Temporal and spatial changes in body size and reproductive state of Nannocalanus minor (Copepoda) females across and along the Gulf Stream

The temporal and spatial trends of cephalothorax length andreproductive state of adult females of the cosmopolitan copepodspecies Nannocalanus minor are described across the Gulf Streamnear Cape Hatteras. NC. Consistent regional differences anda cross-stream gradient in size were found, with the largestanimals in the Slope Water, animals of intermediate size inthe Gulf Stream and the smallest animals in the Sargasso Sea.Distinct seasonal patterns in size were observed within eachcross-stream region (Slope Water, Gulf Stream and Sargasso Sea)and these patterns followed seasonal changes in environmentalconditions (temperature and sea surface pigment concentration).Body sizes of the Slope Water animals were inversely and linearlyrelated to temperature with no clear relationship to sea surfacepigment concentration, while body sizes of the Sargasso Seaanimals were correlated with pigment concentration (asymptotic,non-linear relationship) with no relationship to temperature.The lack of consistent regional and seasonal trends in reproductivestate suggested that continuous reproduction was a life historytrait for adults of this species. The potential effect of short-term,episodic events on the size distribution and reproductive stateof this species was investigated across and along a Gulf Streammeander during the BIOSYNOP project in 1988. Upwelling of nutrientsmay occur on the trailing flank of a meander, with enhancedprimary and, possibly, secondary production occurring downstreamin the leading flank of the meander. No specific meander effectwas observed for body size. More individuals were reproducingdownstream of the upwelling region, suggesting a response tothe enhanced primary production characteristic of the upwellingregion.     

Oligotrophic waters of the Northwest Atlantic support taxonomically diverse diatom communities that are distinct from coastal waters

Diatoms are important components of the marine food web and one of the most species-rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems.     

Organisms associated with stimulated epipelagic bioluminescence in the Sargasso Sea and the Gulf Stream

On three cruises, vertical profiles of stimulated bioluminescencewere measured during the late evening in the upper 200 m ofthe Sargasso Sea using a submarine photometer. On one cruise,organisms were collected in a 25 µm porosity net afterpassing through the photometer where the intensity and lightcontent of their bioluminescence were recorded. Correlationsof bioluminescence and organisms suggested that the majorityof the stimulated bioluminescence produced in the Sargasso Seawas from zooplankton: crustaceans (ostracods, copepods, copepodlarvae, euphausid larvae), larvaceans and colonial radiolarians.In addition, the photosynthetic dinoflagellate Pyrocystis noctilucaappeared to produce 5–30% of the measured bioluminescenceat some stations. Other dinoflagellates, although numerous,were dim and thus produced less than a few percent of the stimulatedbioluminescent light. The subsurface peaks in the Gulf Streamand northern Sargasso Sea were due primarily to ostracods andlarvaceans. In the Anegada Passage in October, and in the northernSargasso Sea and the Gulf Stream in August, there were pronouncedsubsurface peaks in bioluminescence associated with the thermocline.In Anegada Passage and the Sargasso Sea just north of PuertoRico in October, and in the Gulf Stream in August, the subsurfacebioluminescence peak was in or slightly above the chlorophyllmaximum. However, at the Sargasso Sea stations in August, itwas 10–40 m above the depth of the chlorophyll maximum.     

Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic

Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators.Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata.High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation.     

Spawning ofConger oceanicus andConger triporiceps (Congridae) in the Sargasso Sea and subsequent distribution of leptocephali

Synopsis Distribution of leptocephali ofConger in the Western North Atlantic Ocean was studied using specimens from our collections, specimens from other collections, and various existing collection records. The presence of leptocephali ofConger oceanicus andConger triporiceps < 30 mm long over deep water in the southwestern Sargasso Sea in autumn and winter implies a protracted spawning period there. The subtropical convergence zone, meandering east-west across the Sargasso Sea, is probably the northern limit of spawning of both species. Spawning may also occur close to the Bahamas and Antilles.C. triporiceps may spawn also in the Caribbean Sea judging by the capture of small leptocephali in the western Caribbean and of the more southerly continental distribution of its juveniles. The claim of Johannes Schmidt in 1931 that the EuropeanC. conger spawns across the North Atlantic into the western Sargasso Sea is probably incorrect, because leptocephali ofConger are rare in the eastern Sargasso Sea and becauseC. triporiceps, with myomere numbers overlapping those ofC. conger, was recently described in the western North Atlantic. With increasing size, leptocephali ofC. oceanicus and a portion ofC. triporiceps spread westward and northward in the Florida Current and Gulf Stream, but larger leptocephali especially ofC. triporiceps are found also in the Caribbean and Gulf of Mexico. Spawning ofC. oceanicus in the Sargasso Sea indicates that adults cross the Florida Current-Gulf Stream, and successful leptocephali cross the current in the opposite direction to colonize juvenile habitat on the continental shelf, a migratory pattern similar to that of the American eelAnguilla rostrata (Anguillidae).     

Distinct protistan assemblages characterize the euphotic zone and deep sea (2500 m) of the western North Atlantic (Sargasso Sea and Gulf Stream)

Protistan diversity was characterized at three locations in the western North Atlantic (Sargasso Sea and Gulf Stream) by sequencing 18S rRNA genes in samples from euphotic (< or = 125 m) and bathypelagic depths (2500 m). A total of 923 partial-length protistan sequences were analysed, revealing 324 distinct operational taxonomic units (OTUs) determined by an automated OTU-calling program set to 95% sequence similarity. Most OTUs were comprised of only one or two sequences suggesting a large but rare pool of protistan diversity. Many OTUs from both depth strata were associated with recently described novel alveolate and stramenopile lineages while many OTUs from the bathypelagic were affiliated with Acantharea, Polycystinea and Euglenozoa and were not observed in euphotic zone libraries. Protistan assemblages from the euphotic zone and the deep sea were largely composed of distinct OTUs; only 28 of the 324 protistan OTUs were detected in both shallow and deep sea clone libraries. The diversity of protistan assemblages in the deep sea was distinctly lower than the diversity of euphotic zone assemblages. Protistan assemblages from the Gulf Stream were the most diverse for either depth strata. Overall, protistan assemblages from different stations but comparable depths were more similar than the assemblages from different depths at the same station. These data suggest that particular groups of protistan OTUs formed distinct 'shallow' and 'deep-sea' assemblages across widely spaced oceanic locales.     

Euphausiid distribution, abundance and succession in North Atlantic warm-core ring 82B

Zooplankton collections were made with a multiple opening/closingnet and environmental sensing system (MOCNESS) both day andnight in warm-core ring 82B in the North Atlantic Ocean andat night in the Slope Water between March and August 1982. Speciessuccession of euphausiids in 82B was presented during the lifespanof this warm-core ring, aiming at providing basic informationon possible response of North Atlantic marine ecosystem to globalwarming. Species succession of euphausiids (32 species) in thislong-lived warm-core ring was investigated. Major physical changesof 82B occurred in March to April by convective mixing and thermostadcooling, in April/May by stratification of the surface watersand in August by the interaction with Gulf Stream. Substantialchanges in species composition were observed that correspondedto these physical changes. Four different patterns were foundin temporal change in abundance of warm-water species. Therewere species that decreased in number from March to August,species that decreased from March to June, but increased inAugust, species that increased from Match to August and speciesthat showed no systematic trend. These differences may be attributableto a species tolerance to the thermostad temperature decreaseand their vertical distribution. There was also a large changefrom April to June with ascendance of the transition species,Thysanoessa gregaria. Cold-water species had variable patternsof abundance in 82B and occurred more abundantly in the SlopeWater than in the ring. The monthly percentage decrease in theabundance of warm-water species in 82B was high compared withthat of cold-water species in cold-core rings as a result ofthe more rapid changes in the physical structure and the shorterlifetimes of warm-core rings in the western North Atlantic.     

Distribution of chiorophylls and phaeopigments in the Northwestern Atlantic Ocean

During a cruise in the Northwestern Atlantic the distributionsof chlorophylls and phaeopigments were investigated in relationto mesoscale features by high-performance liquid chromatography.Samples were collected from hydrographic Stations and from twoContinuous underway sampling systems fished simultaneously atthe surface (2 m) and at deeper (30–90 m) depths. Highestpigment concentrations were associated with the transitionalzones between Slope Water, the Gulf Stream and Northern SargassoSea. Chlorophyll b occurred in 47% of the samples analyzed andthe dominant chlorophyll a degradation product was phaeophorbidea. Chlorophyllide a was usually found at low concentrationsand phaeophytin a levels were undetectable. Concentrations ofthe various chlorophylls and their degradation products wereused to infer Changes in taxonomic composition, phytoplanktonbiomass and zooplankton grazing pressure across several frontalzones.     

Detection of glycolate oxidase gene glcD diversity among cultured and environmental marine bacteria

Of eight laboratory cultures of marine gamma- and alpha-Proteobacteria tested, growth on glycolate as a sole carbon source was detected for only three species: Pseudomonas stutzeri, Oceanimonas doudoroffii and Roseobacter sp. isolate Y3F. Degenerate polymerase chain reaction (PCR) primers were designed to amplify glcD, which encodes the D-subunit of the enzyme glycolate oxidase; glcD could be amplified only from those cultures that grew on glycolate. The PCR primers were used to explore glcD diversity in four field samples collected from different ocean environments: an Atlantic Gulf Stream Ring, sampled above and below the thermocline and two Pacific coastal sites, Parks Bay and San Juan Channel, WA. Environmental glcD sequences belonged to six major bacterial phylogenetic groups, with most sequences forming novel clades with no close relatives. Different patterns of glcD diversity were observed within and between the two nutrient regimes. Comparison of glcD and 16S rDNA diversity and analyses of available bacterial genomes and a metgenomic library from the Sargasso Sea show that glycolate-utilizing potential exists in only a subset of bacteria. Glycolate is produced in marine environments mainly by phytoplankton. Examination of glcD diversity will aid in understanding the influence of phytoplankton on bacterial community structure.     

Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak,southern Norway

The European eel (Anguilla anguilla L.) is distributed in coastal and inland habitats all over Europe, but spawns in the Sargasso Sea and is thus affected by both continental and oceanic factors. Since the 1980s a steady decline has been observed in the recruitment of glass eels to freshwater and in total eel landings. The eel is considered as critically endangered on the International Union for the Conservation of Nature and Natural Resources Red List of species. The Skagerrak beach seine survey from Norway constitutes the longest fishery-independent dataset on yellow/silver eels (starting in 1904). The Skagerrak coastal region receives larvae born in the Sargasso Sea spawning areas that have followed the Gulf Stream/North Atlantic Drift before they penetrate far into the North Sea. The Skagerrak coastal time series is therefore particularly valuable for exploring the impacts of oceanic factors on fluctuations in eel recruitment abundance. Analyses showed that Sargasso Sea surface temperature was negatively correlated with eel abundance, with a lag of 12 years revealing a cyclic and detrimental effect of high temperatures on the newly hatched larvae. The North Atlantic Oscillation index and inflow of North Atlantic water into the North Sea were negatively correlated with eel abundance, with a lag of 11 years. Increased currents towards the North Atlantic during high North Atlantic Oscillation years may send larvae into the subpolar gyre before they are ready to metamorphose and settle, resulting in low recruitment in the northern part of the distribution area for these years. The Skagerrak time series was compared with glass eel recruitment to freshwater in the Netherlands (Den Oever glass eel time series), and similar patterns were found revealing a cycle linked to changes in oceanic factors affecting glass eel recruitment. The recent decline of eels in the Skagerrak also coincided with previously documented shifts in environmental conditions of the North Sea ecosystem.     

Entry of migrating American eel leptocephali into the Gulf Stream system

Abundance, size and depth distributions of American eel(Anguilla rostrata) leptocephali collected in four transects across the Florida Current between the Straits of Florida and Cape Hatteras, North Carolina, were examined in order to assess the relative importance of two migration routes into the Gulf Stream system. A nine-fold increase in the abundance and an increase in the mean length of leptocephali collected in the Florida Current north of the Straits of Florida indicate that most leptocephali enter the Gulf Stream system directly from the Sargasso Sea rather than by a more southern route through the Bahama Islands. Leptocephali were concentrated in the upper 140 m at night and upper 350 m during the day. Wide vertical ranges in daytime collections precluded more refined estimation of vertical distribution. The collection of eight small leptocephali (11 to 17 mm total length) in the western Caribbean Sea is discussed in relation to the origin of leptocephali taken in the Straits of Florida.     

Organization of Squamata (Reptilia) assemblages in Mediterranean archipelagos

Mediterranean islands have complex reptile assemblages, but little is known about the factors that determine their organization. In this study, the structure of assemblages of Squamata was evaluated based on their species richness and two measures of phylogenetic diversity (variability and clustering). I evaluated the composition of the assemblages comparing distinct biogeographic subregions within the Mediterranean: Adriatic, Aegean, Balearic, Corsica–Sardinia, Crete, Gulf of Gabés, Ionian Sea, Ligurian Sea, Malta, Sicily, and Tyrrhenian Sea. The effect of island environments and geographical isolation on the diversity metrics was assessed using generalized linear models. The analyses indicated that species richness was mostly influenced by island area and geographical isolation. Assemblages on smaller islands were poorer in species and phylogenetically dispersed, possibly as an effect of interspecific competition. The species composition of the assemblages was determined by similar environmental drivers within the biogeographic subregions, including island area, island elevation, geographical isolation, and aridity. In several subregions, significant patterns of phylogenetic attraction were found in species co‐occurrences, caused by the limits imposed by the island size on large predatory species.     

Effect of iron on phytoplankton production in the Sargasso Sea

Addition of chelated iron at concentrations of 0.5, 1.0, 10 and 100μg Fe·l^?1 to Sargasso Sea phytoplankton assemblages did not affect the rate of photosynthesis during 4-h incubation experiments. In experiments with 72-h incubations, however, iron enhanced carbon assimilation. The enhancement was independent of iron concentration.     

Day/night differences in the grazing impact of marine copepods

Day/night differences in the removal rate of phytoplankton can occur as a result of increased copepod grazing rates at certain times of the day and diel vertical migration of animals. We conducted shipboard grazing experiments and fine-scale vertical zooplankton sampling to resolve these behaviors. Day/night feeding differences were compared in the center of several warm-core Gulf Stream rings, under conditions of no lateral water mass exchange, in the mesohaline portion of Chesapeake Bay and when following drogues in the Chesapeake Bay plume. Day/night variations in copepod biomass in the surface mixed layer were greater in neritic waters as compared to the open ocean stations. Day/night differences in weight-specific copepod filtration rates varied less than biomass. At the neritic stations copepod grazing was often higher at night, whereas at the oceanic stations day/night grazing rates were similar or daytime grazing rates were highest. The night/day ratio of zooplankton grazing impact on the phytoplankton community (the product of zooplankton biomass and their weight-specific grazing rate) averaged 4.8 in the Chesapeake Bay plume and 1.6 in warm-core Gulf Stream rings. Our results suggest that at lower food levels, there often are less day/night differences in the removal rate of phytoplankton by the copepod community.     

Patterns of sarcodine feeding in epipelagic oceanic plankton

The range of in situ prey composition was determined in marineplanktonic acantharia, foraminifera and radiolana collectedby divers, and quantitatively compared with the prey available,as determined by surface plankton hauls on cruises in the FloridaCurrent, Gulf Stream and Sargasso Sea. A relatively large percentageof the sarcodines (60% of acantharia, 48% of foraminifera and46% of radiolana) had no detectable prey Of those which hadfed on identifiable prey, there was considerable overlap betweensarcodine species m the types of prey captured. Nevertheless,some partitioning of food resources was evident Foraminiferaconsumed greater numbers of diatoms and copepods than otherprey types, radiolana consumed more tintinnids and mollusc larvae,and acantharia consumed mostly tintinnids. Copepods and theirnauplii dominated the biomass consumed for all three groups,though mollusc larvae were significant for both acantharia andradiolana. The results of parameteric univariate statisticalanalyses earned out on each major predator group and multivariateanalysis on a species-by-speaes basis confirmed that there wasevidence for some partitioning of prey resources among the majorsarcodine predators. The partitioning appeared to follow primarilymorphological rather than taxonomic criteria, however, and mayhave been at least partially a mechanical effect.     

The Distribution and Ecology of Ariosoma Balearicum (Congridae) Leptocephali in the Western North Atlantic

A total of 4589 leptocephali of the congrid eel, Ariosoma balearicum, were examined from 17 cruises in the western North Atlantic Ocean. Myomere counts made on 915 of these indicated there were two ranges of number of myomeres that appear to be associated with separate spawning populations. Those with the higher range (high count: 128–137) were consistently 70–100mm in length in the Sargasso Sea from February to April and 20–80mm in length in the northern Sargasso Sea and Gulf Stream from September to October. Those with the lower range (low count: 120–130) were rare in the northern and eastern Sargasso Sea where they had consistently greater lengths than high count leptocephali and were most abundant in the Florida Current and Providence Channel. The geographic distributions of size and myomere ranges in relation to hydrography provide strong support for the hypothesis that high count eels found along the South Atlantic Bight (SAB) migrate across the Florida Current to spawn in the northwest Sargasso Sea. This migratory pattern is similar to those of Anguilla rostrata and Conger oceanicus, which use the southern Sargasso Sea for development as larvae. However, the distribution of high count leptocephali suggests that they use the entire Sargasso Sea gyre as a development area as larvae before crossing the Florida Current and recruiting to the SAB. The low count eels inhabiting the Bahamas appear to spawn near the banks and their abundance in the Providence Channel and southwest Sargasso Sea suggests most are retained close to the Bahamas. These two distinct styles of spawning, distribution and recruitment of larvae are hypothesized to be related to the different hydrographic regimes of the two juvenile habitats and the resulting constraints on growth and recruitment of larvae. Vertebral and myomere counts reported from other areas suggest there are distinct populations in other regions of the North Atlantic Ocean.     

Live and dead benthic foraminiferal assemblages from coastal environments of the Aegean Sea (Greece): Distribution and diversity

Benthic foraminiferal composition assemblages and their temporal changes, ecological indices and foraminiferal densities are used to compare three coastal environments with different physicogeographical features in the Aegean Sea (coastal environment of Avdira–Vistonikos Gulf and Kitros–Thermaikos Gulf and open lagoonal environment of Vravron–South Evoikos Gulf). Three main foraminiferal assemblages have been recognized: a) “Assemblage A”; high degree of similarity between living and dead foraminiferal species, dominated by Ammonia beccarii, Elphidium spp. and relatively abundant and diverse miliolids, b) “Assemblage B1”; intermediate degree of similarity between live and dead assemblages, characterized by highly-abundant and well-diversified foraminiferal assemblages including the algal symbiont bearing Peneroplis pertusus together with Ammonia tepida and several small epiphytic rotaliids and miliolids, and c) “Assemblage B2”; absence of living individuals, strongly dominated by the opportunistic species A. tepida. Our results suggest a good comparison between living and dead assemblages from different coastal environments in the Aegean Sea, however the prevailing environmental conditions (vegetation cover, hydrodynamics, fresh water influx) have a strong impact on the taphonomic processes.     

Distribution patterns of epilithic diatoms along climatic,spatial and physicochemical variables in the Baltic Sea

The species richness and community composition of the diatom communities were studied in the Baltic Sea, Northern Europe, to enhance knowledge about the diversity of these organisms in a brackish water ecosystem. Many organisms in the Baltic Sea have been studied extensively, but studies investigating littoral diatoms are scarce. The goal of this study was to examine the importance of climatic, spatial and water physicochemical variables as drivers of epilithic diatoms in the Gulf of Finland and the Gulf of Bothnia. The variation in species richness was best explained by pH, total phosphorus and total nitrogen. Redundancy Analysis indicated that the most important factors correlating with species composition were air temperature, silicon, total phosphorus, water temperature, salinity and pH. Variation Partitioning showed that the species composition was mostly affected by climatic and spatial variables, whereas physicochemical variables had little impact. However, the strongest factor was the combined influence of climatic, spatial and physicochemical variables. The results suggest that diatom species richness in the northern Baltic Sea is primarily regulated by local factors, while climatic and spatial variables have little impact on richness. Species composition is mostly affected by climatic and spatial variables. We conclude that understanding the distribution patterns of Baltic Sea diatoms requires the inclusion of climatic, spatial and water chemistry variables.     

Diurnal variations in photosynthetic rates: comparisons of ultraphytoplankton with a larger phytoplankton size fraction

We compared the composition and photosynthetic activity of twosize fractions of phytoplankton during a cruise from the Gulfof Maine to the Sargasso Sea in August 1983. At every station,and at every depth, ultraplankton (defined here as cells passingthrough 3 µm pores in filters) made a major contributionto both the standing crop of chlorophyll and the rate of primaryproduction. Ultraphytoplankton assemblages were dominated byphycoerythrin-rich cyanobacteria. Overall, the ultraplanktoncontribution to total primary production was greatest at lowphoton fluxes: (i) at the beginning and end of the photoperiod;(ii) with increasing depth in the euphotic zone; and (iii) whendaily irradiance was low. The composition of ultraphytoplanktonvaried with depth. Surface (2 m) ultraphytoplankton assemblageswere almost exclusively composed of phycoerythrin-rich cyanobacteriawith smaller (0.2–0.8 µm) cyanobacteria predominating.Below the surface mixed-layer, the proportion of larger (0.8–30 µm) to smaller cyanobacteria increased and the eukaryoticcomponent of the ultraphytoplankton often became important.At two Sargasso Sea stations, the greatest numbers of cyanobacteriawere below the mixed layer at the 1% light level, while themaximum numbers of eukaryotic ultraphytoplankters occurred deeperstill, at the 0.5% light level, coincident with the chlorophyllmaximum. At the bottom of the euphotic layer in the SargassoSea. eukaryotes numerically dominated the ultraphytoplanktonand made a major contribution to the chlorophyll maximum.     

Speciation and demographic history of Atlantic eels (Anguilla anguilla and A. rostrata) revealed by mitogenome sequencing

Processes leading to speciation in oceanic environments without obvious physical barriers remain poorly known. European and American eel (Anguilla anguilla and A. rostrata) spawn in partial sympatry in the Sargasso Sea. Larvae are advected by the Gulf Stream and other currents towards the European/North African and North American coasts, respectively. We analyzed 104 mitogenomes from the two species along with mitogenomes of other Anguilla and outgroup species. We estimated divergence time between the two species to identify major events involved in speciation. We also considered two previously stated hypotheses: one where the ancestral species was present in only one continent but was advected across the Atlantic by ocean current changes and another where population declines during Pleistocene glaciations led to increasing vicariance, facilitating speciation. Divergence time was estimated to ∼3.38 Mya, coinciding with the closure of the Panama Gateway that led to reinforcement of the Gulf Stream. This could have advected larvae towards European/North African coasts, in which case American eel would be expected to be the ancestral species. This scenario could, however, not be unequivocally confirmed by analyses of dN/dS, nucleotide diversity and effective population size estimates. Extended bayesian skyline plots showed fluctuations of effective population sizes and declines during glaciations, and thus also lending support to the importance of vicariance during speciation. There was evidence for positive selection at the ATP6 and possibly ND5 genes, indicating a role in speciation. The findings suggest an important role of ocean current changes in speciation of marine organisms.