Phylogeography of the diamond turbot (Hypsopsetta guttulata) across the Baja California Peninsula

We compared morphology and sequenced nuclear and mitochondrial genes from 11 populations of a previously genetically unstudied “Baja California disjunct” species, the diamond turbot (Hypsopsetta guttulata). This species exhibits very limited adult movement and restriction to soft-bottom habitats but has a moderately long pelagic larval duration. Therefore, if pelagic larval duration is correlated with gene flow between Gulf of California and Pacific populations, we expect a reduced level of genetic and morphological differentiation. However, if adult habitat and ecology have more effect on gene flow, we expect the populations in the two bodies of water to be more highly differentiated. We used logistic regression to compare morphological features and phylogenetic and population genetic analyses to compare nucleotide sequence data. Gulf of California H. guttulata are different from Pacific populations in morphology and both mitochondrial and nuclear gene sequences. MtDNA shows reciprocal monophyly, and nuclear sequences from the Gulf of California formed a monophyletic group. Population genetic analyses also suggest further population subdivision within the Pacific and within the Gulf of California. We argue that adult ecology has a significant effect on migration rates among populations in the Pacific Ocean and the Gulf of California.     

Geographical subdivision,demographic history and gene flow in two sympatric species of intertidal snails, <Emphasis Type="Italic">Nerita scabricosta</Emphasis> and <Emphasis Type="Italic">Nerita funiculata</Emphasis>, from the tropical eastern Pacific

The patchy distribution of rocky intertidal communities in the tropical eastern Pacific (TEP) may impose severe constraints on the genetic connectivity among populations of marine invertebrates associated with this habitat. In this study, we analyzed a portion of the mitochondrial cytochrome c oxidase subunit I (COI) gene in two sympatric species of marine snails, Nerita scabricosta and Nerita funiculata, common inhabitants of the rocky intertidal from the Gulf of California (Sea of Cortez) and outer Pacific coast of the southern Baja California (Baja) peninsula to northern South America, to assess genetic connectivity among populations of each species. One of our aims was to determine whether the morphological, behavioral, and ecological differences observed among populations of both species throughout their range in the TEP corresponded to population genetic differences. In addition, we were interested in elucidating the demographic history of both species. We found no evidence of genetic structure throughout the Gulf of California and outer coast of the Baja peninsula region for either species. Comparisons between Gulf of California/Baja and Panama populations, however, showed significant genetic differentiation for N. scabricosta, but not for N. funiculata. The genetic differences between Mexican and Panamanian populations of N. scabricosta were consistent with previously reported ecological and behavioral differences for this species between these two distant regions. However, previously reported size differences between northern and central/southern Gulf of California individuals of N. scabricosta do not correspond with our findings of genetic connectivity among these populations. Results from neutrality tests (Tajima’s D and Fu’s F _S), the mismatch distribution, and Bayesian skyline analyses suggested that both species have experienced dramatic population expansions dating to the Pleistocene.     

Cryptic isolation of Gulf of California shovelnose guitarfish evidenced by mitochondrial DNA

The shovelnose guitarfish Rhinobatos productus is an evolutionarily, ecologically, and economically important ray, with a continuous distribution from San Francisco, California (USA), to Mazatlan, Sinaloa, and in the Gulf of California (Mexico). Regional studies have revealed morphometric differences between shovelnose from the Gulf of California and the Pacific coast of Baja California, which may result from phenotypic plasticity in the presence of high levels of gene flow or from a degree of genetic differentiation in the presence of cryptic isolation within a continuous distribution. We used PCR-RFLP of the mitochondrial control region to assess the degree of genetic differentiation between Gulf of California and Pacific shovelnose guitarfish. We found very high levels of molecular diversity (averages: h=0.77, =1.19%), which may be associated with historically large and stable populations, as well as very significant levels of genetic differentiation between gulf and Pacific samples (²=64, P<0.0001; _ST=0.63, P<0.0001, mean nucleotide divergence d=2.47%). We found a deep phylogeographic break between haplotypes from the gulf and the Pacific, which may suggest the existence of cryptic species but clearly indicates more than one evolutionarily significant unit of R. productus. Our results show a pattern of genetic structure and levels of differentiation consistent with the geological history of the region. Furthermore, these findings have wide-ranging implications for the management and conservation of cartilaginous fish in Mexico, as they reveal the existence of biological diversity that will go unnoticed without the genetic scrutiny of intraspecific variation and that is highly relevant for much needed management and conservation efforts.Communicated by P.W. Sammarco, Chauvin     

Population genetic structure of North American <Emphasis Type="Italic">Ophiactis</Emphasis> spp. brittle stars possessing hemoglobin

With the discovery of previously unreported populations of hemoglobin-possessing Ophiactis from the Texas coast in the Gulf of Mexico, an investigation into its population structure, including populations of O. simplex from the Pacific coast of California and O. rubropoda from the Atlantic coast of Florida, was undertaken using DNA sequence data from the mitochondrial COI gene. The reconstructed haplotype network suggests that California populations contain the ancestral source of mtDNA variation, and there is no evidence of recent introductions into Texas. Population genetic analyses reveal the California, Florida, and Texas Ophiactis populations to each be significantly differentiated from one another. Sequence divergence among the three areas is shallower than would be predicted given biogeographic history. Texas and Florida populations are equally genetically diverged from California populations as they are to one another, despite the greater potential for gene flow between these areas. The genetic distinctiveness of the Texas populations and the concordance of this pattern with phylogeographic patterns in other brittle star systems indicate an isolated and independent evolutionary history and we hypothesize that the three geographic regions included in this study each serve as hypotheses of population-level lineages that remain to be tested with independent sources of data.     

Identification of genetic populations of the Pacific blue shrimp Penaeus stylirostris of the Gulf of California, Mexico

Species are often composed of discrete breeding units (i.e. populations or stocks) which, while not reproductively isolated from other such groups, may have limited opportunities to exchange genetic material because of geographic distance, barriers to migration, or spawning asynchrony. Low levels of gene flow between stocks may result, over time, in their genetic divergence, and species that are subdivided into morphologic or genetically distinct stocks are said to be structured. The aim of our investigation was to test whether or not Penaeus stylirostris from the Gulf of California (Mexico) was structured into genetically distinct populations. Shrimp samples were collected in 1996 from six regions of the Gulf where specimens with distinct morphologic characteristics had previously been identified. Statistical analysis of 324 RAPD loci (randomly amplified polymorphic DNA), resolved through polyacrylamide gels and scored for each of 78 specimens, permitted the quantification and comparison of between-stock genetic differences. The finding that genetically discrete stocks of P. stylirostris can be found in a small portion of the geographic distribution range of the species, disagrees with the long-held perception that this resource is panmictic in nature. This new evidence is not only of interest for selective breeding programs in the shrimp aquaculture industry, but is also relevant to the management of the Mexican shrimp fishery which, at present, is perceived and managed as a single stock. Received: 10 May 1999 / Accepted: 27 July 2000     

Genetic structure and phylogeography of the lined shore crab, Pachygrapsus crassipes, along the northeastern and western Pacific coasts

Marine invertebrates with high larval dispersal capacity typically exhibit low degrees of population differentiation, which reflects both contemporary and historical processes. We sampled 346 individuals from seven populations of the lined shore crab, Pachygrapsus crassipes Randall, along the northeastern Pacific Coast and Korea during summer 2003. DNA sequence analysis of 613 bp of the mitochondrial COI gene showed that overall gene diversity (h) was high (0.92±0.01), whereas overall nucleotide diversity (π) was low (0.009±0.005). A total of 154 mtDNA haplotypes were identified; however, 114 were present in only one individual. Analysis of molecular variance revealed significant genetic structuring at Point Conception, CA, USA, that is likely due to the oceanographic circulation patterns, which result in asymmetrical migration of haplotypes. However, genetic variation among eastern Pacific populations was generally low, probably because of high contemporary gene flow and recent common ancestry of haplotypes. Mismatch analysis and nested clade analysis suggested that the population history of this region is characterized by two contiguous northwards range expansions, which are congruent with Late Pleistocene glacial cycles. Highly significant genetic differentiation was detected between eastern Pacific populations and Korea, indicating transpacific gene flow is restricted. Time of divergence between the two transpacific lineages was estimated between 0.8 and 1.2 Myrs ago. The small, recently founded population of P. crassipes at Bamfield, BC, Canada, did not appear to have undergone a founder effect.     

Genetic analysis of sympatric morphotypes of common dolphins (genus Delphinus)

Sympatric populations of two forms of the common dolphin, currently recognized collectively as Delphinus delphis Linnaeus, occur in several areas of the world's oceans. A molecular genetic study was initiated to determine whether these forms are genetically distinct in the Northeast Pacific. We compared mitochondrial DNA sequences from the control region and cytochrome b gene between specimens of the long-beaked and the short-beaked morphotypes collected between 1986 and 1989 off the coast of southern California. Additional short-beaked specimens collected from the eastern tropical Pacific (in 1978 and 1982) and the Black Sea (in 1989) were also compared. There were no shared mitochondrial DNA haplotypes between the two morphotypes, and both gene regions exhibited frequency and fixed nucleotide substitutions between the two morphotypes. This genetic differentiation, coupled with unique morphological characters of the short-beaked and long-beaked morphotypes determined in a parallel study, indicate that although sympatric, these populations of common dolphin are reproductively isolated from one another and may represent separate species.     

Interspecific variation in thermal denaturation of proteins in the congeneric musselsMytilus trossulus andM. galloprovincialis: evidence from the heat-shock response and protein ubiquitination

The genetic population structure of the precominant zooplankter, the copepod Calanus finmarchicus (Gunnerus), was examined to determine whether genetically distinct populations exist in the Gulf of Maine. C. finmarchicus was sampled in three regions of the Gulf of Maine (Great South Channel, spring 1989; northern Gulf of Maine, winter 1990; Great South Channel and Georges Bank, spring 1990). Copepods from seven locations in the Great South Channel, five in the northern Gulf of Maine and four on or near Georges Bank were assayed for allozyme variation and mitochondrial DNA variation of amplified 16S rRNA and cytochrome b genes. Restriction fragment length polymorphism (RFLP) analyses of both mitochondrial DNA genes revealed no variation among any of the individuals assayed. Analysis of five polymorphic allozyme loci revealed that genetic variation among the three geographic regions was low, and genetic identities were high between all locations (I>0.97). Most of the genetic variation was among locations regardless of region. Chi-square tests were used to examine genetic similarity between specific pairs of locations within and between regions. In the northern Gulf of Maine, genetic homogeneity occurred over larger spatial scales (hundreds of km) than in either the Great South Channel or Georges Bank (tens of km). Only copepods from the Bay of Fundy and Nova Scotian Shelf locations were genetically distinct from Wilkinson Basin copepods at two loci. Copepod populations from the northern locations may have been partially isolated or they may represent immigrant populations (e.g., from the Gulf of St. Lawrence). Several pairs of locations were genetically distinct at one or more loci in the two southern regions. Differences between locations in these regions may represent distinct populations advected into the areas at different times or from different sources (e.g., genetic variation may represent a mixture of genetically distinct northern and southern copepod populations). These results suggest extensive gene flow among populations of C. finmarchicus in the Gulf of Maine with some evidence of genetic population subdivision near the Gulf's northeastern and southern boundaries.     

Genetic Diversity and Tests of the Hybrid Origin of the Endangered Yellow Larkspur

Abstract: Delphinium luteum ( Ranunculaceae), an endangered larkspur, is restricted to two wild populations near Bodega Bay, California. The total number of individuals in these two populations is estimated to be <100. We used allozyme and random amplified polymorphic DNA ( RAPD) markers to (1) assess levels and patterns of genetic diversity in one wild population and two cultivated populations and (2) test the hypothesis that D. luteum is of hybrid origin between D. decorum and D. nudicaule . These data will be used to aid in developing a management plan to conserve the species. The wild population maintains high levels of genetic diversity. Genetic data indicate that both cultivated populations, especially the north Sonoma population, have several allozymes and RAPD markers not found in the wild population and could be used to establish new populations of D. luteum or to enhance the diversity and size of the wild population. The allozyme data did not reveal any fixed differences between D. decorum and D. nudicaule , although allele frequencies of the putative parental populations differed. At these loci, D. luteum resembled D. nudicaule more than D. decorum  . Many unique RAPD markers distinguish each of the three species. The diagnostic markers from populations of D. nudicaule and D. decorum were not additive in the putative hybrid, and these data indicate that D. luteum is not of recent hybrid origin. Conservation of the yellow larkspur should include strategies that use the cultivated populations of D. luteum , but hybridizing D. decorum and D. nudicaule to "recreate" D. luteum is not recommended.     

Co-existing populations of Pacific ocean perch, Sebastes alutus, in Queen Charlotte Sound, British Columbia

Variation at five microsatellite loci (Sal1, Sal2, Sal3, Sal4 and Sal5) was examined in approximately 1300 Pacific ocean perch (Sebastes alutus) sampled from 14 coastal sites in British Columbia, Canada. Mean observed heterozygosities by locus ranged from 71% to 88%, and by sample ranged from 75% to 84%. Theta values ranged from 0 to 0.04 over the five loci, and averaged 0.015. Among Pacific ocean perch samples, Š ranged from 0.001 to 0.056. Canonical discriminate analysis of multilocus genotypes and neighbour-joining analysis of pairwise genetic distances between samples both indicated the presence of three populations, one off the west coast of Vancouver Island (the Vancouver Island population) and two co-existing populations in Queen Charlotte Sound, Dixon Entrance and along the west coast of the Queen Charlotte Islands (the eastern and western QCI populations). Pacific ocean perch of the eastern and western QCI populations were caught in close proximity to each other, but individual samples showed little evidence of admixture. Fall and spring samples collected within geographic areas were genetically similar, indicating seasonally stable population structure. Restricted gene flow between the Vancouver Island and the two more northerly populations may result from limited adult dispersal and larval retention within the California Current and Alaska Gyre, respectively, but the presence of two populations within Queen Charlotte Sound cannot be explained entirely by larval retention hypotheses. The presence of two Pacific ocean perch populations in central British Columbia has implications for fisheries management.     

Genetic diversity and relatedness of the mangrove Rhizophora mangle L. (Rhizophoraceae) using amplified fragment polymorphism (AFLP) among locations in Florida, USA and the Caribbean

Rhizophora mangle L. is a widespread mangrove species in the Western Hemisphere. Mangrove habitat loss and their importance to coastal and reef ecosystems make greater understanding of their genetic structure useful for conservation and management. An amplified fragment polymorphism (AFLP) analysis was performed on samples from Florida and the Caribbean to discover the genetic structure present. R. mangle had variable genetic diversity not related to latitude; P ranged 7 %–92 %. Some other factor, perhaps human impact, has caused low genetic diversity in some populations. Across Florida R. mangle populations varied in genetic diversity with less diversity (G_st?=?0.195) and greater gene flow on the Atlantic coast (Nm =2.07) than on the Gulf coast (G_st?=?0.717, Nm?=?0.197). Gene flow between Caribbean islands was low (Nm?=?0.386) compared to continental populations (Nm?=?1.40), indicating that long distance dispersal is not common between islands. Analysis of molecular variance (AMOVA) analysis showed significant deviations from Hardy-Weinberg expectations at the level of region among subpopulations and overall genetic difference among subpopulations for R. mangle. One implication for management is that small continental populations and island populations may be genetically isolated and distinct from each other.     

Assessment of community-based initiatives in sustainable management of Indian dry deciduous forests

SUMMARY

This work is devoted to analyzing the pros and cons of the proposed San Ignacio Lagoon (SIL) salt production project in Baja California Sur, México. The analysis of the project was done taking into account the technical data of the main potential effects of the SIL project, and by comparing the SIL proposed actions with the data of Ojo de Liebre Lagoon, where an almost identical salt production project has been operating for more than 44 years. The analysis was done from the following perspectives: biodiversity, biosphere reserve management, and socioeconomic development. The resulting analysis of the pros and cons of the SIL salt production project addresses key aspects of sustainable development in Third World nations. The resulting analysis indicates that the SIL salt production project is compatible with the biodiversity conservation of the zone. The SIL salt production project seems to be one of the best options to attain the sustainable use of natural resources of the most arid and isolated zone of all México.     

Primary productivity in the Gulf of California

Primary productivity data from 6 cruises in the Gulf of California are summarized. A total of 31 stations with 133 samples was available. The Gulf of California represents a subtropical area with exceptionally high rates of primary productivity. This fact may be accounted for by temporary seasonal upwelling, a wind-mixed water column down to the shallow thermocline, especially in the northern part of the Gulf, and by tidal mixing, especially in the Ballenas Channel between Baja California and the island Angel de la Guardia. Rates of primary productivity in the Gulf of California are comparable to those in areas such as the Bay of Bengal, the upwelling areas off the west coast of Baja California, or North Africa. They are about 2 to 3 times greater than that in the open Atlantic or the open Pacific at similar latitudes.Contribution from the Scripps Institution of Oceanography, University of California, San Diego. This work was part of the Scripps Tuna Oceanography Research Program. It was supported by the Bureau of Commercial Fisheries under contract number 14-17-0007-963. Shiptime was provided from funds of the National Science Foundation.     

Effects of habitat fragmentation on the genetic structure and connectivity of the black-lipped pearl oyster Pinctada margaritifera populations in French Polynesia

Habitat destruction leading to increased fragmentation is detrimental to species by reducing population size and genetic diversity and by restraining population connectivity. However, little is known about the effects of naturally fragmented habitats on wild populations, especially when it comes to marine benthic invertebrates with long pelagic larval duration. In this framework, we investigated the connectivity and genetic diversity variation among nine wild populations of the black-lipped pearl oyster, Pinctada margaritifera, throughout French Polynesia using ten microsatellite DNA markers. Despite the naturally fragmented habitat (South Pacific oceanic islands), we found high values of genetic diversity and population admixture, indicating connectivity at small and large spatial scales within sampled sites of the Tuamotu, and between the Society and Tuamotu Archipelagos. In the meantime, habitat geomorphology increased genetic drift in populations occurring in small, closed lagoons. Significant genetic structure not correlated to geographic distance was detected mainly between closed and open lagoons. The Marquesas Islands hosted the most divergent populations, likely a result of vicariance. Our results also highlight that migration patterns among lagoons are not symmetrical. Altogether, the general pattern of gene flow, nonsymmetrical migration rates among populations, absence of isolation by distance and absence of recent extinction events found in our study strongly suggest that P. margaritifera populations of French Polynesia follow an asymmetrical island model of dispersal.     

Gene flow among vestimentiferan tube worm (Riftia pachyptila) populations from hydrothermal vents of the eastern Pacific

Allozyme data are presented for six discrete populations of the giant hydrothermal vent tube worm Riftia pachyptila Jones, 1981 collected throughout the species' known range along mid-ocean spreading ridges of the eastern Pacific Ocean. Contrary to an earlier report, levels of genetic variation are relatively high in this species. Estimates of gene flow based on F-statistics revealed that dispersal throughout the surveyed region is sufficiently high to counter random processes that would lead to losses of genetic diversity and significant population differentiation. R. pachyptila, like other species of tube worms, displays considerable morphologic variation among populations, but this diversity is not reflected in allozyme variation. Vestimentifera, in general, appear to show extensive phenotypic plasticity. In the light of the available genetic data, caution is warranted when making inferences about the taxonomic status of collections based on morphological variation alone. A general decrease in estimated rates of gene flow between geographically more distant populations supports the hypothesis that dispersal in this species follows a stepping-stone model, with exchange between neighboring populations in great excess of long-distance dispersal. High levels of gene flow have been recorded in a variety of vent fauna and may be a prerequisite for success of species found in the ephemeral habitats associated with regions of sea-floor hydrothermal activity.     

An investigation of genetic population structure in blue crabs,Callinectes sapidus,using nuclear gene sequences

Spatial and temporal genetic heterogeneity in the sequences of nuclear protein-coding genes were investigated in populations of the blue crab, Callinectes sapidus. Haplotype frequencies of these novel markers were determined for adult crabs collected along 300 km in the northern Gulf of Mexico in two different years (2010 and 2011), as well as for megalopal recruits collected in 2010. Tests of genetic differentiation among all locations and between locations spanning known genetic breaks in other species were conducted. In addition, samples from distant locations within the range of C. sapidus were used to assess genetic divergence on a broader geographic scale. Significant between-year differences were found for adults at one location and near significant spatial differentiation was found across northern Gulf of Mexico locations in 2010. These results suggest that although the large population sizes and meroplanktonic life history of blue crabs promote widespread gene flow on a regional scale, genetic composition can change over just one year. Substantial divergence between the northern Gulf of Mexico and Venezuela suggests the possibility that temporal shifts in haplotype frequencies could result from variation in the rate of immigration from genetically distinct source populations. The possible effects of the Deepwater Horizon Oil Spill and attendant fisheries closures during the sampling phase of this study are also considered.     

Colonization of the northwest Atlantic by the blue mussel, <Emphasis Type="Italic">Mytilus trossulus</Emphasis> postdates the last glacial maximum

Blue mussels in the genus Mytilus first arrived in the Atlantic Ocean from the Pacific during the Pliocene, following the opening of the Bering Strait. Repeated periods of glaciation throughout the Pleistocene led to re-isolation of the two ocean basins and the allopatric divergence of Mytilus edulis in the Atlantic and M. trossulus in the Pacific. Mytilus trossulus has subsequently colonized the northwest Atlantic (NW Atlantic) so that the two species are presently sympatric and hybridize throughout much of the Canadian Maritimes and the Gulf of Maine. To estimate when M. trossulus arrived in the NW Atlantic, we have examined sequence variation within a portion of the female mtDNA lineage large untranslated region (F-LUR) for 156 mussels sampled from three Pacific and eleven Atlantic populations of M. trossulus. Although we found no evidence of reciprocal monophyly for Pacific and NW Atlantic M. trossulus, limited gene flow between ocean basins has led to the divergence of unique sequence clades within each ocean basin. In contrast, relative genetic homogeneity indicates high levels of gene flow within each basin. Coalescence-based analysis of the F-LUR sequences suggests that M. trossulus recolonized the NW Atlantic from the northeast Pacific subsequent to a demographic expansion in the Pacific that occurred ~96,000 years before present (ybp). Estimates of timing of divergence for Pacific and NW Atlantic populations and the time since expansion among NW Atlantic sequence clades indicate that M. trossulus arrived in the NW Atlantic more recently, between 20,000 and 46,000 ybp. Given that these estimates overlap with the dates of peak ice in the NW Atlantic during the last glacial maximum (LGM, ~18,000–21,000 ybp), we suggest that colonization of the NW Atlantic by M. trossulus occurred during, but more likely just subsequent to, the LGM and was followed by rapid temporal and spatial expansion in the region.     

Genetic structure of wild European populations of the invasive Pacific oyster Crassostrea gigas due to aquaculture practices

As a result of aquaculture activities, Pacific oysters Crassostrea gigas (Thunberg, 1793) have invaded European coasts. Using seven microsatellites, we found virtually no genetic differentiation between natural populations throughout the European range (from the south of the Wadden Sea (the Netherlands) to the south of France) and French cultivated oysters. The genetic homogeneity of Pacific oyster samples appears to be the result of repeated transfers from same seed stocks made for aquaculture and, to a lesser extent, widespread dispersal due to specific biological traits of this species. The only genetic differentiation of Sylt population in the north of the Wadden Sea (Germany) suggests a stronger, persistent impact of ongoing supply of new genetic material from hatchery production, corresponding to seeds selection made by breeders. All of our genetic data highlighted the importance of aquaculture practices on the genetic structure of the keystone invader C. gigas in Europe.     

Phylogeography of California and Galápagos sea lions and population structure within the California sea lion

We investigate the phylogeography of California (Zalophus californianus) and Galápagos (Z. wollebaeki) sea lions and describe within-population structure for the California sea lion based on mitochondrial DNA. Fifty control-region haplotypes were found, 41 from Z. californianus and 9 from Z. wollebaeki, with three fixed differences between the two species. Ranked population boundaries along the range of Z. californianus were defined based on the Monmonier Maximum Difference Algorithm, resulting in five genetically distinct populations, two in the Pacific Ocean and three inside the Gulf of California. A Minimum Spanning Network showed a strong phylogeographic signal with two well-defined clusters, Z. californianus and Z. wollebaeki, separated by six base-pair differences, supporting the existence of two genetically distinct species with an estimated divergence time of ~0.8 Ma. Results are discussed in the context of the historical geologic and paleoceanographic events of the last 1 Ma in the eastern Pacific.     

Population structure of the widely dispersing marine bryozoan Membranipora membranacea (Cheilostomata): implications for population history, biogeography, and taxonomy

Morphologically plastic, cryptic, or geographically widespread species pose similar challenges to the evolutionary biologist: their taxonomic status is often unclear yet must be known to study almost any aspect of their biology, ecology, evolution, or biogeography. The marine bryozoan Membranipora membranacea (L.) is morphologically plastic and geographically widespread in temperate oceans of the Northern and Southern Hemispheres, and its taxonomy is unclear. This study examined genetic relationships among allopatric populations and sympatric morphs of this species, or species complex. Colonies were collected from 1992 to 1995. Allozymes were used to elucidate the relationships among four widely separated populations, two in the North Atlantic and two in the North Pacific Ocean. Allozymes and mtDNA sequencing were used to clarify the genetic relationships among three sympatric morphs that might correspond to the species M. villosa Hincks and M. membranacea in the northeastern Pacific (Washington State). Populations in the North Atlantic and North Pacific had no fixed allelic differences at the loci tested but were separated by an average Nei's genetic distance of 0.581, suggesting their near-sibling species status. Populations from Friday Harbor (Washington) and Catalina Island (California) were not significantly differentiated, which was attributed to high gene flow. Populations on either side of the North Atlantic were genetically indistinguishable, which is most likely due to the recent establishment of the West Atlantic populations from European founders. At Friday Harbor, sympatric morphs varying in their spination and spine inducibility were genetically indistinguishable, supporting the hypothesis that M. villosa is an induced phenotype of M. membranacea and not a distinct species in the northeastern Pacific. Since such phenotypic plasticity is common in cheilostome bryozoans, the morphospecies concept must be used with caution. Received: 31 August 1998 / Accepted: 10 August 1999