Evidence of a human-mediated invasion of the tropical western Atlantic by the 'world's most common brittlestar'

Approximately three million years ago the Isthmus of Panama formed an impenetrable land barrier between the tropical eastern Pacific Ocean and the tropical western Atlantic Ocean. Since this time, isolated geminate species have evolved from once contiguous populations, either side of the barrier. One such organism whose distribution is divided by the Isthmus is the tropical brittlestar Ophiactis savignyi, once suggested to be the most common brittlestar in the world. Rather than showing a genetic pattern consistent with a history of isolation, we show that this species has recently dispersed between the Pacific Ocean and the western Atlantic Ocean. This conclusion is based upon a phylogenetic analysis using sequences of the COI mitochondrial DNA gene from these populations. Identical haplotypes between oceans, and a genetic signature of population expansion, provide compelling evidence that the western Atlantic contains at least one cluster of haplotypes recently derived from the Indo-Pacific. Inadvertent human-aided translocation of individuals, presumably in ballast water or fouling communities, is strongly implicated as a mechanism for dispersal between oceans. We believe that cryptic marine invasions are likely to be common and our awareness of them will rapidly increase as systematic and phylogeographic knowledge of marine taxa grow.     

Relationships among Egg Size, Composition, and Energy: A Comparative Study of Geminate Sea Urchins

Egg size is one of the fundamental parameters in the life histories of marine organisms. However, few studies have examined the relationships among egg size, composition, and energetic content in a phylogenetically controlled context. We investigated the associations among egg size, composition, and energy using a comparative system, geminate species formed by the closure of the Central American Seaway. We examined western Atlantic (WA) and eastern Pacific (EP) species in three echinoid genera, Echinometra, Eucidaris, and Diadema. In the genus with the largest difference in egg size between geminates (Echinometra), the eggs of WA species were larger, lipid rich and protein poor compared to the smaller eggs of their EP geminate. In addition, the larger WA eggs had significantly greater total egg energy and summed biochemical constituents yet significantly lower egg energy density (energy-per-unit-volume). However, the genera with smaller (Eucidaris) or no (Diadema) differences in egg size were not significantly different in summed biochemical constituents, total egg energy, or energy density. Theoretical models generally assume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life history evolution. We show that even among closely-related taxa, large eggs cannot be assumed to be scaled-up small eggs either in terms of energy or composition. Although our data comes exclusively from echinoid echinoderms, this pattern may be generalizable to other marine invertebrate taxa. Because egg composition and egg size do not necessarily evolve in lockstep, selective factors such as sperm limitation could act on egg volume without necessarily affecting maternal or larval energetics.     

Fossil calibration of molecular clocks and the divergence times of geminate species pairs separated by the isthmus of panama

Calibration of nucleotide sequence divergence rates provides an important method by which to test many hypotheses of evolution. In the absence of an adequate fossil record, geological events, rather than the first appearances of sister taxa in the geological record, are often used to calibrate molecular clocks. The formation of the Isthmus of Panama, which isolated the tropical western Atlantic and eastern Pacific oceans, is one such event that is frequently used to infer rates of nucleotide sequence divergence. Isthmian calibrations assume that morphologically similar "geminate" species living now on either side of the isthmus were isolated geographically by the latest stages of seaway closure 3.1-3.5 MYA. Here, I have applied calibration dates from the fossil record to cytochrome c oxidase-1 (CO1) and nuclear histone-3 (H3) divergences among six pairs of geminates in the Arcidae to test this hypothesis. Analysis of CO1 first and third positions yield geminate divergences that predate final seaway closure, and on the basis of CO1 first positions, times for all six geminates are significantly greater than 3.5 Myr. H3 sequences produce much more recent geminate divergences, some that are younger than 3.1 Myr. But H3-derived estimates for all arcid geminates are not significantly different from both 0 and 15 Myr. According to CO1, one of the two most divergent pairs, Arca mutabilis and A. imbricata, split more than 30 MYA. This date is compatible with the fossil record, which indicates that these species were morphologically distinct at least 16-21 MYA. Across all CO1 nucleotide sites, divergence rates for arcids are slower than the rates reported for other taxa on the basis of isthmian calibrations, with the exception of rates determined from the least divergent species pair in larger surveys of multiple transisthmian pairs. Rate differences between arcids and some taxa may be real, but these data suggest that divergence rates can be greatly overestimated when dates corresponding to final closure of the Central American Seaway are used to calibrate the molecular clocks of marine organisms.     

Species-level phylogeography and evolutionary history of the hyperdiverse marine gastropod genus Conus

Phylogenetic and paleontological analyses are combined to reveal patterns of species origination and divergence and to define the significance of potential and actual barriers to dispersal in Conus, a species-rich genus of predatory gastropods distributed throughout the world's tropical oceans. Species-level phylogenetic hypotheses are based on nucleotide sequences from the nuclear calmodulin and mitochondrial 16S rRNA genes of 138 Conus species from the Indo-Pacific, eastern Pacific, and Atlantic Ocean regions. Results indicate that extant species descend from two major lineages that diverged at least 33 mya. Their geographic distributions suggest that one clade originated in the Indo-Pacific and the other in the eastern Pacific + western Atlantic. Impediments to dispersal between the western Atlantic and Indian Oceans and the central and eastern Pacific Ocean may have promoted this early separation of Indo-Pacific and eastern Pacific + western Atlantic lineages of Conus. However, because both clades contain both Indo-Pacific and eastern Pacific + western Atlantic species, migrations must have occurred between these regions; at least four migration events took place between regions at different times. In at least three cases, incursions between regions appear to have crossed the East Pacific Barrier. The paleontological record illustrates that distinct sets of Conus species inhabited the Indo-Pacific, eastern Pacific + western Atlantic, and eastern Atlantic + former Tethys Realm in the Tertiary, as is the case today. The ranges of <1% of fossil species (N=841) spanned more than one of these regions throughout the evolutionary history of this group.     

Phylogenetic relationships of tropical western Atlantic snappers in subfamily Lutjaninae (Lutjanidae: Perciformes) inferred from mitochondrial DNA sequences

Phylogenetic relationships among 20 nominal species of tropical lutjanine snappers (Lutjanidae) (12 from the western Atlantic, one from the eastern Pacific, and seven from the Indo‐Pacific) were inferred based on 2206 bp (712 variable, 614 parsimony informative) from three protein‐coding mitochondrial genes. Also included in the analysis were DNA sequences from two individuals, identified initially as Lutjanus apodus, which were sampled off the coast of Bahia State in Brazil (western Atlantic), and from three individuals labelled as ‘red snapper’ in the fish market in Puerto Armuelles, Panama (eastern Pacific). Bayesian posterior probabilities and maximum‐likelihood bootstrap percentages strongly supported monophyly of all lutjanines sampled and the hypothesis that western Atlantic lutjanines are derived from an Indo‐Pacific lutjanine lineage. The phylogenetic hypothesis also indicated that oceans where lutjanines are distributed (western Atlantic, eastern Pacific, and Indo‐Pacific) are not reciprocally monophyletic for the species distributed within them. There were three strongly supported clades that included all western Atlantic lutjanines: one included six species of Lutjanus from the western Atlantic, two species of Lutjanus from the eastern Pacific, and the monotypic genera Rhomboplites and Ocyurus (western Atlantic); one that included three, probably four, species of Lutjanus in the western Atlantic; and one that included Lutjanus cyanopterus (western Atlantic), an unknown species of Lutjanus from the eastern Pacific, and three species of Lutjanus from the Indo‐Pacific. Molecular‐clock calibrations supported an early Miocene diversification of an Indo‐Pacific lutjanine lineage that dispersed into the western Atlantic via the Panamanian Gateway. Divergent evolution among these lutjanines appears to have occurred both by vicariant and ecological speciation: the former following significant geographic or geological events, including both shoaling and closure of the Panamanian Gateway and tectonic upheavals, whereas the latter occurred via phenotypic diversification inferred to indicate adaptation to life in different habitats. Taxonomic revision of western Atlantic lutjanines appears warranted in that monotypic Ocyurus and Rhomboplites should be subsumed within the genus Lutjanus. Finally, it appears that retail mislabelling of ‘red snapper’ in commercial markets extends beyond the USA. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 915–929.     

A DWARF FORM OF THE SPINNER DOLPHIN (STENELLA LONGIROSTRIS) FROM THAILAND

A very small form of the spinner dolphin has been found to inhabit the Gulf of Thailand. Ten specimens taken incidentally in a local shrimp fishery differ from specimens of this species collected elsewhere in body size and shape, skull size and shape, number of teeth and numbers of thoracic and lumbar vertebrae. Four cranially adult males were 129–137 cm long, well below the ranges for the Indian Ocean, western Pacific, central Pacific, eastern Pacific and Atlantic. The skull is also very small. Tooth counts and vertebral counts average lower than in other series. The color pattern is not significantly different from that of spinner dolphins in the central and western Pacific, Atlantic and Indian Oceans but differs from that of the small eastern spinner of the eastern Pacific. The Gulf of Thailand specimens are morphologically separable from all other specimens, but it is to be expected that when larger samples are available there will be some overlap. The dwarf form may overlap in body and skull size with small spinner dolphins taken incidentally in a gillnet fishery for sharks off northern Australia. The geographic range of the dwarf spinner may be restricted. The size and status of the population and the impact of the shrimp fishery are unknown and should be investigated. The dwarf spinner may have an ecology different from that of other spinners, feeding mainly on reef-associated and benthic organisms rather than mesopelagic animals.     

Evolution of poecilogony from planktotrophy: cryptic speciation, phylogeography, and larval development in the gastropod genus Alderia

Poecilogony, a rare phenomenon in marine invertebrates, occurs when alternative larval morphs differing in dispersal potential or trophic mode are produced from a single genome. Because both poecilogony and cryptic species are prevalent among sea slugs in the suborder Sacoglossa (Gastropoda: Opisthobranchia), molecular data are needed to confirm cases of variable development and to place them in a phylogenetic context. The nominal species Alderia modesta produces long-lived, feeding larvae throughout the North Atlantic and Pacific, but in California can also produce short-lived larvae that metamorphose without feeding. We collected morphological, developmental, and molecular data for Alderia from 17 sites spanning the eastern and western Pacific and North Atlantic. Estuaries south of Bodega Harbor, California, contained a cryptic species (hereafter Alderia sp.) with variable development, sister to the strictly planktotrophic A. modesta. The smaller Alderia sp. seasonally toggled between planktotrophy and lecithotrophy, with some individuals differing in development but sharing mitochondrial DNA haplotypes. The sibling species overlapped in Tomales Bay, California, but showed no evidence of hybridization; laboratory mating trials suggest postzygotic isolation has arisen. Intra- and interspecific divergence times were estimated using a molecular clock calibrated with geminate sacoglossans. Speciation occurred about 4.1 million years ago during a major marine radiation in the eastern Pacific, when large inland embayments in California may have isolated ancestral populations. Atlantic and Pacific A. modesta diverged about 1.7 million years ago, suggesting trans-Arctic gene flow was interrupted by Pleistocene glaciation. Both Alderia species showed evidence of late Pleistocene population expansion, but the southern Alderia sp. likely experienced a more pronounced bottleneck. Reduced body size may have incurred selection against obligate planktotrophy in Alderia sp. by limiting fecundity in the face of high larval mortality rates in warm months. Alternatively, poecilogony may be an adaptive response to seasonal opening of estuaries, facilitating dispersal by long-lived larvae. An improved understanding of the forces controlling seasonal shifts in development in Alderia sp. may yield insight into the evolutionary forces promoting transitions to nonfeeding larvae.     

Low worldwide genetic diversity in the basking shark (Cetorhinus maximus)

The basking shark (Cetorhinus maximus) is found in temperate waters throughout the world's oceans, and has been subjected to extensive exploitation in some regions. However, little is known about its current abundance and genetic status. Here, we investigate the diversity of the mitochondrial DNA control region among samples from the western North Atlantic, eastern North Atlantic, Mediterranean Sea, Indian Ocean and western Pacific. We find just six haplotypes defined by five variable sites, a comparatively low genetic diversity of pi=0.0013 and no significant differentiation between ocean basins. We provide evidence for a bottleneck event within the Holocene, estimate an effective population size (Ne) that is low for a globally distributed species, and discuss the implications.     

PHYLOGENY AND BIOGEOGRAPHY OF THE ATLANTIC AND EASTERN PACIFIC HYPSELODORIS STIMPSON, 1855 (NUDIBRANCHIA, CHROMODORIDIDAE) WITH THE DESCRIPTION OF A NEW SPECIES FROM THE CARIBBEAN SEA

A new species, based on two specimens from Guana Island, BritishVirgin Islands, is described. The new species agrees with thegenus Hypselodoris in having a high body profile, a large vestibulargland and mantle glands. It differs from other members of thegenus in the Atlantic Ocean by having a reddish background bodycolour. In addition, dorsal colour patterns such as a broadcentral white line with lateral extensions and the lack of yellowlines or spots further differentiate this species. The radularformula of 52x41.0.41 and a smaller seminal receptacle are alsodistinctive. The phylogenetic relationships of 34 species andsubspecies of Hypselodoris from the eastern Pacific and Atlanticare examined using morphological characters. With the exceptionof the new species, these are characterized by a dark blue backgroundbody colour. The phylogenetic analysis of the data matrix resultedin eight most-parsimonious trees. The resulting consensus treeshows that eastern Pacific and Atlantic species of Hypselodorisconstitute a monophyletic group that is basally split into twosister clades. One clade contains the eastern Pacific speciesand most of the Caribbean species, whereas the other clade containsthe eastern Atlantic species. The new species is the sisterto the rest of the Caribbean species, which also form a monophyleticgroup. This phylogenetic hypothesis suggests that two consecutivevicariant events have affected the biogeography of Hypselodoris:(1) the closure of communication between the tropical Indo-Pacificregion and the Atlantic and eastern Pacific, completed withthe formation of the East Pacific Barrier; and (2) the riseof the Panama isthmus. (Received 19 December 2003; accepted 12 October 2005)     

Phylogeography of the pantropical sea urchin Tripneustes: contrasting patterns of population structure between oceans

To understand how allopatric speciation proceeds, we need information on barriers to gene flow, their antiquity, and their efficacy. For marine organisms with planktonic larvae, much of this information can only be obtained through the determination of divergence between populations. We evaluated the importance of ocean barriers by studying the mitochondrial DNA phylogeography of Tripneustes, a pantropical genus of shallow water sea urchin. A region of cytochrome oxidase I (COI) was sequenced in 187 individuals from locations around the globe. The COI phylogeny agreed with a previously published phylogeny of bindin that barriers important to the evolution of Tripneustes are: (1) the cold water upwelling close to the tip of South Africa, (2) the Isthmus of Panama, (3) the long stretch of deep water separating the eastern from the western Atlantic, and (4) the freshwater plume of the Orinoco and the Amazon rivers between the Caribbean and the coast of Brazil. These barriers have previously been shown to be important in at least a subset of the shallow water marine organisms in which phylogeography has been studied. In contrast, the Eastern Pacific Barrier, 5000 km of deep water between the central and the eastern Pacific that has caused the deepest splits in other genera of sea urchins, is remarkably unimportant as a cause of genetic subdivision in Tripneustes. There is also no discernible subdivision between the Pacific and Indian Ocean populations of this genus. The most common COI haplotype is found in the eastern, central, and western Pacific as well as the Indian Ocean. Morphology, COI, and bindin data agree that T. depressus from the eastern Pacific and T. gratilla from the western Pacific are, in fact, the same species. The distribution of haplotype differences in the Indo-Pacific exhibits characteristics expected from a sea urchin genus with ephemeral local populations, but with high fecundity, dispersal, and growth: there is little phylogenetic structure, and mismatch distributions conform to models of recent population expansion on a nearly global scale. Yet, comparisons between local populations produce large and significant F(ST) values, indicating nonrandom haplotype distribution. This apparent local differentiation is only weakly reflected in regional divergence, and there is no evidence of isolation by distance in correlations between F(ST) values and either geographical or current distance. Thus, Tripneustes in the Indo-Pacific (but not in the Atlantic) seems to be one large metapopulation spanning two oceans and containing chaotic, nonequilibrium local variation, produced by the haphazard arrival of larvae or by unpredictable local extinction.     

Phylogenetic relationships among calyptraeid gastropods and their implications for the biogeography of marine speciation

Although calyptraeid gastropods are not well understood taxonomically, in part because their simple plastic shells are the primary taxonomic character, they provide an ideal system to examine questions about evolution in the marine environment. I conducted a phylogenetic analysis of calyptraeid gastropods using DNA sequence data from mitochondrial cytochrome oxidase I (COI) and 16S genes and the nuclear 28S gene. The resultant phylogeny was used to examine the biogeographic patterns of speciation in the Calyptraeidae. Parsimony and Bayesian analyses of the combined data sets for 94 calyptraeid operational taxonomic units and 24 outgroups produced well-resolved phylogenies. Both approaches resulted in identical sister-species relationships, and the few differences in deeper topology did not affect biogeographic inferences. The geographic distribution of the species included here demonstrate numerous dispersal events both between the Pacific and Atlantic oceans and across the equator. When parsimony is used to reconstruct the movement from the Pacific to the Atlantic oceans on the phylogeny, there are 12 transitions between oceans, primarily from the Pacific to the Atlantic. When the latitude is coded as north versus south of the equator, the most-parsimonious reconstruction gives the origin of calyptraeids in the north followed by 15 dispersal events to regions south of the equator and no returns to the north. Many clades of the most closely related species are either sympatric or occur along a single coastline. Closely related species can, however, occur in such divergent regions as Southern California and South Africa. There is little evidence for sister-species pairs or larger clades having been split by the Isthmus of Panama or the Benguela upwelling, but the East Pacific Barrier appears to separate the most basal taxa from the rest of the family.     

Speciation and diversity on tropical rocky shores: a global phylogeny of snails of the genus Echinolittorina

A phylogenetic approach to the origin and maintenance of species diversity ideally requires the sampling of all species within a clade, confirmation that they are evolutionarily distinct entities, and knowledge of their geographical distributions. In the marine tropics such studies have mostly been of fish and reef-associated organisms, usually with high dispersal. In contrast, snails of the genus Echinolittorina (Littorinidae) are restricted to rocky shores, have a four-week pelagic development (and recorded dispersal up to 1400 km), and show different evolutionary patterns. We present a complete molecular phylogeny of Echinolittorina, derived from Bayesian analysis of sequences from nuclear 28S rRNA and mitochondrial 12S rRNA and COI genes (nodal support indicated by posterior probabilities, maximum likelihood, and neighbor-joining bootstrap). This consists of 59 evolutionarily significant units (ESUs), including all 50 known taxonomic species. The 26 ESUs found in the Indo-West Pacific region form a single clade, whereas the eastern Pacific and Atlantic species are basal. The earliest fossil occurred in the Tethys during the middle Eocene and we suggest that the Indo-West Pacific clade has been isolated since closure of the Tethyan seaway in the early Miocene. The geographical distributions of all species (based on more than 3700 locality records) appear to be circumscribed by barriers of low temperature, unsuitable sedimentary habitat, stretches of open water exceeding about 1400 km, and differences in oceanographic conditions on the continuum between oceanic and continental. The geographical ranges of sister species show little or no overlap, indicating that the speciation mode is predominantly allopatric. Furthermore, range expansion following speciation appears to have been limited, because a high degree of allopatry is maintained through three to five branching points of the phylogeny. This may be explained by infrequent long-distance colonization, habitat specialization on the oceanic/continental gradient, and perhaps by interspecific competition. In the eastern Pacific plus Atlantic we identify five cases of divergence on either side of the Isthmus of Panama, but our estimates of their ages pre-date the emergence of the Isthmus. There are three examples of sister relationships between species in the western Atlantic and eastern Atlantic, all resulting from dispersal to the east. Within the Indo-West Pacific, we find no geographical pattern of speciation events; narrowly endemic species of recent origin are present in both peripheral and central parts of the region. Evidence from estimated divergence times of sister species, and from a plot of the number of lineages over time, suggest that there has been no acceleration of diversification during the glacio-eustatic cycles of the Plio-Pleistocene. In comparison with reefal organisms, species of Echinolittorina on rocky shores may be less susceptible to extinction or isolation during sea-level fluctuations. The species richness of Echinolittorina in the classical biogeographic provinces conforms to the common pattern of highest diversity (11 species) in the central "East Indies Triangle" of the Indo-West Pacific, with a subsidiary focus in the eastern Pacific and western Atlantic, and lowest diversity in the eastern Atlantic. The diversity focus in the East Indies Triangle is produced by a mosaic of restricted allopatric species and overlap of a few widespread ones, and is the result of habitat specialization rather than historical vicariance. This study emphasizes the plurality of biogeographic histories and speciation patterns in the marine tropics.     

A new look at geographic and phylogenetic relationships within the species group surrounding Octopus vulgaris (Mollusca, Cephalopoda): indications of very wide distribution from mitochondrial DNA sequences

The distribution of Octopus vulgaris has not yet been completely clarified. For a long time, a cosmopolitan distribution with unknown distribution limits had been assumed. This assumption has recently been questioned and it has been postulated that the distribution is restricted to the Mediterranean and the northeastern Atlantic. However, as our previous studies show, the existence of O. vulgaris can be confirmed for the Mediterranean and the whole eastern Atlantic, and evidence is provided for its occurrence in the western Atlantic. The aim of the present work is to extend our previous data matrix and to clarify whether O. vulgaris exists in the northwestern Pacific. Therefore, the sequence variation in ostensible O. vulgaris from 13 localities in the Mediterranean (France), the Atlantic Ocean [Lanzarote, Senegal, South Africa (Atlantic, Indian Ocean), Tristan da Cunha, north, middle and south Brazil], the Caribbean Sea (Venezuela) and the Pacific Ocean (Taiwan, Japan and Costa Rica) was examined using the mitochondrial genes coding for the 16S rRNA and cytochrome oxidase subunit III (COIII). Sequence divergence was relatively low between populations of O. vulgaris from the Mediterranean, the eastern and western Atlantic (except north Brazil), Venezuela, Taiwan and Japan compared with other species of the genus Octopus . Trees constructed by using maximum likelihood, neighbour joining and maximum parsimony algorithms (PAUP) show the above-mentioned populations from the Mediterranean, the western and eastern Atlantic, Venezuela and the northwestern Pacific (Japan and Taiwan) as a monophyletic cluster. Thus, even if the Octopus vulgaris -like octopus from north Brazil should turn out a cryptic species, the data of this work not only support our hypothesis of the distribution of O. vulgaris in the Mediterranean, the eastern and western Atlantic but also show that O. vulgaris is present in the northwestern Pacific, namely in the waters of Taiwan and Japan.     

Dispersal barriers in tropical oceans and speciation in Atlantic and eastern Pacific sea urchins of the genus Echinometra

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).     

Gordiichthys combibus, a new species of eastern Pacific sand-eel (Anguilliformes: Ophichthidae)

A new species of bascanichthyin ophichthid, Gordiichthys combibus, is described from shallow water along the Pacific coast of Colombia. It is the first known eastern Pacific species of Gordiichthys and is very similar to G. randalli from Puerto Rico. It differs from its other western Atlantic congeners in vertebral number and other characters. A key to the genus is provided.     

Worldwide phylogeography of the blacktip shark (Carcharhinus limbatus) inferred from mitochondrial DNA reveals isolation of western Atlantic populations coupled with recent Pacific dispersal

Although many coastal shark species have widespread distributions, the genetic relatedness of worldwide populations has been examined for few species. The blacktip shark, (Carcharhinus limbatus), inhabits tropical and subtropical coastal waters throughout the world. In this study, we examined the genetic relationships of blacktip shark populations (n = 364 sharks) throughout the majority of the species' range using the entire mitochondrial control region (1067-1070 nucleotides). Two geographically distinct maternal lineages (western Atlantic, Gulf of Mexico, and Caribbean Sea clades, and eastern Atlantic, Indian, and Pacific Ocean clades) were identified and shallow population structure was detected throughout their geographic ranges. These findings indicate that a major population subdivision exists across the Atlantic Ocean, but not the Pacific Ocean. The historical dispersal of this widespread, coastal species may have been interrupted by the rise of the Isthmus of Panama. This scenario implies historical dispersal across the Pacific Ocean (supported by the recovery of the same common haplotype from the Philippines, Hawaii, and the Gulf of California reflecting recent/contemporary dispersal abilities) and an oceanic barrier to recent migration across the Atlantic. Genetic structure within the eastern Atlantic/Indo-Pacific (Phi(ST) = 0.612, P < 0.001) supports maternal philopatry throughout this area, expanding previous western Atlantic findings. Eastern Atlantic/Indo-Pacific C. limbatus control region haplotypes were paraphyletic to Carcharhinus tilstoni haplotypes in our maximum-parsimony analysis. The greater divergence of western Atlantic C. limbatus than C. tilstoni from eastern Atlantic/Indo-Pacific C. limbatus reflects the taxonomic uncertainty of western Atlantic C. limbatus.     

A new species of pikeblenny (Blenniiformes: Chaenopsidae: Chaenopsis) from the tropical eastern Pacific and a key to all species

The genus Chaenopsis presently includes 10 species, four in the eastern Pacific and six in the western Atlantic. Five individuals of an undescribed species of this genus were obtained at Gorgona Island in the eastern Pacific of Colombia, in depths between 3 and 5 m. This new species differs from all the Eastern Pacific species in an array of traits including meristic, coloration and morphometric characters. Chaenopsis celeste new species differs from its only sympatric species, C. deltarrhis, in having fewer pectoral rays, fewer dorsal spines and more dorsal-fin soft rays and anal-fin elements. This new species is found over shallow sandy, rubble and small rocks bottoms from Costa Rica to Colombia.     

Maternal Influences on Variation in Egg Sizes in Temperate Marine Fishes

SYNOPSIS. We examine the variation in egg sizes of marine teleostsand evaluate the maternal contribution to this variability.At the species level, egg sizes in 309 North Atlantic fishesrange from 0.3 to 18.0 mm diameter (median = 1.1), size at hatchingvaries directly with egg size, and large adult size is associatedwith large eggs but the relationship is weak. Within populations,egg sizes are distributed normally with a median coefficientof variation of 4% (n = 56 species). Egg size varied among femalesin all cases for which female-level data were found. Estimatesof the variance components of egg size due to females were foundfor three species and, as a percentage of total variance, are71 % for capelin, Mallotus villosus, 46% for winter flounder,Pleuronectes americanus, and 35% for Atlantic cod, Gadus morhua.For cod, which spawn multiple egg batches per year, an additionalbatches-within-females component was estimated to be 26%. Sizeat hatching also differs among sibgroups and is generally directlyrelated to egg size at the individual level. We modelled fishgrowth by allowing individuals to grow at exponential ratesfrom a normal distribution of initial sizes. Comparing sizevariation in model fish to empirical evidence suggests thatvariation in initial sizes, propagated by growth, could accountfor a large fraction of the size variation observed months afterhatching in natural populations. We view size variation in youngmarine fishes to be largely of maternal origin and environmentallymodulated, which if true has special consequences for fisheriesand aquaculture.     

Marine species formation along the rise of Central America: The anomuran crab Megalobrachium

The evolution of marine neotropical shallow water species is expected to have been greatly affected by physical events related to the emergence of the Central American Isthmus. The anomuran crab Megalobrachium, a strictly neotropical porcellanid genus, consists of four species in the West Atlantic (WA) and nine in the East Pacific (EP). Dispersal is limited to a relatively short planktonic phase, which lasts approximately two weeks. We obtained DNA sequences of three mitochondrial and two nuclear genes of all but one species of Megalobrachium to construct a time‐calibrated phylogeny of the genus and its historical phylogeography, based on the reconstruction of ancestral areas. The topology of the phylogenetic trees of Megalobrachium produced by Bayesian Inference (BI) and Maximum Likelihood (ML) were virtually congruent. The genus is monophyletic with respect to other porcellanids. Ancestral area reconstruction indicates that it arose in the eastern Pacific 18 million years ago and diversified into at least 13 species that are currently formally recognized and three additional species indicated by our data. Most morphological variation appears to have followed phylogenetic differentiation, though some cryptic speciation has also occurred. Four geminate clades in this genus implicate the gradual emergence of the Central American Isthmus in this diversification, but events preceding the final separation of the oceans as well as within‐ocean events after the cessation of water connections were also important.     

Panbiogeographical analysis of distribution patterns in hagfishes (Craniata: Myxinidae)

Aim To analyse the worldwide distribution patterns of hagfishes using panbiogeographical track analysis, and to attempt to correlate these patterns with the tectonic history of the ocean basins. Location Atlantic and Pacific oceans. Method The distributions of 47 out of 70 species of hagfish (in the genera Eptatretus, Myxine, Nemamyxine, Neomyxine, and Paramyxine) were studied by the panbiogeographical method of track analysis. The analysis was performed using distributional data obtained from the collections included in the Ocean Biogeographic Information System (OBIS, http://www.iobis.org ) and FishBase ( http://www.fishbase.org ), with additional records from the literature. Individual tracks were obtained for each species by plotting localities and connecting them by minimum‐spanning trees. Generalized tracks were determined from the spatial overlap between individual tracks. Results Six generalized tracks were found: in the Gulf of Mexico, Caribbean Sea, South‐eastern Atlantic, Western Pacific, North‐eastern Pacific and South‐eastern Pacific. Main conclusions The distribution patterns of myxinids are marked by a high degree of endemism and vicariance, and are correlated with the tectonic features involved in many of the events that led to the development of oceanic basins. The main massing of the group is around the Pacific Basin. In the Atlantic Ocean, the distribution of Myxine glutinosa seems to correspond to a classic trans‐oceanic track and vicariance resulting from the opening of the Atlantic Ocean during the Cretaceous. In the Pacific Ocean, the distribution of the Eptatretus and Paramyxine species is clearly associated with the margins of the Pacific tectonic plate. The generalized tracks of hagfishes are shared by several other groups of marine organisms, including many from shallow tropical waters, implying a common history for this marine biota. Overall, vicariance is a major feature of hagfish distribution, suggesting vicariant differentiation of widespread ancestors as a result of sea‐floor spreading between continents in connection with ocean formation.