Global phylogeography of the fissiparous sea-star genus <Emphasis Type="Italic">Coscinasterias</Emphasis>

The fissiparous starfish genus Coscinasterias (Verrill) is represented in shallow waters around many of the world's continents. This wide distribution could be explained by dispersal, vicariance, or translocation associated with shipping, and represents an excellent system for marine biogeographic research. We conducted a global phylogeographic analysis of 42 Coscinasterias mtDNA cytochrome oxidase 1 sequences (15 haplotypes) from 18 sites, including representatives of all four recognised species. Phylogenetic analysis yielded a robust phylogeny, with strong support for the monophyly of the genus (90% bootstrap support) and of each separate species (99-100%). Haplotypes exhibited strong phylogeographic structure, with robust mtDNA clades often associated with distinct land masses. A general lack of genetic differentiation within sites may reflect fissiparity. However, shared haplotypes over larger distances (e.g. across Japan), and the presence of related haplotypes on adjacent land masses (e.g. Tasmania, New Zealand; 1.6-1.8%) suggest that long-distance dispersal is an important biogeographical process for Coscinasterias. The 4.0-4.4% divergence between Japanese and South African sister groups may relate to transequatorial dispersal around the onset of the Pleistocene. Divergent Atlantic and Mediterranean populations of C. tenuispina (maximum 1.5%) may deserve subspecific recognition, and high divergences within Australian C. muricata (maximum 8.0%) suggest a species complex.     

Extreme mitochondrial DNA divergence within populations of the deep-sea gastropod Frigidoalvania brychia

The deep sea supports a diverse and highly endemic invertebrate fauna, the origin of which remains obscure. Little is known about geographic variation in deep-sea organisms or the evolutionary processes that promote population-level differentiation and eventual speciation. Sequence variation at the 16 S rDNA locus was examined in formalin-preserved specimens of the common upper bathyal rissoid Frigidoalvania brychia (Verrill, 1884) to examine its population genetic structure. The specimens came from trawl samples taken over 30 years ago at depths of 457-1,102 m at stations in the Northwest Atlantic south of Woods Hole, Massachusetts, USA. Near the upper boundary of its bathymetric range (500 m), extremely divergent haplotypes comprising three phylogenetically distinct clades (average uncorrected sequence divergence among clades ~23%, ~3% within clades) were found at stations separated by a maximum distance of ~80 km, suggesting the presence of high levels of intraspecific divergence or the possibility of morphologically cryptic species. Only one of these clades was found at two stations in the mid- to lower part of F. brychia's depth distribution (800-1,100 m), suggesting lower clade diversity with increasing depth, although among-sample divergence, with a single exception, was minimal. One station was genetically divergent from all others sampled, containing a unique suite of haplotypes including two found only at this site. Steep vertical selective gradients, major oceanographic changes during the late Cenozoic, and habitat fragmentation by submarine canyons might have contributed to an upper bathyal region that is highly conducive to evolutionary change.     

Sympatry of distinct mitochondrial DNA lineages in a copepod inhabiting estuarine creeks in the southeastern USA

The population genetic structure of the meiobenthic harpacticoid copepod Microarthridion littorale (Poppe) was examined with a geographic survey of a 348 bp fragment of the mitochondrial cytochrome b gene. Copepods were collected from ten locations on the coast from North Carolina to Georgia, USA, from January 1997 to November 1998. Sequence divergence among 198 individuals was as much as 4.3%, and three divergent mitochondrial clades were uncovered that differed by six to nine nucleotide changes. A rapid assay was developed to distinguish among mitochondrial clades, and an additional 333 specimens were surveyed. The three lineages co-occurred in seven of ten sampling locations. Data analyses were carried out separately for individuals assayed by DNA sequencing as well as for a combined data set that included individuals typed by restriction endonuclease digestion. An analysis of molecular variance indicated that a significant proportion of the total genetic variance could be partitioned among populations, although no significant correlation between geographical and genetic distance was detected.     

Genetic structure of round scad mackerel <Emphasis Type="Italic">Decapterus macrosoma</Emphasis> (Carangidae) in the Indo-Malay archipelago

Round scad mackerel sampled in 1995-1998 were analysed for genetic variation using mitochondrial-DNA and nuclear-DNA markers. Sequence variation for a fragment of the cytochrome b gene (mitochondrial), amplified by polymerase chain reaction, was screened across individuals using single strand conformation polymorphism (SSCP). Sequence analysis of all SSCP haplotypes indicated two mitochondrial clades separated by, on average, 2.3% nucleotide divergence. The geographic distribution of haplotypes was homogeneous (Weir and Cockerham's [^(q)] \hat \theta =-0.002). Also, no geographic heterogeneity was detected for length polymorphism of Intron 1 of the gene encoding aldolase B ([^(q)] \hat \theta =0.005). Although homogeneity in allele frequencies throughout the Indo-Malay archipelago conformed to the expectations for a widely distributed pelagic fish in a highly connected habitat, this was at variance with the sharp geographic structure previously uncovered in Indian scad mackerel, Decapterus russelli, a fish with life-history characteristics similar to D. macrosoma. A remarkable similarity, however, was the occurrence of two similarly distinct clades within each species, suggesting a common history of geographic isolation. Low sea levels in the Pleistocene might have caused the separation and vicariance of populations within both D. macrosoma and D. russelli. Subsequent genetic exchange between populations would then have erased allele-frequency differences at the cytochrome b and aldolase B loci in D. macrosoma while some barrier to gene flow was maintained in D. russelli.     

Sympatry and allopatry in the deeply divergent mitochondrial DNA clades of the estuarine pulmonate gastropod genus <Emphasis Type="Italic">Phallomedusa</Emphasis> (Mollusca,Gastropoda)

DNA sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene were collected from estuarine snails in the genus Phallomedusa to examine the effects of estuarine isolation on population structure and gene flow. Three clades were recovered, one corresponding to Phallomedusa austrina and two others with the morphology of Phallomedusa solida. The haplotype diversity in all three clades indicated recent population expansion. Phallomedusa austrina was restricted to the west of a previous land bridge in the Bass Strait between mainland Australia and Tasmania, and P. solida to its east and to northern Tasmania. Phylogeographic analysis of P. austrina and P. solida shows strong geographic separation of species, but no local genetic structure indicative of regional or estuarine isolation. The clades of P. solida exhibit substantial genetic divergence and were sympatric across their entire distribution in eastern Tasmania and mainland Australia. Such a situation, which has not previously been observed in phylogeographic studies of southeast Australia, suggests that P. solida has had a complex refugial history during periods of environmental challenge.     

Phylogenetic relationship among several genera of Dictyotaceae (Dictyotales, Phaeophyceae) based on 18S rRNA and partial rbcL gene sequences

The partial sequences of the 18S rRNA gene and the rbcL gene, RuBisCo spacer region of representatives of the Dictyotaceae were determined and compared to resolve their phylogenetic relationships. A total of 1,861 base pairs of 18S rDNA sequences were aligned and examined. There was a high similarity of nucleotide sequences within the tribe Dictyoteae (99.4-99.6%) and the tribe Zonarieae (92.8-99.8%). The largest intergeneric divergence within the Dictyotaceae was 7.31%, between Dilophus and Zonaria. The aligned 422 bases of partial rbcL gene sequences gave a similar phylogenetic relationship among taxa to those of the 18S rDNA sequences, although our alignment covered less than 30% of the whole gene. All genera of Dictyotales consisted of several clades forming a polychotomy in MP trees based on partial rbcL gene sequences and on 18S rDNA sequences. The genera within the tribe Dictyoteae formed a clade, a Dictyota-Phacydictyon-Dilophus clade, while genera of the tribe Zonarieae formed four clades: Padina; Distromium-Lobophora-Zonaria; Spatoglossum-Lobospira; and Dictyopteris-Spatoglossum. All genera of the Dictyotaceae included in these sequence analyses are classified into five groups, which have little phylogenetic resolution among them. The genera classified in the tribe Zonarieae according to Womersley's criteria do not appear to be monophyletic, based on our analyses. Although not all genera of Dictyotaceae were analysed, our data suggest that the status of the meristem cells should not be a criterion for separating tribes within the family Dictyotaceae and that other hierarchical systems should be adopted to properly reflect phylogenetic relationships.     

Evidence of different reproductive traits in the transisthmian sister species, <Emphasis Type="Italic">Alpheus saxidomus</Emphasis> and <Emphasis Type="Italic">A. simus</Emphasis> (Decapoda,Caridea, Alpheidae): description of the first postembryonic stage

The emergence of the Isthmus of Panama, approximately 3.0-3.5 million years ago, established two very different marine systems (the Caribbean and the eastern Pacific) and separated previously continuous marine populations. The geographic isolation of transisthmian sister species provides an excellent basis for the study of divergence and speciation processes. Here we describe the morphology of the first postembryonic stage of Alpheus saxidomus and A. simus, two rock-boring alpheids; the estimated time since divergence for this transisthmian pair ranges from 4.4-6.1 to 3-9 million years. The presence of a first zoeal stage in A. saxidomus, e.g., without pleopods and a telson with 7+7 setae, indicates a prolonged developmental sequence. In contrast, hatchlings of A. simus are substantially more developed and resemble juveniles. Thus, the developmental modes of A. saxidomus and A. simus are strikingly different with abbreviated, most probably direct, development in the latter species. Reduced food availability in the Caribbean compared to the Pacific coast is likely to be a possible reason for the evolution of such important differences in life history traits of the two transisthmian sister species. It is suggested that a transition from prolonged to abbreviated development evolved gradually during the estimated time since divergence; however, such a shift may have taken place within a substantially smaller time span.     

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

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

Morphological, reproductive, and genetic variability among three populations of Crucibulum quiriquinae (Gastropoda: Calyptraeidae) in northern Chile

Crucibulum quiriquinae (Lesson, 1830) is the only species of Crucibulum currently recognized in northern Chile. Recent analysis of three Crucibulum populations obtained in northern Chile demonstrates the existence of morphological, genetic, and reproductive differences among populations. Two populations present in Bahía Tongoy (30°15'S), one inhabiting the shells of the snail Turritella cingulata and the other shells of the pectinid Argopecten purpuratus, showed morphological differences. However, both had planktonic larval development and show low genetic divergence (D=0.002). A third population from Bahía La Herradura (29°58'S), which also inhabits the shells of T. cingulata, did not show morphological differences compared with its counterpart from Bahía Tongoy. However the Bahía La Herradura population had intracapsular development and metamorphosis, and a larger genetic distance (D=0.06) from both Tongoy populations. The results of the reproductive and genetic analyses strongly suggested that the two Tongoy populations, although showing morphological differences, are biologically the same species, Crucibulum quiriquinae, whereas the La Herradura population is a new species.     

Cryptic species of Clavelina (Ascidiacea) in two different habitats: harbours and rocky littoral zones in the northwestern Mediterranean

Marinas and harbours provide ideal sites for the study of population genetics of marine invertebrates with restricted dispersal capabilities. They combine a confinement effect, particular ecological conditions (pollution, turbidity), and the possibility of high gene flow through ship-borne propagules, which greatly increases the natural dispersal capability of sexual and asexual propagules in many species with short-lived larvae. We studied the genetic structure of populations of the ascidian Clavelina lepadiformis living inside and outside harbours in the north-western Mediterranean. A 500-bp segment of the cytochrome c oxidase subunit I (COI) mitochondrial gene was sequenced in three populations from inside harbours (interior form) and in three populations from the rocky littoral (exterior form). Two congeneric Mediterranean species, Clavelina sp. and C. dellavallei, were used for comparison. We found that the interior and exterior forms of C. lepadiformis belong to two distinct clades, with a genetic divergence of 5%. Gene-flow values among these forms were insignificant. The lack of gene flow and the genetic divergence suggest that the interior and exterior forms of C. lepadiformis are in fact cryptic species rather than differentiated populations of the same species. Levels of gene flow were higher among interior habitats than among exterior habitats, a pattern likely maintained by genetic exchange through ships. We discuss the possible origins of the present-day distribution of these cryptic species. We contend that the study of species living both inside and outside these particular habitats will reveal more instances of genetic discontinuities allowing local adaptations.     

DNA sequence evidence for speciation,paraphyly and a Mesozoic dispersal of cancellothyridid articulate brachiopods

Because the classification of extant and fossil articulate brachiopods is based largely upon shell characters observable in fossils, it identifies morphotaxa whose biological status can, in practice, best be inferred from estimates of genetic divergence. Allozyme polymorphism and restriction fragment length polymorphism of mitochondrial DNA (mtDNA RFLP) have been used to show that nuclear and mitochondrial genetic divergence between samples of the cancellothyridid brachiopods Terebratulina septentrionalis from Canada and T. retusa from Europe is compatible with biological speciation, but the genetic distances obtained were biased by methodological limitations. Here, we report estimates of divergence in 12S rDNA mitochondrial sequences within and between samples of these brachiopods. The sequence-based genetic distance between these samples (5.98ǂ.07% SE) is at least 10 times greater than within them and, since they also differ in a complex life-history trait, their species status is considered to be securely established. Divergence levels between 12S rDNA genes of three other cancellothyridids, T. unguicula from Alaska, T. crossei from near Japan, and Cancellothyris hedleyi from near Australia are higher than between the two North Atlantic species, and the mean nucleotide distance between all these cancellothyrids is similar to the mean distance between species of Littorina (Mollusca: Gastropoda). Sequences of both 12S and 16S genes from cancellothyridids and other short-looped brachiopod species show neither saturation nor lineage-specific rate differences and, when analysed with different outgroups, either separately or together, yield one unexpected, but well-supported, tree with Alaskan T. unguicula basal and C. hedleyi nested within Terebratulina, i.e. these genera are paraphyletic. A geologically dated divergence between Antarctic and New Zealand species of the short-looped brachiopod Liothyrella is used to calibrate the rate of 12S divergence at ca. 0.1% per million years (MY), and this rate is used to infer that T. septentrionalis and T. retusa have been diverging for ca. 60 MY and that they and T. unguicula have been diverging from their last common ancestor for ca. 100 MY. This indicates a Mesozoic origin for the present-day distribution of cancellothyridids and the basal position of T. unguicula suggests a possible North Pacific centre of origin, with separate Atlantic and Pacific radiations. The inclusion of Cancellothyris within Terebratulina also shows that adult shell characters such as umbo, foramen and symphytium shape, whilst probably indispensible for the practical classification of fossils, are not reliable guides to genealogy.     

Post-glacial population history and genetic structure of the northern clingfish (Gobbiesox maeandricus), revealed from mtDNA analysis

Mitochondrial d-loop sequences were analyzed to characterize the phylogeographic and population genetic structure of the northern clingfish (Gobbiesox maeandricus Girard). Sequence analysis of 378 bp from 111 individuals sampled in 14 localities along the northeast Pacific coast and within the Strait of Georgia from 1996 to 1999 revealed marked genetic differentiation (Φ_ct=0.247) among regional population groupings. The gene genealogy distinguished two major clades of haplotypes separated by at least 1.1% sequence divergence. One clade with very low haplotype diversity (h=0.2095, n=18) occurred only within the recently unglaciated Strait of Georgia. The other clade had high haplotype diversity (h=0.8808, n=93) and was found in all populations. High haplotype diversity was found in open coastal populations, both north and south of the maximum extant of the Wisconsin ice sheet, suggesting that the clingfish range was not pushed to a southern refugium during the last glacial maximum. A nested clade analysis also did not detect a large northward expansion from a single southern refugium. The level of sequence divergence and coalescent-based analyses suggest that the observed patterns of polymorphism are the result of Pleistocene diversification within multiple refugia, followed by population expansion and asymmetrical lineage introgression. Received: 5 February 2000 / Accepted: 31 August 2000     

Population dynamics and secondary production of the wedge clam <Emphasis Type="Italic">Donax</Emphasis><Emphasis Type="Italic">hanleyanus</Emphasis> (Bivalvia: Donacidae) on a high-energy,subtropical beach of Brazil

The population biology of Donax hanleyanus (Philippi, 1845) (Bivalvia: Donacidae) was studied by monthly sampling from June 1998 through June 2000, at Restinga da Marambaia Beach, Brazil. Two transects were established and divided into ten strata parallel to the waterline, and five replicates were taken with a 0.04 m² sampler in each stratum. The highest densities of D. hanleyanus were recorded in winter (September 1998, July 1999) and autumn (April 2000). A stratified distribution was observed: recruits were found mainly in the middle swash zone, while juveniles and adults occurred across the tidal gradient up to the retention zone. Mortality rates did not differ significantly between years. Annual production (in ash-free dry mass) ranged from 0.76 g AFDM m^-2 year^-1 (1998) to 3.67 g AFDM m^-2 year^-1 (1999), while the production-to-biomass (P/B) ratio varied from 1.45 to 1.59. Life span was ca. 17 months. Of all variables tested, only two, one biological and one physical, seem to have influenced the population dynamics of this species. A significant negative correlation between the densities of the suspension-feeders D. hanleyanus and Emerita brasiliensis (Crustacea: Decapoda) indicated possible intraguild competition, and there was a significant exponential correlation between beach slope and the mean across-shore position of D. hanleyanus. Data compiled from literature suggests a latitudinal gradient in population parameters, with mortality and renewal rate (P/B) of different species of Donax increasing and life span decreasing from temperate to tropical regions. Other factors influencing population dynamics, such as food availability, and the contributions of individual and community biomass to energy and nutrient cycling are also discussed.     

Evidence of a spawning area of <Emphasis Type="Italic">Anguilla</Emphasis><Emphasis Type="Italic">marmorata</Emphasis> in the western North Pacific

Leptocephali of the tropical eel Anguilla marmorata have been consistently collected in the same area of the North Equatorial Current (NEC) in the western North Pacific during three consecutive cruises in June and July of 1991 (N=28) and 1994 (N=20), and July and September of 1995 (N=27), indicating that this is a spawning area of this species. These leptocephali were collected from 130°E to 142°E and 12°N to 20°N, to the west of the Mariana Islands, in 20 tows in 1991, in 13 tows in 1994 and in 17 tows in 1995, indicating a widespread presence, but a relatively low abundance. Six of these specimens (16.3-36.0 mm total length) from the 1995 cruise, which were of the typical size range of these leptocephali, were genetically confirmed to be A. marmorata in a previous study. The consistent presence of recently spawned A. marmorata leptocephali (9-20 mm) in all 3 years, suggests that the western region of the NEC is the spawning area of the northern population of A. marmorata that was identified in a recent population genetics study. These leptocephali would thus be transported westward by the NEC and then transported north into the Kuroshio Current and toward Taiwan and Japan, or south toward the southern Philippines and into the Celebes Sea by the Mindanao Current. Available evidence indicates that A. marmorata may have potentially year-round spawning, and the presence of a spawning area of this species in the same region as that of Anguilla japonica suggests that the northern population of A. marmorata has evolved a spawning migration from East Asia, the Philippines and the Celebes Sea region to the NEC area, but differs from A. japonica by having some individuals that recruit to more southern areas.     

Ultrastructural,biochemical, and immunological characterization of two populations of the mytilid mussel <Emphasis Type="Italic">Bathymodiolus</Emphasis><Emphasis Type="Italic">azoricus</Emphasis> from the Mid-Atlantic Ridge: evidence for a dual symbiosis

A recently described species of mytilid mussel, Bathymodiolus azoricus Von Cosel et al., 1999, was observed to be the dominant organism at the hydrothermal vents off the Azores, at both the Lucky Strike and Menez Gwen sites. Evidence suggests this species of Bathymodiolus represents yet another example of the intriguing dual symbiosis known in three other species of deep-sea mytilid mussels. Transmission electron micrographs (TEM) show the majority of gill bacteriocytes in mussels sampled from both populations to contain two distinct symbiont morphotypes. One morphotype is characterized by large size (mean diameter, 1.25 µm), coccoid shape, and stacked intracytoplasmic membranes that are consistent with the morphology of type I methanotrophs. The second morphotype is smaller (mean diameter, 0.35 µm) and was observed in coccoid or rod shapes. Immunoblots revealed the presence of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and methanol dehydrogenase (MeDH) in both populations of mussels. Activities of these enzymes, as well as sulfate adenylyl transferase (ATP sulfurylase) and adenylyl sulfate reductase (APS reductase), were detected in gill extracts. The activities measured for the two populations were highly variable, though the population sampled from Lucky Strike showed higher RubisCO activity. Stable carbon isotope values (Lucky Strike, '¹³C=-32.6ǂ.3‰; Menez Gwen, '¹³C=-22.8ǂ.4‰) are in the range of previously reported stable carbon isotope measurements for mytilid mussels hosting a dual symbiosis. Collectively, these results provide evidence for the activity of both sulfur-oxidizing and methane-oxidizing metabolic pathways in B. azoricus. Furthermore, evidence for a greater dependence on methanotrophy in the Menez Gwen mussel population is offered by analysis of cell counts from TEMs. Higher methanotroph numbers, and putatively activity, in this population of mussels are further supported by published geochemical data indicating higher methane concentrations in the vent fluids at Menez Gwen. This finding suggests that environmental conditions may regulate a balance between the physiological activities of different symbiont populations associated with these mussels. The existence of a dual symbiosis could thus confer greater environmental tolerance and increased niche space to the mytilid host in the stochastic hydrothermal vent habitat.     

Evolutionary relationships of deep-sea vent and cold seep clams (Mollusca: Vesicomyidae) of the "<Emphasis Type="Italic">pacifica/lepta</Emphasis>" species complex

Vesicomyid clams are among the dominant invertebrates of chemosynthesis-based communities found at deep-sea cold seeps and hydrothermal vents, yet there is considerable taxonomic confusion within the family Vesicomyidae. The present study examined phylogenetic relationships among vesicomyid clams belonging to a cryptic-species complex that includes Vesicomya (previously Calyptogena) pacifica (Dall, 1891) and Vesicomya lepta (Dall, 1896). Mitochondrial (mt) COI sequences from clams collected between 1991 and 2001 along the western margin of North America from the San Clemente Basin (32°N latitude) to the Juan de Fuca Ridge (48°N latitude) revealed five discrete evolutionary lineages: V. pacifica (mt-type I), V. lepta (mt-type IV), and three undescribed species defined by mt-types II, III, and V. Separation of the three lineages defined by mt-types I, II, and III was also revealed by an independent nuclear gene (ribosomal ITS-1), and by DNA sequences from vertically transmitted bacterial endosymbionts found in the gills of these clams. Lineages I, II, and III occur in Monterey Bay, California, and the data suggest that bathymetric segregation may have played a role in their isolation and divergence.     

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

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

Mechanisms of range expansion in the intertidal brown alga <Emphasis Type="Italic">Fucus</Emphasis><Emphasis Type="Italic">serratus</Emphasis> in northern Spain

The intertidal brown alga Fucus serratus is naturally expanding its distributional range along the north shore of Spain. Shores being colonised by F. serratus harbour saturated algal communities, with almost no free space. Processes responsible for the successful expansion were investigated in a locality where the alga was experimentally introduced. After a short period, the number of F. serratus plants increased exponentially at a monthly intrinsic rate of 0.111. At early and intermediate stages of invasion, colonisation occurred by the growth of patches of the invader around the transplant points. F. serratus is a bad disperser, with most of propagules settling in near proximity to parent plants. Colonisation of manipulated and control areas was influenced by the proximity of propagule sources. Experimental removals of canopies, irrespective of whether the turf was removed or left intact, accelerated the expansion, as this component of the community had a negative influence on recruitment of both F. serratus and F. vesiculosus. The unmanipulated algal beds were, however, invasible, as some control quadrats close (<10 m) to F. serratus patches had a 100% cover of F. serratus at the end of the study. Prior or concurrent to increases of F. serratus, a decrease in cover of F. vesiculosus and other resident macroalgae was evident. No formal competition experiment was done but results suggest that F. serratus outcompetes F. vesiculosus and Bifurcaria bifurcata at mid-tidal levels. In scraped quadrats, in which both Fucus species recruited at large densities, F. serratus grew at a higher rate than F. vesiculosus and eventually monopolised the experimental surfaces.     

Phylogeography and evolution of the triplefin <Emphasis Type="Italic">Tripterygion delaisi</Emphasis> (Pisces,Blennioidei)

The genus Tripterygion (Risso 1826) is restricted to the eastern Atlantic and the Mediterranean, and comprises only three species. T. melanuros and T. tripteronotus are essentially endemic to the Mediterranean, while Tripterygion delaisi occurs in the Atlantic and in the Mediterranean. Two subspecies of T. delaisi have been described (T. d. xanthosoma in the Mediterranean and T. d. delaisi in the Atlantic). Several scenarios have been proposed for the evolution of T. delaisi subspecies, but so far its subspeciation process is not clear. In this study we present a population survey of T. delaisi including specimens from the two recognized subspecies. We combined a phylogeographic approach with estimates of the direction of migration (between the Atlantic and the Mediterranean) and of the coalescence time of the two subspecies, using polymorphic mitochondrial and nuclear genes. The results of this study clearly support the existence of two Tripterygion delaisi clades, one in the eastern Atlantic islands and another in the Atlantic coasts of Europe and in the Mediterranean. Historical migration between the islands and Western Europe plus Mediterranean was restricted, and showed a westbound trend, with a higher number of migrants going from the Western Europe plus Mediterranean into the islands. We estimated the time of coalescence of both groups of T. delaisi to be more recent than the onset of Pleistocene glaciations (1.7 Mya). Our results are consistent with previous hypothesis that consider successive dispersal events of a Tripterygion ancestor from the western African coast colonizing the Atlantic islands and the Mediterranean, promoting the evolutionary divergence between these areas.