and survival were strongly affected by temperature, family, the interaction of family and temperature, and the three-way interaction of family, temperature, and food. Family means for days to 20 % survival ranged from 10.2 to 20.0 and did not cluster by species. The two M. trossulus families were intermediate in survival and ranked first and third for growth. The temperature by family interaction effect for survival was very strong, and the two M. trossulus families responded very differently to temperature manipulations. One M. trossulus family exhibited highest survival at 13 °C, while the other M. trossulus family and all three M. edulis families exhibited highest survival at 10 °C. By contrast, greatest growth consistently occurred at 17 °C, indicating that higher mortality in warmer water may be at least partially offset (at a population level) by more rapid growth in body size. The results illustrate the importance of assaying both growth and survival when evaluating environmental effects on larvae and the need to ensure a high level of genetic diversity when pools of larvae are used to study genetically similar species.

Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.

Due to slow rates of molecular evolution, DNA sequences used to identify and build phylogenies of algal species involved in harmful algal blooms (HABs) are generally invariant at the intraspecific level. This means that it is unknown whether HAB events result from the growth of a single clone, a few dominant clones, or multiple clones. This is true despite the fact that several physiological and demographic traits, as well as toxicity, are known to vary across clones. We generated AFLP fingerprints from a set of 6 clonal isolates, taken from a bloom of Prymnesium parvum at a striped bass mariculture facility. This new haptophyte bloom was recently implicated in fish kills at several sites in the United States. The AFLP fragments were highly reproducible and showed that all isolates were distinguishable due to abundant AFLPs unique to single isolates. These results demonstrate that blooms can be genetically diverse outbreaks and indicate that AFLP can be a powerful molecular tool for characterizing and monitoring this diversity.

The Waccamaw Darter Etheostoma perlongum is one of three teleost fish species and several molluscs endemic to Lake Waccamaw, a large Carolina Bay in southeastern North Carolina. We assessed species status by sequencing most of the mitochondrial cytochrome b (cyt b) gene in E. perlongum and its putative sister species, E. olmstedi, from the adjoining Waccamaw River and streams in the nearby Pee Dee and Cape Fear Drainages. Phylogeographic analysis shows strongly supported river- and drainage-specific clades with divergence times dating to the Pleistocene. Haplotypes of Etheostoma perlongum are very similar and not phylogenetically distinct from sequences from the Waccamaw River population of E. olmstedi. However, strong haplotype frequency differentiation shows that free genetic exchange does not occur between the river and lake, and along with life history distinctiveness, suggests reproductive isolation despite contact between these populations. Coalescent simulations showed that incomplete line...

Historical hydrographic data, spanning the period 1896–2006, are used to examine the annual mean and seasonal variations in the distribution of freshwater along and across the shelf/slope boundary along the Labrador and Newfoundland Shelves and the Grand Banks of Newfoundland. Particular attention is paid to the export of freshwater along the eastern Grand Banks, between Flemish Cap and the Tail of the Grand Banks, as this has long been identified as a preferential region for the loss of mass and freshwater from the boundary. The data are combined into isopycnally averaged long-term annual and monthly mean gridded property fields and the evolving distribution of fresh arctic-origin water is analyzed in fields of salinity anomaly, expressed as departures from the “central water” temperature–salinity relation of the Gulf Stream. The climatology confirms that cold/fresh northern-source waters are advected offshore within the retroflecting Labrador Current along the full length of the boundary between Flemish Cap and the Tail of the Grand Banks. In fact, it is estimated that most of the equatorward baroclinic transport at the boundary must retroflect back toward the north in order to explain the annual mean distribution of salinity in the climatology. While the retroflection of the Labrador Current appears seasonally robust, the freshwater distribution within the retroflection region varies in response to (1) the freshness of the water available for export which is set by the arrival and rapid flushing of the seasonal freshwater pulse at the boundary, (2) seasonal buoyancy forcing at the surface which alters the vertical stratification across the retroflection region, restricting certain isopycnal export pathways, and (3) the density structure along the eastern Grand Banks, which defines the progressive retroflection of the Labrador Current.

A 5-year-long time series of meridional transport below 1180 dbar—zonally integrated across a section spanning, the western basin of the tropical North Atlantic—is analyzed. It has been inferred from (i) zonally integrated meridional geostrophic transports derived from density and bottom pressure measurements at the end points of a 1000 km wide section bounded by the base of the western continental rise and the Mid-Atlantic Ridge and (ii) mooring-based direct current meter measurements over the steep Lesser Antilles continental rise. The southward time mean transport of North Atlantic Deep Water (NADW) transport is 15.9 Sv. The vertical shear of the geostrophic transport profiles in the western and eastern part of the section each show two layers of maximum southward transport within the NADW. The transport time series reveals changes of 7.7 Sv rms at periods of 1 month and longer, at times showing changes of up to 40 Sv within a month's time. The baroclinic (internal) contribution of the geostrophic flow (relative to 4950 dbar), yields fluctuations of 6.6 Sv rms. Adding transports over the steep continental rise reduces the overall transport variability to 5.2 Sv rms. As a result of this reduction in shorter-period variability, the lower-frequency variability becomes more pronounced, part of which is expected to be linked to the meridional overturning circulation (MOC). The transport variability is consistent with baroclinic Rossby waves (at periods between 3 and 9 months), dominating in the eastern and central part of the section, and with changes in deep western boundary current (DWBC) strength, DWBC re-circulation patterns and eddies that become important in the western part of the section. The reference-level (external) geostrophic transport variability displays long-wavelength (>2000 km) fluctuations of 7.5 Sv rms on periods less than 2 weeks that are consistent with barotropic Rossby waves. Numerical model simulations imply that the observed zonally integrated deep transport changes in the western basin have moderate skill in sensing changes in the MOC and in meridional heat transport, and that a now implemented extension of the array's integration scale into the eastern basin of the Atlantic would substantially improve the performance of the array as an MOC observing system.

Recent demonstrations of positive selection on genes controlling gamete compatibility have resulted in a proliferation of hypotheses concerning the sources of selec- tion. We tested a prediction of one prominent hypothesis, selection to avoid hybridization (i.e., reinforcement), by comparing heterospecific gamete compatibility in two Myti- lus edulis populations: one population in Cobscook Bay, Maine, in which the close congener, M. trossulus, is abun- dant (a region of sympatry), and one population in Kittery, Maine, in which M. trossulus is absent (a region of allo- patry). Three diagnostic nuclear DNA markers were used to identify mussels to species and to estimate the frequency of both species and their hybrids in the two populations. Con- trolled crosses were then conducted by combining eggs of M. edulis females with a range of M.edulis and M. trossulus sperm concentrations. Results were not consistent with the reinforcement hypothesis. M. edulis females collected from the region of sympatry were no more incompatible with M. trossulus males than were M. edulis females collected from the region of allopatry. A trend in the opposite direction, toward greater compatibility in sympatry, suggests that in- trogression of M. trossulus genes that control egg compat- ibility, such as those encoding receptors for sperm, may influence evolution of gametic isolation in hybridizing pop- ulations.

The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circulation north and cyclonic circulation south of the Antarctic Circumpolar Current. This image mostly follows from geostrophic estimates from hydrography using a bottom level of no motion for the eastward flow regime which typically yield transports near 170 Sv. Net eastward transport of about 145 Sv for this region results from subtracting those westward flows. This estimate is compatible with the canonical 134 Sv through Drake Passage with augmentation from Indonesian Throughflow (around 10 Sv). A new image is developed from World Ocean Circulation Hydrographic Program sections I8S and I9S. These provide two quasi-meridional crossings of the South Australian Basin and the Australian–Antarctic Basin, with full hydrography and two independent direct-velocity measurements (shipboard and lowered acoustic Doppler current profilers). These velocity measurements indicate that the belt of eastward flow is much stronger, 271 ± 49 Sv, than previously estimated because of the presence of eastward barotropic flow. Substantial recirculations exist adjacent to the Antarctic Circumpolar Current: to the north a 38 ± 30 Sv anticyclonic gyre and to the south a 76 ± 26 Sv cyclonic gyre. The net flow between Australia and Antarctica is estimated as 157 ± 58 Sv, which falls within the expected net transport of 145 Sv. The 38 Sv anticyclonic gyre in the South Australian Basin involves the westward Flinders Current along southern Australia and a substantial 33 Sv Subantarctic Zone recirculation to its south. The cyclonic gyre in the Australian–Antarctic Basin has a substantial 76 Sv westward flow over the continental slope of Antarctica, and 48 ± 6 Sv northward-flowing western boundary current along the Kerguelen Plateau near 57°S. The cyclonic gyre only partially closes within the Australian–Antarctic Basin. It is estimated that 45 Sv bridges westward to the Weddell Gyre through the southern Princess Elizabeth Trough and returns through the northern Princess Elizabeth Trough and the Fawn Trough – where a substantial eastward 38 Sv current is hypothesized. There is evidence that the cyclonic gyre also projects eastward past the Balleny Islands to the Ross Gyre in the South Pacific. The western boundary current along Kerguelen Plateau collides with the Antarctic Circumpolar Current that enters the Australian–Antarctic Basin through the Kerguelen–St. Paul Island Passage, forming an energetic Crozet–Kerguelen Confluence. Strongest filaments in the meandering Crozet-Kerguelen Confluence reach 100 Sv. Dense water in the western boundary current intrudes beneath the densest water of the Antarctic Circumpolar Current; they intensely mix diapycnally to produce a high potential vorticity signal that extends eastward along the southern flank of the Southeast Indian Ridge. Dense water penetrates through the Ridge into the South Australian Basin. Two escape pathways are indicated, the Australian–Antarctic Discordance Zone near 125°E and the Geelvinck Fracture Zone near 85°E. Ultimately, the bottom water delivered to the South Australian Basin passes north to the Perth Basin west of Australia and east to the Tasman Basin.

Studies of population connectivity have largely focused on along-shelf, as opposed to across-shelf, processes. We hypothesized that a discontinuity in across-shelf mixing caused by the divergence of the Eastern Maine Coastal Current (EMCC) from shore acts as an ecological barrier to the supply of mussel larvae to the coast. Existing data on the relative abundance of two congeneric blue mussels, Mytilus edulis and M. trossulus , were analysed to quantify the association of M. trossulus with the colder temperature signal of the EMCC and generate larval distribution predictions. We then sampled the across-shelf distribution of larvae along two transects during 2011. Larvae were identified using restriction digests of PCR amplicons from the mitochondrial 16S rDNA. Mytilus edulis larvae were consistently abundant on either the inshore and offshore transect ends, but not homogeneously distributed across the shelf, while M. trossulus larvae were less common throughout the study area. The divergence of the EMCC from shore appears to create a break in the connectivity of M. edulis populations by isolating those inshore of the EMCC from upstream larval sources. Across-shelf transport processes can thus produce connectivity patterns that would not be predicted solely on the basis of along-shelf processes.

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

A blue crab (Callinectes sapidus) expressed sequence tag project was designed for multiple purposes including discovery of genes for cuticular (exoskeletal) proteins, some of which may regulate mineralization. One of the expression libraries sequenced was from the hypodermis (the epithelium depositing the cuticle). RNAs used for cDNA synthesis were pooled from arthrodial and mid-dorsal hypodermis at both pre-ecdysis and post-ecdysis. This ensured representation from both calcifying and non-calcifying regions and from layers of cuticle deposited both before and after ecdysis. The EST database was mined for cuticular protein sequences in three ways. First, we searched for sequences coding for known cuticle-specific motifs like the Rebers-Riddiford chitin-binding sequence and a motif known only from proteins extracted from mineralized exoskeletons of other decapods. Second, we checked the associated annotations in the EST project for similarity to known cuticular proteins, often from insects. Third, BLAST was used to search the EST data for significant homology to published cuticular protein sequences from other crustaceans. In all, the database contains at least 73 contigs or singlets representing transcripts of cuticular proteins. Forty-five of these distribute among ten clusters of very similar transcripts, possibly representing alternative splicing or recent gene duplications. The rest share less similarity. We have obtained complete sequences for 25 of the transcripts, have produced phylogenetics trees comparing them with similar proteins from insects and other crustaceans, and have determined expression patterns across the molt in calcifying versus non-calcifying cuticle. The combination of homology analysis and gene expression analysis allows us to infer putative functions in cuticle synthesis and calcification.

Bindin is a gamete recognition protein known to control species-specific sperm-egg adhesion and membrane fusion in sea urchins. Previous analyses have shown that diversifying selection on bindin amino acid sequence is found when gametically incompatible species are compared, but not when species are compatible. The present study analyzes bindin polymorphism and divergence in the three closely related species of Echinometra in Central America: E. lucunter and E. viridis from the Caribbean, and E. vanbrunti from the eastern Pacific. The eggs of E. lucunter have evolved a strong block to fertilization by sperm of its neotropical congeners, whereas those of the other two species have not. As in the Indo-West Pacific (IWP) Echinometra, the neotropical species show high intraspecific bindin polymorphism in the same gene regions as in the IWP species. Maximum likelihood analysis shows that many of the polymorphic codon sites are under mild positive selection. Of the fixed amino acid replacements, most have accumulated along the bindin lineage of E. lucunter. We analyzed the data with maximum likelihood models of variation in positive selection across lineages and codon sites, and with models that consider sites and lineages simultaneously. Our results show that positive selection is concentrated along the E. lucunter bindin lineage, and that codon sites with amino acid replacements fixed in this species show by far the highest signal of positive selection. Lineage-specific positive selection paralleling egg incompatibility provides support that adaptive evolution of sperm proteins acts to maintain recognition of bindin by changing egg receptors. Because both egg incompatibility and bindin divergence are greater between allopatric species than between sympatric species, the hypothesis of selection against hybridization (reinforcement) cannot explain why adaptive evolution has been confined to a single lineage in the American Echinometra. Instead, processes acting to varying degrees within species (e.g., sperm competition, sexual selection, and sexual conflict) are more promising explanations for lineage-specific positive selection on bindin.

The aragonite skeleton of massive reef-building corals contains a record of the oceanic environment in which they grow. However, reading of the record requires understanding of how it is archived, a process complicated by the elaborate skeletal construction and seasonal growth patterns that characterize many species. In this study, we assess the utility of the massive brain coral Diploria labyrinthiformis as an archive of sea surface temperature (SST) variability in the western North Atlantic. In situ staining of live colonies combined with microscale analysis of skeletal chemistry indi- cate that D. labyrinthiformis grows throughout the year on Bermuda and records the full annual cycle of SST variability. However, skeleton accreted during the summer is overlain (thickened) by skeleton accreted during the subsequent fall and winter. As a result, conventional coarse sampling for δ 18 O enables seasonal δ 18 O cycles to be resolved but these do not capture the full amplitude of the annual SST cycle. Our data show that the shallow gradient of the δ 18 O-SST regression equation derived for D. labyrinthiformis (-0.113‰ oC -1 ) relative to the expected -0.22‰ oC -1 for marine skeletons results from dampening of the summertime peak in δ 18 O. In contrast, skeleton accreted during the winter is not thickened and wintertime δ 18 O captures the interannual wintertime SST variability at this site. Using SIMS ion microprobe to analyse strontium to calcium ratios (Sr/Ca), we avoided the thickening deposits and were able to resolve the full amplitude of the annual Sr/Ca cycle. The Sr/Ca-SST relationship obtained for D. labyrinthiformis (-0.0843 mmol/mol oC -1 ) corresponds to that derived from fast-growing tropical reef corals. X-ray intensity ratios, used as a proxy for skeletal density, reveal the expected seasonal changes associated with growth banding as well as variability on inter- annual and decadal timescales. These variations are well correlated with wintertime SST variability in the subtropical gyre and may be a valuable proxy thermometer for the North Atlantic.

Species of the sea urchin genus Echinometra found on the two coasts of Panamaare recently diverged and only partially isolated by incomplete barriers to inter- specific fertilization. This study confirms previous work that revealed incompatibility between the eggs of the Atlantic E. lucunter and the sperm of the other two neotropical species, whereas eggs of its sympatric congener E. viridis and allo- patric E. vanbrunti are largely compatible with heterospe- cific sperm. Here we quantify fertilization using a range of sperm dilutions. We demonstrate a much stronger block to cross-species fertilization of E. lucunter eggs than was previously shown at fixed sperm concentrations, and mild incompatibility of the other two species' eggs where previ- ous crosses between species were not distinguishable from within-species controls. Additionally, we present evidence for intraspecific variation in egg receptivity towards het- erospecific sperm. Our findings here again discount the "reinforcement model" as a viable explanation for the pat- tern of prezygotic isolation. Gamete incompatibility in these Echinometra has appeared recently—within the last 1.5 million years— but is weaker in sympatry than in allopatry. Accidents of history may help explain why incompatibility of eggs emerged in one species and not in others. Compen- satory sexual selection on sperm in this species could fol- low, and promote divergence of proteins mediating sperm- egg recognition.

Observational evidence is presented for interannual to interdecadal variability in the intensity of the North Atlantic gyre circulation related to the atmospheric North Atlantic Oscillation (NAO) patterns. A two-point baroclinic pressure difference between the subtropical and subpolar gyre centers—an oceanic analogue to the much-used sea level pressure (SLP)-based atmospheric NAO indices—is constructed from time series of potential energy anomaly (PEA) derived from hydrographic measurements in the Labrador Basin and at Station S near Bermuda. Representing the upper 2000-db eastward baroclinic mass transport between the two centers, the transport index indicates a Gulf Stream and North Atlantic Current that gradually weakened during the low NAO period of the 1960s and then intensified in the subsequent 25 years of persistently high NAO to a record peak in the 1990s. The peak-to-peak amplitude difference was 15–20 megatons per second (MT s−1) with a 43-yr mean of about 60 MT s−1 a change of 25%–33%...

Variability of the North Atlantic Oscillation and the Tropical Atlantic dominate the climate of the North Atlantic sector, the underlying ocean and surrounding continents on interannual to decadal time scales. Here we review these phenomena, their climatic impacts and our present state of understanding of their underlying cause. Copyright © 2001 Royal Meteorological Society.

Warm and saline waters enter the Nordic Seas from the south as part of the warm-to-cold water transformation of the thermohaline circulation of the northern North Atlantic. One explanation for the origin of the Nordic Seas Inflow is a “shallow source hypothesis” under which the Inflow waters are a modification of upper ocean subtropical waters. Warm waters from the subtropical gyre are carried to the eastern North Atlantic by the North Atlantic Current and branch northwards, joined by poleward upper thermocline flow along the upper continental slope, to provide the Nordic Seas Inflow. Along this pathway the upper water column is progressively cooled and freshened by winter convection, the subpolar mode water transformation process, and this sets the Inflow characteristics. A “deep source hypothesis” provides an alternative explanation for the characteristics of the Nordic Seas Inflow and the pathway delivering the waters to the Inflow. Under this hypothesis Inflow waters originate from the core of the Mediterranean Overflow Waters in the Gulf of Cadiz carried northward at mid-depth by the eastern boundary undercurrent in the subtropics, continuing into the subpolar gyre along the eastern boundary, and rising from depths near 1200m in the Rockall Trough to less than 600m to cross the Wyville-Thomson Ridge into the Faroe-Shetland Channel and thence to the Nordic Seas. The deep source hypothesis focus is on lower thermocline source waters beneath the sill depth for the Nordic Seas Inflow, in contrast to the shallow source hypothesis focus of transformation of upper thermocline waters above the sill depth. On the basis of regional water mass distributions, geostrophic shear, and direct current measurements, we reject the deep source hypothesis in favor of the shallow source hypothesis. Rather than a flow of deep Mediterranean Overflow Water along the eastern boundary rising from depth to feed the Nordic Seas, the Inflow is supplied directly by transformed North Atlantic Current waters from the same depth range as the Inflow. Mediterranean Overflow Water is a constituent of the Inflow, but only through its contribution to defining the temperature-salinity relationship of the interior of the subtropical gyre from which the North Atlantic Current draws its water, rather than by direct northward advection of Mediterranean Outflow Water via the eastern boundary undercurrent crossing from the subtropics into the subpolar domain. Instead, the eastern boundary undercurrent wholly expels its transport of Mediterranean Overflow Water into the eastern edge of the subtropical gyre, creating the mid-depth westward extending subtropical salinity plume. For the lower thermocline isopycnal range of the core of the Mediterranean Overflow Water, measurements show that at the Wyville-Thomson Ridge the flow is southward and descending (rather than northward and rising), representing cold spillover from the Faroese Channels entraining warm upper ocean waters as the spillover plume descends and warms into the northern Rockall Trough. No evidence is found in Rockall Trough northward of 54°N (Porcupine Bank) for direct advection of Mediterranean Overflow Waters via an eastern boundary undercurrent extension, and the very dilute Mediterranean Overflow Waters in the Trough reflect the interaction of the Wyville-Thomson Ridge plume descending into the Trough from the North, and lower thermocline waters delivered to the Trough from the south.

Echinometra is a pantropical sea urchin made famous through studies of phylogeny, speciation, and genetic structure of the Indo-West Pacific (IWP) species. We sequenced 630 bp of the cytochrome oxidase I (COI) mitochondrial gene to provide comparable information on the eastern Pacific and Atlantic species, using divergence between those separated by closure of the Isthmus of Panama 3.1 million years ago (Ma) to estimate dates for cladogenic events. Most recently (1.27-1. 62 Ma), the Atlantic species E. lucunter and E. viridis diverged from each other, at a time in the Pleistocene that sea levels fell and Caribbean coral speciation and extinction rates were high. An earlier split, assumed to have been coincident with the completion of the Isthmus of Panama, separated the eastern Pacific E. vanbrunti from the Atlantic common ancestor. Transisthmian COI divergence similar to that in the sea urchin genus Eucidaris supports this assumption. The most ancient split in Echinometra occurred between the IWP and the neotropical clades, due to cessation of larval exchange around South Africa or across the Eastern Pacific Barrier. Gene flow within species is generally high; however, there are restrictions to genetic exchange between E. lucunter populations from the Caribbean and those from the rest of the Atlantic. Correlation between cladogenic and vicariant events supports E. Mayr's contention that marine species, despite their high dispersal potential, form by means of geographical separation. That sympatric, nonhybridizing E. lucunter and E. viridis were split so recently suggests, however, that perfection of reproductive barriers between marine species with large populations can occur in less than 1.6 million years (Myr).

The Deep Western Boundary Current (DWBC) of the North Atlantic is a principal conduit between the formation region for Labrador Sea Water (LSW) and the oceanic interior to the south. Time series (1985–1997) of hydrographic properties obtained in the DWBC at 26.5°N show that prior to 1994, temperature, salinity, and transient tracer properties within the LSW density range showed little indication of recently formed parcels. Properties characteristic of a newer version of LSW (cooler, fresher, and higher tracer concentrations) were observed beginning in 1994 and continuing through 1997. Longer time series of temperature and salinity, developed from a regional data base, show both the 1994 and a 1980–1981 event in the Abaco region. Both events are consistent with anomalies in the Labrador Sea that occurred some 10 years earlier. The 10-year transit time from the Labrador Sea to 26.5°N is less than the 18-year transit time inferred from earlier studies.

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssal currents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from the Brazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv [ 106 m3 s21), comprising organized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated by a quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transports occur in September‐October, while minimum transports occur in February‐March. Allowing for this quasiannual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated mean AABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water (LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speed through the array, 9.9 cm s21 into the Brazil Basin. The LNADW records also have a quasi-annual cycle with strong LNADW flow episodes occurring in April‐May. Time series of temperature indicate that the LNADW/ AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (uplifted transition layer during strong AABW transport periods). An observed overall warming trend appears to be accompanied by a decline in AABW transport.

Marine invertebrate gamete recognition proteins (GRPs) are classic examples of rapid adaptive evolution of reproductive proteins, and hybridizing Mytilus blue mussels allow us to study the evolution of GRPs during speciation following secondary contact. Even with frequent hybridization, positive selection drives divergence of M7 lysin, one of the three Mytilus egg vitelline envelope (VE) lysins. Mytilus trossulus and M. edulis form a broad hybrid zone in the Canadian Maritimes and eastern Maine, isolated by strong (but partial) gamete incompatibility. M7 lysin, however, is an unlikely GRP controlling this gametic incompativility, as earlier studies showed either weak or no positive selection and extensive introgression between the two species. We used reverse transcriptase-polymerase chain reaction and cloned several alleles of M3 lysin, a potent VE lysin encoded by a nonhomologous gene whose evolution has not been studied. McDonald-Kreitman and HKA tests reveal strong positive selection, which PAML branch-site models detect in 19.7% of the codons. Protein structure predictions show that replacements map exclusively to one face of the carbohydrate recognition domain (CRD) of this C-type lectin, with codons under positive selection localizing to CRD regions known to control ligand specificity. Polymorphism/divergence analyses show that selective sweep has purged M. edulis but not M. trossulus of polymorphism, and unique to M3 is an absence of fixed substitutions and broad haplotype sharing between M. edulis and Mediterranean M. galloprovincialis. Taken together, these results suggest that different lysins serve as GRPs in different Mytilus hybrid zones, with M3 likely co-opted to play this role in the western Atlantic.

The nucleotide diversity across 1705 bp of the G6pd gene is studied in 50 Drosophila melanogaster and 12 D. simulans lines. Our earlier report contrasted intraspecific polymorphism and interspecific differences at silent and replacement sites in these species. This report expands the number of European and African lines and examines the pattern of polymorphism with respect to the common A/B allozymes. In D. melanogaster the silent nucleotide diversity varies 2.8-fold across localities. The B allele sequences are two- to fourfold more variable than the derived A allele, and differences between allozymes are twice as among B alleles. There is strong linkage disequilibrium across the G6pd region. In both species the level of silent polymorphism increases from the 5′ to 3′ ends, while there is no comparable pattern in level of silent site divergence or fixation. The neutral model is not rejected in either species. Using D. yakuba as an outgroup, the D. melanogaster lineage shows a twofold greater rate of silent fixation, but less than half the rate of amino acid replacement. Lineage-specific differences in mutation fixation are inconsistent with neutral expectations and suggest the interaction of species-specific population size differences with both weakly advantageous and deleterious selection.

Data collected in 1988–1989, as part of the South Atlantic Ventilation Experiment, have been combined with the historical database to study the circulation and water mass variability of the abyssal water in the Argentine Basin. A map of potential temperature at 4000 m used as an indication of geostrophic shear defines a south and western intensified crescent-shaped abyssal recirculation. Within this recirculation, and its northward extension to the Brazil Basin, Antarctic Bottom Water (AABW) properties have undergone two modifications during the 1980s: (1) The water mass cooled (0.05°C) and freshened (0.008 in salinity ratio) on surfaces of constant density. (2) The densest layer of AABW was altered to less dense water through mixing or advection out of the study area. This water mass change does not appear to have affected the flow pattern. Data collected in 1983 and 1988 to the north in the Brazil Basin show penetration of the freshwater mass in the deep western boundary current to between 18°S and 10°S, indicating very rapid propagation of the anomaly from the Argentine Basin into the Brazil Basin as a deep western boundary current. It is suggested that open ocean convective events within the Weddell Sea contributed to the change in AABW documented here.

Strongylocentrotus sea urchins are common subjects for studies in developmental and cell biology, reproductive biology, ecology, and evolution. We report 14 microsatellite loci from the red urchin, S. franciscanus , isolated for the purpose of estimating paternal success of males in experimental group spawns. Most of these loci were found to be highly polymorphic in a population from British Columbia. A high frequency of null alleles appears responsible for heterozygote deficit at a majority of these loci, but if used with appropriate caution, these microsatellites should be effective markers for studies of Strongylocentrotus populations.

Small-scale population densities of tidal creek eastern mudsnails, Ilyanassa obsoleta Say (studied in 1986 and in 1992 at West Meadow Creek, Stony Brook, New York) corresponded more to variation in water flow velocity than to surface sediment chlorophyll a. Higher densities were found at low flow sites. Short-term behavioral responses are likely to be responsible for density variation. Experiments using laboratory flumes and field observations both demonstrated that the snails responded to strong flow by burrowing into the substratum. Burrowing may prevent dislodgment from the sediment surface, but it also appears to be disadvantageous since burrowed mudsnails have smaller amounts of food in their guts. Snails released in sites of periodic high flow conditions moved greater distances and were soon found near the quiet-water periphery of the creek, whereas snails released at the quiet-water periphery moved far less. It is not clear whether movement from the high flow site was through crawling or through hydrodynamic transport. Laboratory flume experiments demonstrated an active crawling movement towards areas of lower current velocity. This evidence suggests that strong bottom flow in the creek center results in a combined response of burial to avoid dislodgment and a net movement towards quiet water, which reduces exposure to the high velocity conditions of the creek center.

Understanding patterns of connectivity among populations of marine organisms is essential for the development of realistic, spatially explicit models of population dynamics. Two approaches, empirical genetic patterns and oceanographic dispersal modelling, have been used to estimate levels of evolutionary connectivity among marine populations but rarely have their potentially complementary insights been combined. Here, a spatially realistic Lagrangian model of larval dispersal and a theoretical genetic model are integrated with the most extensive study of gene flow in a Caribbean marine organism. The 871 genets collected from 26 sites spread over the wider Caribbean subsampled 45.8% of the 1900 potential unique genets in the model. At a coarse scale, significant consensus between modelled estimates of genetic structure and empirical genetic data for populations of the reef-building coral Montastraea annularis is observed. However, modelled and empirical data differ in their estimates of connectivity among northern Mesoamerican reefs indicating that processes other than dispersal may dominate here. Further, the geographic location and porosity of the previously described east-west barrier to gene flow in the Caribbean is refined. A multi-prong approach, integrating genetic data and spatially realistic models of larval dispersal and genetic projection, provides complementary insights into the processes underpinning population connectivity in marine invertebrates on evolutionary timescales.

A comparison of a recently assembled hydrographic database for the North Atlantic with the Lovitus atlas shows striking differences in the vicinity of the Gulf Stream and the North Atlantic Current. On isopycnal surfaces in the main thermocline, isolated pools of warm, saline water are found in the Levitus database but are absent in the new database. Using synoptic data as a proxy for temporally averaged climatological data, it is shown that the anomalous features can be accounted for by the differences in the averaging process. To produce a gridded database from irregularly spaced station data, Levitus averaged the data on pressure surfaces while the new database was prepared with averaging an potential density surfaces. It is shown that averaging on a pressure surface in an area of sharply sloping isopycnals produces a water mass with a θ–S signature uncharacteristic of the local water mass(es). The anomalous potential temperatures and salinities that result are compared to the large-scale wate...

A long CTD/hydrographic section with closely spaced stations was made in February-April 1989 in the western Atlantic Ocean between 0”40’N and South Georgia (545) along a nominal longitude of 25W. Vertical sections of various properties from CTD and discrete water-sample measurements are presented and discussed in terms of the large-scale circulation of the South Atlantic Ocean. One of the most important results is the identification of various deep-reaching fronts in relation to the large-scale circulation and the distribution of mode waters. Five major fronts are clearly defined in the thermal and salinity fields. These are the Polar (49SS), Subantarctic (45S), Subtropical (41-425), Brazil Current (355) Fronts, and an additional front at 20-22s. The first three are associated with strong baroclinic shear. The Brazil Current Front is a boundary between the denser and lighter types of the Subantarctic Mode Water (SAMW), and the 20-223 front marks the boundary between the anticyclonic subtropical and cyclonic subequatorial gyres. The latter front coincides with the northern terminus of the high-oxygen tongue of the Antarctic Intermediate Water (AAIW) and also with the abrupt shift in density of the high-silica tongue originating in the Upper Circumpolar Water and extending northward. Two pycnostads with temperatures 20-24°C are observed between 10s and 2% with the denser one in the subtropical and the other lighter one in the subequatorial gyre. A weak thermostad centered at 4°C occurs in the AAIW between the Subtropical Front and the Subantarctic Front and shows characteristics similar to the densest variety of the SAMW. Another significant result is a detailed description of the complex structure of the deep and bottom waters. The North Atlantic Deep Water (NADW) north of 25s contains two vertical maxima of oxygen (at 2000 m and 3700 m near the equator) separated by intervening low-oxygen water with more influence from the Circumpolar Water. Each maximum is associated with a maximum of salinity and minima of nutrients. The deeper salinity maximum is only weakly defined and is limited to north of 18S, appearing more as vertically uniform salinity. South of 25s the NADW shows only a single maximum of salinity, a single maximum of oxygen, and a single minimum of each nutrient, all lying close together. The salinity maximum south of 25s and the deeper oxygen/salinity maximum north of 11s are derived from the same source waters. The less dense NADW containing the shallower extrema of characteristics turns to the east at lower latitudes and does not reach the region south of 25s. The

Hybridizing populations of blue mussels, Mytilus edulis and Mytilus trossulus, in Cobscook Bay (eastern Maine) have been used by our laboratory to study the evolution of gamete incompatibility and molecular evolution of the vitelline coat lysin proteins expressed in sperm. The M7 lysin locus has been the most studied of the three lysins, but evidence for positive selection necessary to help confirm its role in gamete recognition in western Atlantic hybrid zones is contradictory. We developed an alternative test, based on rates of introgression at M7 lysin. Contrary to expectations, introgression at this locus is much higher (instead of much lower) than is introgression at neutral markers. In this article, we present simulations, constructed using synthetic populations of combinations of admixed genotypes, representing various hybrid offspring categories. Simulations produced variation in introgression across loci, but did not generate the massive introgression at M7 lysin observed in natural populations in Cobscook Bay. We consider these results in the context of selection operating on gamete recognition loci, both within and between species, during the third stage of allopatric speciation in Mytilus.

Property distributions and geostrophic shear from a hydrographic section near 37°W in the Atlantic Ocean show the deep western boundary current (DWBC) in the North Atlantic Deep Water (NADW) established against the western boundary of the Brazil Basin immediately south of the equator (between 2° and 5°S). The DWBC thus has directly crossed the equator to the South Atlantic following the east-southeast trend of the continental slope isobaths. The estimated DWBC transport of NADW is 35 × 106 m3 s−1, similar to other estimates from the tropics discussed here. These large DWBC transports are opposed by flow of deep water to the North Atlantic immediately offshore of the DWBC, with as much as two-thirds of the DWBC transport being returned as these recirculations. One recirculation center is the Guiana Basin north of the equator but extends at least a few hundred kilometers south of the equator; another is visible at 11°S in the Brazil Basin. The degree of connection of these two observed recirculatio...