Genetic variation in deep-sea invertebrate populations: The bathyal gastropod Bathybembix bairdii

Five populations of the bathyal trochid gastropod Bathybembix bairdii (Dall) taken from depths of 579 to 1156 m in the Southern California Continental Borderland were surveyed for levels of genetic variability at 18 presumptive gene loci, using techniques of starch gel electrophoresis. All the populations were highly similar genetically; none of the populations possessed unique alleles. Four of 5 polymorphic loci scored in all the populations displayed statistically significant heterogeneity of allelic frequencies among stations. Only the diallelic fumarase (FUM) locus displayed a trend with depth. The geographic and depth patterns of the genetic variability in these populations do not support the hypothesis that the high levels of genetic variability observed in bathyal populations are an artifact of gene flow between populations differentiated with depth. The 16.2% of the loci heterozygous per individual of B. bairdii is of the same order as the average heterozygosities reported for other deep-sea invertebrate taxa. These reports of high genetic variability in a physically constant and seasonless environment are reviewed and discussed in the context of the trophic-stability hypothesis of Ayala and Valentine and the time, size and divergence hypothesis of Soulé. Evidence is presented that despite the observed correlation of average heterozygosities in benthic marine invertebrates with seasonality of the trophic regime, this is not apparent for an enzyme system (leucine amino peptidase) which might be expected to display this trend if it reflected a genetic strategy to cope with trophic seasonality. The time, size and divergence hypothesis, focusing on population size, accounts for the general features of levels of genetic variability in deep-sea invertebrates.     

Analysis of the structure of decapod crustacean assemblages off the Catalan coast (North-West Mediterranean)

We sampled the communities of decapod crustaceans inhabiting the depth zone between 3 and 871 m off the Catalan coast (North-West Mediterranean) from June 1981 to June 1983. The 185 samples comprised 90 species differing widely in their depth distributions. Multivariate analysis revealed four distinct faunistic assemblages, (1) littoral communities over sandy bottoms, (2) shelf communities over terrigenous muds, (3) upper-slope communitics, and (4) lower-slope or bathyal communities. The brachyuran crab Liocarcinus depurator is the most abundant species of the shelf assemblage, although L. vernalis dominates over the shallow sandy bottoms of the shelf. The dominant species of the upper-slope assemblage are nektobenthic species (Solenocera membranacea, Plesionika heterocarpus, Processa canaliculata), pelagic species (Pasiphaea sivado, Sergestes arcticus), and benthic species (Macropipus tuberculatus, Munida intermedia, Nephrops norvegicus). Aristeus antennatus comprise most of the biomass of the lower-slope community, which supports a greater diversity than the other assemblages. The main assemblages appear to be related to different hydrological characteristics, the extent of seasonal fluctuations, and to the changes in sediment structure associated with changes in the steepness of the bottom.     

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.     

Population structure of red porgy, <Emphasis Type="Italic">Pagrus pagrus</Emphasis>, in the Atlantic Ocean

The red porgy, Pagrus pagrus (L.), is a protogynous sparid associated with reefs and hard bottom habitat throughout the warm-temperate Atlantic Ocean. In this study, the degree of geographic population differentiation in Atlantic populations was examined with microsatellite and mitochondrial DNA markers (mtDNA). Six microsatellite loci were amplified and scored in 690 individuals from the eastern North Atlantic (Crete, Madeira, and Azores), western North Atlantic (North Carolina to Florida, and the eastern Gulf of Mexico), and Brazil. At two loci, fixed allelic differences were found among the three major geographic areas, while frequency differences were observed at three other loci. The DNA of 371 individuals was amplified at the mtDNA control region, and 526 bp were sequenced. Tamura–Nei’s D was used as a measure of nucleotide diversity and divergence: diversity averaged 0.011 within samples, while the corrected divergence averaged 0 between samples within the same area and 0.061 between samples from different areas. Transversion haplotype minimum spanning networks, nucleotide divergence, and F _ST values all show that the western Atlantic samples were more closely related to each other than any was to samples from the eastern North Atlantic. Within the western North Atlantic, no significant population differentiation was observed, and within the eastern North Atlantic, only the Azores sample showed detectable differences from Crete and Madeira. These data indicate general homogeneity within large areas, and deep divisions between these areas. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Spatial changes in the distributions of deep-sea “Cerviniidae” (Harpacticoida, Copepoda) and their associations with environmental factors in the bathyal zone around Sagami Bay, Japan

To estimate species turnover rates on scales of several tens of km in deep-sea benthic animals, we analyzed spatial and inter-annual changes in species diversity and composition of cerviniids, a typical group of deep-sea harpacticoids, at stations in and around Sagami Bay, central Japan. Associations with environmental factors were also investigated. Generally, bathymetrical patterns in diversity of benthos are unimodal and peak at depths of 2,000–3,000 m. In Sagami Bay, cerviniid diversity did not follow this trend; both species richness and evenness were negatively correlated with water depth. Multivariate analyses [detrended correspondence analysis (DCA) and non-metric multi-dimensional scaling] suggested that temporal changes in species composition of cerviniids are smaller than spatial changes that occur on horizontal scales of several tens of km. Community structure does not change completely on these scales in the bathyal zone around Sagami Bay. DCA also showed that bathymetrical changes in species composition can be regulated by certain factors associated with water depth.     

Population structure of the planktonic copepod Calanus pacificus in the North Pacific Ocean

The pelagic copepod Calanus pacificus ranges nearly continuously across temperate-boreal regions of the North Pacific Ocean and is currently divided into three subspecies—C. pacificus oceanicus, C. pacificus californicus, C. pacificus pacificus—based on subtle morphological differences and geographic location. The relation between geography and genetic differentiation was examined for 398 C. pacificus individuals sampled from six widely distributed locations across the North Pacific, including an open ocean site and coastal sites on both sides of the North Pacific basin. For each individual copepod, the DNA sequence was determined for a 421-bp region of the mitochondrial coxI gene (mtCOI). A total of sixty-three different mtCOI sequences, or haplotypes, were detected, with a sequence divergence between haplotypes of 0.2–3.1%. The number and distribution of haplotypes varied with sampling location; 12 haplotypes were distributed across multiple sampling locations, and 51 occurred at only one location. Five genetically distinct populations were detected based on F _ST values. Haplotype minimum spanning networks, nucleotide divergence and F _ST values indicated that individuals from coastal sites in the North Pacific Ocean were more closely related to each other than to individuals from the open ocean site at Station P. These results provide genetic support for the designation of two subspecies—a coastal subspecies that consists of what is currently referred to as C. p. pacificus and C. p. californicus and an open ocean subspecies C. p. oceanicus. This work also indicates that planktonic copepods with potentially high dispersal capacity can develop genetically structured populations in the absence of obvious geographic barriers between proximate locales within an ocean basin.     

Population structure of the planktonic copepod <Emphasis Type="Italic">Calanus pacificus</Emphasis> in the North Pacific Ocean

The pelagic copepod Calanus pacificus ranges nearly continuously across temperate-boreal regions of the North Pacific Ocean and is currently divided into three subspecies—C. pacificus oceanicus, C. pacificus californicus, C. pacificus pacificus—based on subtle morphological differences and geographic location. The relation between geography and genetic differentiation was examined for 398 C. pacificus individuals sampled from six widely distributed locations across the North Pacific, including an open ocean site and coastal sites on both sides of the North Pacific basin. For each individual copepod, the DNA sequence was determined for a 421-bp region of the mitochondrial coxI gene (mtCOI). A total of sixty-three different mtCOI sequences, or haplotypes, were detected, with a sequence divergence between haplotypes of 0.2–3.1%. The number and distribution of haplotypes varied with sampling location; 12 haplotypes were distributed across multiple sampling locations, and 51 occurred at only one location. Five genetically distinct populations were detected based on F _ST values. Haplotype minimum spanning networks, nucleotide divergence and F _ST values indicated that individuals from coastal sites in the North Pacific Ocean were more closely related to each other than to individuals from the open ocean site at Station P. These results provide genetic support for the designation of two subspecies—a coastal subspecies that consists of what is currently referred to as C. p. pacificus and C. p. californicus and an open ocean subspecies C. p. oceanicus. This work also indicates that planktonic copepods with potentially high dispersal capacity can develop genetically structured populations in the absence of obvious geographic barriers between proximate locales within an ocean basin.     

Population genetic structure of the stalked barnacle <Emphasis Type="Italic">Pollicipes pollicipes</Emphasis> (Gmelin, 1789) in the northeastern Atlantic: influence of coastal currents and mesoscale hydrographic structures

Within its distribution range in the northeastern Atlantic, the stalked barnacle Pollicipes pollicipes shows a well-defined pattern of genetic variation, comprising (a) a subtropical/temperate northern assemblage, made up of populations distributed between 47°N and 28°N along the French, Iberian, North African and Canary Islands coastlines, and (b) a single isolated and highly divergent tropical population in the Cape Verde Islands (16°N), at the southernmost limit of the species’ distribution. However, within the northern assemblage several populations show a level of genetic differentiation that allows rejection of the hypothesis of genetic homogeneity. The congruence observed between genetic and hydrographic patterns suggests a crucial role of hydrodynamics, and of the dispersal of the planktonic larvae, in the determination of population structure. Along the southern European Atlantic coast, the Iberian Poleward Current and mesoscale hydrographic structures are, respectively, facilitating gene flow at the regional level and genetic differentiation at the local level. On the Atlantic coast of North Africa, the homogenizing equatorward flow of the Canary Current does not extend as far as the Cape Verde Islands. A demographic expansion is dated to the late Pleistocene, preceding the Eemian interglacial, and is oldest in the case of the long-standing Cape Verde population, sustained by a stable tropical habitat. The divergence between the Cape Verde population and the remaining populations is thus ancient, and suggests that oceanic current patterns may constitute a generalized physical barrier to the dispersal of marine organisms between Cape Verde and the rest of Macaronesia.     

Biochemical composition and trophic strategies of the amphipod Eurythenes gryllus at hadal depths (Atacama Trench,South Pacific)

Amphipods Eurythenes gryllus were collected at 7800?m depth in the Atacama Trench (South Pacific) for studying their biochemical composition (in terms of proteins, lipids and carbohydrates and fatty acid content) and to gather information on bioenergetic strategies and trophic habits of organisms living in this extreme environment. The effect of long-term formalin storage on the biochemical determinations was also determined. Proteins were the dominant biochemical class of organic compounds (39–53%D.W.), whereas carbohydrates accounted for a very small fraction (1–2%D.W.). Lipid concentrations of E. gryllus accounted for 7–18%D.W. and were much lower than those reported for other deep-sea amphipods. These differences are likely to be more dependent upon food availability in the Atacama Trench rather than upon temperature. Lipid composition of E. gryllus revealed the dominance of monounsaturated fatty acids with polyunsaturated fatty acids accounting for a very small fraction, suggesting that hadal amphipods are higher dependent upon lipid reserves than amphipods inhabiting at shallow depths. The ratio of C18:1Δ⁹ to C18:1Δ¹¹ was >11 confirming the necrophagous trophic habits of this hadal amphipod.     

Phylogeography of the Antarctic planktotrophic brittle star Ophionotus victoriae reveals genetic structure inconsistent with early life history

In the marine environment, connectivity is influenced by physical oceanography as well as life history and behavioral traits, which in combination with historical events such as geology, physical oceanography, and climate, determine population structure. The Antarctic brittle star Ophionotus victoriae develops via a feeding planktonic larval stage, and therefore has potential for long-distance dispersal throughout its Antarctic/subantarctic range. To evaluate this hypothesis, phylogeography of this ecologically dominant species was elucidated by sequence analysis of two mtDNA genes from individuals collected throughout the Antarctic Peninsula and from two subantarctic islands. Counter to expectations of genetic homogeneity, mtDNA data revealed substantial levels of genetic differentiation as well as diversity. Although there were some genetically homogeneous populations, such as those throughout Bransfield Strait, we found O. victoriae to have significant population structure throughout much of the Antarctic Peninsula, with evidence of potential cryptic speciation between the western and eastern Antarctic Peninsula. Furthermore, Antarctic Peninsula populations were genetically distinct from subantarctic island populations. The low levels of connectivity implied for O. victoriae contrast with those found for many other Antarctic benthic taxa, and suggest a complex interplay between oceanography, recent climate history, and larval ecology.     

Mitochondrial DNA analyses of the red rock lobsterJasus edwardsii supports an apparent absence of population subdivision throughout Australasia

Nucleotide sequence polymorphism in the mitochondrial genomes of 132 adult lobsters (Jasus edwardsil) collected from widespread locales across southern Australia and from New Zealand (April 1989 to June 1990) was assayed, using six restriction endonucleases, to test the hypothesis of a lack of genetic subdivision in a marine species with a long-lived planktonic larva. The mean amount of mtDNA diversity among the 132 mitochondrial genomes was 0.77%. Phenetic clustering and gene-diversity analyses, as well as pairwise comparison of the genetics of specimens from each, or grouped, locales did not detect the presence of genetic subdivision across approx 4600 km of Southern Ocean habitats. The inability of this study to detect population subdivision does not preclude fortutitous, active or habitat-specific larval settlement from producing and maintaining hidden groupings. If genetic homogeneity is maintained in this species by larval dispersal in ocean currents flowing to the east, then westerly populations may deserve special conservation status.     

Genetic and reproductive differentiation of<Emphasis Type="Italic"> Artemia franciscana</Emphasis> in a new environment

The nature and speed of intraspecific genetic differentiation between the Artemia population native to San Francisco Bay (SFB) and populations resulting from introduction of SFB material into Vietnam (Vinh Chau, VC) have been investigated from a multidisciplinary perspective using reproductive characters, allozymes and mitochondrial DNA. Rapid divergence between SFB and VC Artemia is evident from an analysis of five reproductive traits at the temperatures of 26°C and 30°C. The VC strain seems to be better adapted to high temperature, as gauged by the significantly higher reproductive output displayed compared with SFB Artemia. Analysis of variance confirms the existence of a strong genetic component involved in the determination of encystment. Allozyme analysis (20 loci) of the same strains reveals considerable genetic differences between samples, cultured at the above temperatures. Combined probability values for tests of allele frequency homogeneity pooled over all polymorphic loci, at 30°C as well as between temperatures, are 0.015 and 0.026, respectively. In addition, cluster analysis shows evidence of differentiation of Vietnamese Artemia from the wild (SFB) stock as early as within a year after inoculation. Mitochondrial DNA RFLP markers show similar patterns of genetic differentiation to those seen with allozymes and reproductive traits. However, unlike allozymes, there is strong evidence of a reduction in haplotype diversity from 40.6% in SFB Artemia to 10.5% in the established VC strain. Although genetic drift could account for much of the depletion in gene diversity, firm indications are obtained of selective forces shaping the gene pool in the tropical conditions of Vietnam. The brine shrimp is an invaluable genetic system for fine-scale studies of microevolutionary divergence, and multidisciplinary studies are likely to be of both fundamental and practical value for such inquiries.Communicated by O. Kinne, Oldendorf/Luhe     

Hydrology influences population genetic structure and connectivity of the intertidal amphipod Corophium volutator in the northwest Atlantic

The mechanisms driving genetic structure in marine systems are elusive due to the difficulty of identifying temporal and spatial barriers to dispersal. By studying marine invertebrate species with limited dispersal potential, genetic structure can be directly related to physical and biological factors restricting connectivity. In the northwest Atlantic, the benthic brood-rearing amphipod Corophium volutator is distributed across basins with distinct circulation patterns and has the potential to disperse passively during its adult stage. We analyzed spatial genetic variation and migration rates across C. volutator’s North American range with sequence data for mitochondrial DNA and three novel nuclear markers using frequency and coalescent-based methods. We found low genetic differentiation within basins, but strong subdivision within the Bay of Fundy and a striking biogeographic break between the Bay of Fundy and Gulf of Maine, suggesting that genetic drift may act on populations in which connectivity is restricted due to the limitation of passive dispersal by hydrological patterns.     

Genetic heterogeneity among <Emphasis Type="Italic">Eurytemora affinis</Emphasis> populations in Western Europe

Evolutionary diversification of the broadly distributed copepod sibling species complex Eurytemora affinis has been documented in the northern hemisphere. However, the fine scale geographic distribution, levels of genetic subdivision, evolutionary, and demographic histories of European populations have been less explored. To gain information on genetic subdivision and to evaluate heterogeneity among European populations, we analyzed samples from 8 locations from 58° to 45°N and 0° to 23°E, using 549 base pairs of the mitochondrial cytochrome oxidase subunit I (COI) gene. We discovered three distinct lineages of E. affinis in Western Europe, namely the East Atlantic lineage, the North Sea/English Channel (NSEC) lineage, and the Baltic lineage. These geographically separated lineages showed sequences divergence of 1.7–2.1%, dating back 1.9 million years (CI: 0.9–3.0 My) with no indication of isolation by distance. Genetic divergence in Europe was much lower than among North American lineages. Interestingly, genetic structure varied distinctively among the three lineages: the East Atlantic lineage was divided between the Gironde and the Loire populations, the NSEC lineage comprised one single population unit spanning the Seine, Scheldt and Elbe rivers and the third lineage was restricted to the Baltic Proper (Sweden). We revealed high haplotype diversity in the East Atlantic and the Baltic lineages, whereas in the NSEC lineage haplotype diversity was comparatively low. All three lineages showed signs of at least one demographic expansion event during Pleistocene glaciations that marked their genetic structure. These results provide a preliminary overview of the genetic structure of E. affinis in Europe.     

Evaluation of hermit crab shell indices

A study on the alimentary ethology of conger eels was carried out over a period of four years. Individuals were captured at a depth ranging from 10 to 800 m. The parameters analysed lead to the following observations: (a) in the Central Mediterranean basin the feeding of Conger conger (L.) populations was mainly based on Osteichthyes, Crustacea and Cephalopoda; (b) the neritic population fed on a higher number of species than the epimesobathyal. The high percentage of empty stomachs in the bathyal population was due to the fact that the neritic females, at the end of trophic phase, set out on a reproductive migration towards the bathyal level.     

Population differentiation of the Atlantic spotted dolphin (Stenella frontalis) in the western North Atlantic, including the Gulf of Mexico

Information about the genetic population structure of the Atlantic spotted dolphin [Stenella frontalis (G. Cuvier 1829)] in the western North Atlantic would greatly improve conservation and management of this species in USA waters. To this end, mitochondrial control region sequences and five nuclear microsatellite loci were used to test for genetic differentiation of Atlantic spotted dolphins in the western North Atlantic, including the Gulf of Mexico (n=199). Skin tissue samples were collected from 1994–2000. Significant heterozygote deficiencies in three microsatellite loci within samples collected off the eastern USA coast prompted investigation of a possible Wahlund effect, resulting in evidence for previously unsuspected population subdivision in this region. In subsequent analyses including three putative populations, two in the western North Atlantic (n=38, n=85) and one in the Gulf of Mexico (n=76), significant genetic differentiation was detected for both nuclear DNA (R _ST=0.096, P≤0.0001) and mitochondrial DNA (Φ _ST=0.215, P≤0.0001), as well as for all pair-wise population comparisons for both markers. This genetic evidence for population differentiation coupled to known biogeographic transition zones at Cape Hatteras, North Carolina and Cape Canaveral, Florida, USA, evidence of female philopatry, and preliminary support for significant genetic differences between previously documented morphotypes of Atlantic spotted dolphins in coastal and offshore waters all indicate that the biology and life history of this species is more complex than previously assumed. Assumptions of large, panmictic populations might not be accurate in other areas where S. frontalis is continuously distributed (e.g., eastern Atlantic), and could have a detrimental effect on long-term viability and maintenance of genetic diversity in this species in regions where incidental human-induced mortality occurs.

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Lack of population subdivision among the minke whales (Balaenoptera acutorostrata) from Icelandic and Norwegian waters based on mitochondrial DNA sequences

The minke whale (Balaenoptera acutorostrata) is subject to commercial whaling, but stock identification and assessment are still uncertain. Mitochondrial DNA (mtDNA) sequences were determined to examine the population structure of minke whales from the central and northeastern parts of the North Atlantic, as well as the Antarctic regions IV and V. The analyses include 345 nucleotide positions of the control region of 110 individuals, and 250 nucleotide positions of the NADH dehydrogenase subunit 2 gene for a representative selection of North Atlantic minke whales. Maximum parsimony analyses and sequence divergence calculations did not reveal any genetic differentiation between individuals from the central and northeastern parts of the North Atlantic. These results do not support the International Whaling Commission's separation of minke whales in this area into different management units, and they are in conflict with previously reported results from allozyme analyses. Comparison of minke whale control region sequences showed that the sequence diversity of North Atlantic minke whales is substantially lower (0.0065) than that of Antarctic minke whales (0.0166), and clearly demonstrated that individuals from these two areas represent genetically distinct populations.     

Genetic population structure in a commercial marine invertebrate with long-lived lecithotrophic larvae: <Emphasis Type="Italic">Cucumaria</Emphasis><Emphasis Type="Italic">frondosa</Emphasis> (Echinodermata: Holothuroidea)

The patterns of genetic diversity and connectivity were investigated in Cucumaria frondosa, the most abundant sea cucumber in the North Atlantic, to assist in the management and conservation of this ecologically important marine invertebrate, which is the target of an emerging fishery. Mitochondrial DNA COI sequences of 334 C. frondosa were obtained and analyzed, mainly from its western North Atlantic range, where the commercial fishery is being developed, with complementary sampling in the mid- and eastern North Atlantic. Analysis of molecular variance showed no significant (P > 0.05) differences among subpopulations in the western region suggesting that it constitutes one panmictic population. The same analysis showed low, but significant differences between eastern and western Atlantic populations. Coalescent analyses using isolation with migration models and a Bayesian skyline plot indicated historical divergence and a general increase in population size prior to the last glacial maximum and highly asymmetric gene flow (nearly 100 times lower from west to east) between sea cucumbers from North America and Norway. Results suggest that subpopulations of C. frondosa within the western North Atlantic have been highly connected. We propose that aided by the high-connectivity local subpopulations can recover rapidly from natural (i.e., ice ages) or anthropogenic (i.e., overfishing) population declines through recruitment from deep refugia.     

Population connectivity in the temperate damselfish <Emphasis Type="Italic">Parma microlepis</Emphasis>: analyses of genetic structure across multiple spatial scales

This study investigated the utility of microsatellite markers for providing information on levels of population connectivity for a low dispersing reef fish in New South Wales (NSW), Australia, at scales ≤400 km. It was hypothesized that the temperate damselfish Parma microlepis, which produces benthic eggs and has limited post-settlement dispersal, would exhibit spatial genetic structure and a significant pattern of isolation-by-distance (IBD). A fully nested hierarchical sampling design incorporating three spatial scales (sites, location and regions, separated by 1–2, 10–50 and 70–80 km respectively) was used to determine genetic variability at seven microsatellite loci. Broad-scale genetic homogeneity and lack of IBD was well supported by single and multi-locus analyses. The proportion of the total genetic variation attributable to differences among regions, locations or sites was effectively zero (Φ/R-statistics ≤0.007). The geographic distribution of genetic diversity and levels of polymorphism (H _E 0.21–0.95) indicate high mutation rates, large effective population sizes, and high rates of gene flow. Significant gene flow may be driven by factors influencing pre-settlement dispersal, including the East Australian Current (EAC) and habitat continuity. Genetic connectivity may not reflect demographically important connectivity, but does imply that P. microlepis populations are well connected from an evolutionary perspective. Total observed genetic diversity was accounted for within 1–2 km of reef and could be represented within small Marine Protected Areas. Reef fishes in NSW which have life histories similar to P. microlepis (e.g. pre-settlement durations ≥2 weeks) are also likely to exhibit genetic homogeneity. Genetic markers are, therefore, most likely to provide information on demographically relevant connectivity for species with lower dispersal capabilities, small population sizes, short life spans, and whose habitats are rare, or patchily distributed along-shore. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mitochondrial evolution and phylogeography in the hydrozoan<Emphasis Type="Italic"> Obelia geniculata</Emphasis> (Cnidaria)

The distribution and genetic structure of many marine invertebrates in the North Atlantic have been influenced by the Pleistocene glaciation, which caused local extinctions followed by recolonization in warmer periods. Mitochondrial DNA markers are typically used to reconstruct species histories. Here, two mitochondrial markers [16S rDNA and cytochrome c oxidase I (COI)] were used to study the evolution of the widely distributed hydrozoan Obelia geniculata (Linnaeus, 1758) from the North Atlantic and the Pacific and, more specifically, in the context of North Atlantic phylogeography. Samples were collected from six geographic localities between 1998 and 2002. Hydroids from the North Atlantic, North Pacific (Japan), and South Pacific (New Zealand) are reciprocally monophyletic and may represent cryptic species. Using portions of the 16S rDNA and COI genes and the date of the last trans-Arctic interchange (3.1–4.1 million years ago), the first calibrated rate of nucleotide substitutions in hydrozoans is presented. Whereas extremely low substitution rates have been reported in other cnidarians, mainly based on anthozoans, substitution rates in O. geniculata are comparable to other invertebrates. Despite a life history that ostensibly permits substantial dispersal, there is apparently considerable genetic differentiation in O. geniculata. Divergence estimates and the presence of unique haplotypes provide evidence for glacial refugia in Iceland and New Brunswick, Canada. A population in Massachusetts, USA, appears to represent a relatively recent colonization event.Communicated by J.P. Grassle, New Brunswick