Molecular population structure of the kuruma shrimp <Emphasis Type="Italic">Penaeus</Emphasis><Emphasis Type="Italic"> japonicus</Emphasis> species complex in western Pacific

In a previous study on the kuruma shrimp Penaeus japonicus from the South China Sea, we detected high genetic divergence between two morphologically similar varieties (I and II) with distinct color banding patterns on the carapace, indicating the occurrence of cryptic species. In the present study, we clarify the geographical distribution of the two varieties in the western Pacific by investigating the genetic differentiation of the shrimp from ten localities. Two Mediterranean populations are also included for comparison. Based on the mitochondrial DNA sequence data, the shrimps are separated into two distinct clades representing the two varieties. Variety I comprises populations from Japan and China (including Taiwan), while variety II consists of populations from Southeast Asia (Vietnam, Singapore and the Philippines), Australia and the Mediterranean. Population differentiation is evident in variety II, as supported by restriction profiles of two mitochondrial markers and analysis of two microsatellite loci. The Australian population is genetically diverged from the others, whereas the Southeast Asian and Mediterranean populations show a close genetic relationship. Variety I does not occur in these three localities, while a small proportion of variety II is found along the northern coast of the South China Sea and Taiwan, which constitute the sympatric zone of the two varieties. The present study reveals high genetic diversity of P. japonicus. Further studies on the genetic structure of this species complex, particularly the populations in the Indian Ocean and Mediterranean, are needed not only to understand the evolutionary history of the shrimp, but also to improve the knowledge-based fishery management and aquaculture development programs of this important biological resource.     

Oocyte development in the kuruma prawn Penaeus japonicus

Female kuruma prawns (Penaeus japonicus Bate) with undeveloped, early developing, developing, nearly ripe and ripe ovaries, were collected from Ise Bay, Japan, in 1984. Oocyte development of the kuruma prawn was classified into ten stages according to morphological characters, namely: (1) synapsis stage, (2) chromatin nucleolus stage, (3) early perinucleolus stage, (4) late perinucleolus stage, (5) oil globule Stage I, (6) oil globule Stage II, (7) yolkless stage, (8) yolk granule stage, (9) prematuration stage, and (10) maturation stage. The synapsis stage is a multiplication stage. The chromatin nucleolus stage, early and late perinucleolus stages are previtellogenesis and primary growth stages. Oil globule Stage I is an initial stage of primary vitellogenesis and secondary growth. Follicle cells on the oil globule Stage I oocytes expand rapidly and reach maximum size during oogenesis. Yolk granule stage oocytes are in the initial stages of secondary vitellogenesis. Strongly acidophilic yolk granules accumulate within basophilic vesicles of the cytoplasm. The yolk granules are first concentrated in the inner part of the cytoplasm, then gradually spread to the periphery. Cortical crypts, which are separated from the oocyte cytoplasm by the cytoplasmic membrane, are situated outside of oocyte cytoplasm. Germinal vesicle breakdown (GVBD) is initiated in the late phase of prematuration and continues until the late phase of maturation immediately prior to spawning. At the beginning of the maturation stage, the oocytes are ovulated, after which the nuclei further shrink and migrate out-wards. After ovulation, meiotic division of the ovarian oocyte progressed up to the metaphase of primary maturation division. Finally, the meiotic metaphase is visible just beneath the cytoplasmic membrane in the mature oocyte. Though ovulation is synchronous within the same ovary, GVBD is not completely synchronous. Ovulated mature oocytes have many club-shaped cortical crypts in the peripheral part of the cytoplasm and contain extensive accumulations of yolk granules dispersed throughout the cytoplasm. The apical end of the club-shaped cortical crypts and cytoplasmic membrane are coated by the vitellin envelope in the mature oocyte.     

Phylogenetic relationships of deep-sea mussels of the genus<Emphasis Type="Italic"> Bathymodiolus</Emphasis> (Bivalvia: Mytilidae)

We examined phylogenetic relationships among three Bathymodiolus species in Japanese waters and Bathymodiolus spp. from the Manus Basin by two different approaches. Two-dimensional gel electrophoresis allowed us to compare 263–407 (average=318) proteins, giving comprehensive information on genetic distances among the species. The neighbor-joining tree presented two clusters: (1) B. japonicus and B. platifrons and (2) B. septemdierum and B. sp. Members of the first cluster contain methanotrophic endosymbiotic bacteria and members of the second cluster contain thioautotrophic endosymbionts. DNA sequencing of a fragment (415 bp) of mitochondrial cytochrome c oxidase subunit I (COI) provided a neighbor-joining tree with the same topology as that derived from protein analysis. Inspection of intraspecific variation in COI in B. japonicus and B. platifrons revealed no genetic differentiation between mussel populations of either species from cold-water seeps versus hydrothermal vents, suggesting high adaptability of these Bathymodiolus species to deep-sea chemosynthetic environments. Our results indicated genetic exchanges between mussels from distant localities, suggesting that a limited dispersal capability of the larvae is not the likely factor leading to speciation events in these Bathymodiolus species.Communicated by T. Ikeda, Hakodate     

Hidden ancient diversification in the circumtropical lancelet Asymmetron lucayanum complex

The tropical lancelet Asymmetron lucayanum (= Epigonichthys lucayanus) is distributed from the western Indian Ocean to the central Pacific Ocean, and the western Atlantic Ocean. Molecular phylogenetic analysis of mitochondrial cytochrome c oxidase subunit I (COI) sequences (1,035 bp) of A. lucayanum (80 specimens from seven localities) showed clearly that this species is genetically distinguished into three major groups of geographical populations based on neighbor-joining tree using maximum likelihood distance (HKY model with invariable sites and gamma correction), suggesting the existence of three cryptic species. Our genetic data show that (1) inter-oceanic divergence time between Clade B (the West-Central Pacific) and Clade C (the Atlantic) (d = 6.6%, ca. 12 million years ago) was smaller than intra-oceanic divergence time between Clade A (the Indo-West Pacific) and Clade B (d=39.5%, ca. 100 million years ago); (2) there are two cryptic species in the West Pacific in sympatry; and (3) high gene flow is implied between the Maldives and the Ryukyus in Clade A (10,000 km distance), the Philippines and Hawaii in Clade B (8,500 km distance), and Barbados and Bermuda in Clade C (2,200 km distance).     

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     

Peculiar growth dynamics of the surfgrass <Emphasis Type="Italic">Phyllospadix japonicus</Emphasis> on the southeastern coast of Korea

The surfgrass Phyllospadix japonicus is endemic to exposed shores of the northeastern Pacific Ocean. Unlike the majority of seagrasses, P. japonicus grows on rocky substrata. The specific physical features of the habitat are likely related to the peculiar ecological characteristics of P. japonicus. However, few studies have been conducted thus far on the growth dynamics of Phyllospadix spp., largely due to the turbulent water conditions in its habitat. P. japonicus is a dominant seagrass species, and it plays critical ecological roles on the eastern coast of Korea. Here, we examined its growth dynamics for the first time on the Korean coast. We measured shoot density, biomass, leaf production, phenology, morphology, tissue nutrient content, as well as environmental factors including underwater photon flux density (PFD), water temperature and water column nutrient concentrations from March 2003 to December 2005. Shoot density, biomass, leaf productivity and morphological characteristics exhibited significant seasonal variations; maximum values of these variables occurred in winter and early spring, and the minima were recorded in late summer and early fall. PFD and water temperature were, respectively, positively and negatively correlated with leaf production. Nutrient availability fluctuated substantially, but there was no evidence of distinct seasonal variation, nor was it correlated with leaf production. Leaf chlorophyll concentrations were correlated strongly with leaf production, whereas tissue nutrient contents were unrelated to leaf production. Maximum potential seed production ranged from 1,200 seeds m⁻² in 2004 to 3,445 seeds m⁻² in 2003; however, seedlings were rarely detected through the observation period. Thus, P. japonicus meadows at the study site appeared to persist through vegetative propagation. Leaf C content varied bimodally, with peaks in spring and fall. Leaf N content was minimal during months in which leaf productivity was lowest. These patterns in tissue nutrient content are clearly different from those of the majority of soft-substratum seagrasses and appear to relate to the reduced physiological tolerance of high temperature in P. japonicus compared to other temperate seagrasses.     

Genetic evidence for two distinct clades in a French Polynesian population of the coral reef three-spot damselfish<Emphasis Type="Italic"> Dascyllus trimaculatus</Emphasis>

Previous studies have shown that the three-spot damselfish species complex [ Dascyllus albisella Gill, D. auripinnis Randall and Randall, D. strasburgi Klausewitz, D. trimaculatus (Ruppell)] is an assemblage of five geographically distinct clades. The one exception was a single D. trimaculatus from French Polynesia, which grouped with "Pacific Rim" individuals. In the present study, an additional 252 individuals from French Polynesia collected between June 1996 and January 2002 were analyzed using PCR amplifications, restriction fragment length polymorphisms, and DNA sequencing of the mitochondrial control region. The French Polynesian D. trimaculatus belong to two distinct clades. One clade comprising 96% of the individuals includes haplotypes found only in French Polynesia. The other clade (4% of the individuals) is comprised of haplotypes that cluster with "Pacific Rim" individuals, a clade with widespread distribution from Japan to the Line Islands and from Wallis to Palau. Present data suggest that a small number of larvae from northwestern reefs (possibly Line Islands) may have occasionally reached and colonized French Polynesian reefs.     

Vertical distribution,population structure and lifecycle of<Emphasis Type="Italic"> Eucalanus bungii</Emphasis> (Copepoda: Calanoida) in the Oyashio region,with notes on its regional variations

Vertical distribution and population structure of Eucalanus bungii were investigated at site H in the Oyashio region (western subarctic Pacific) from September 1996 through October 1997 to evaluate the species lifecycle pattern and associated ontogenetic vertical migration. Additional temporary samplings were also made at several stations covering the entire subarctic Pacific, Okhotsk Sea and Japan Sea, as a basis for regional comparison of lifecycle features of this species. At site H, a marked phytoplankton bloom occurred from mid-March to June, and E. bungii spawned in April/May in the surface layer. Resulting nauplii and copepodite stage 1 (C1) formed a prominent abundance peak in early June. The C1 developed and reached C5 by August. The development of nauplii through C4 occurred in the surface layer. From August onwards, C5 and a small fraction of C3–C4 sank gradually deeper, and entered diapause to overwinter at >500 m depth. The C5 molted to C6 males and females in February and March, respectively. The C6 males and females mated at 250–500 m depth, and only mated C6 females ascended to the surface layer in April for spawning. Judging from the size of lipid droplets in the body, the C3–C5 specimens deposited lipids in the body through the phytoplankton bloom period, and the lipids were consumed gradually during overwintering. Taking account of sampling season, temporal changes in population structure, and vertical distribution, the data collected from the western subarctic Pacific and Okhotsk Sea are consistent with a 1-year lifecycle for the site H population, while the data from the central and eastern subarctic Pacific were consistent with a 2-year lifecycle. The populations from the southern and southeastern Japan Sea did not fit the features of either lifecycle scenario, and because of their very small population size it is suggested that they originated from the northern Japan Sea. Regional comparison of the prosome length of C6 females, including those in the Bering Sea, indicated significantly larger specimens from the Japan Sea and Okhotsk Sea, and smaller specimens in the eastern subarctic Pacific, as compared with those in the western subarctic Pacific (including site H) and Bering Sea. A possible overwintering mechanism of E. bungii is discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Genetic differentiation ofMytilus edulis in eastern North America

There is significant differentiation at five polymorphic loci ofMytilus edulis among certain geographical areas of the Atlantic coast of North America. Non-metric multidimensional numerical methods distinguished three population groups: (I) populations south of Cape Cod, (II) populations throughout the Gulf of Maine, Gulf of St. Lawrence, areas of both southern and northern Newfoundland, and southern Hudson Bay, and (III) populations in southeastern Nova Scotia, northern Newfoundland and Hudson Strait, Quebec. Each subset consists of populations that exhibit characteristic multilocus, multiple allele genotypes. Populations in Groups II and III are spatially interdigitated among each other. At least one geographical area of mixing between genetically distinct populations occurs in northeastern Newfoundland. There is no evidence for interbreeding among genetically distinct individuals in mixed populations, suggesting the possibility that populations in the Atlantic Canadian Provinces and areas of northern Canada may consist of two distinct species.     

DNA markers indicate that distinct spawning cohorts and aggregations of Patagonian squid,<Emphasis Type="Italic"> Loligo gahi</Emphasis>, do not represent genetically discrete subpopulations

The Patagonian squid, Loligo gahi DOrbigny, has been described as having temporally and spatially identifiable spawning aggregations. Variation at six microsatellite loci was used to assess whether seasonal and geographical spawning groups around the Falkland Islands represent distinct sub-populations. Genetic variation at these loci is high in this species (mean expected heterozygosity=0.87; mean number of alleles=14.7). No evidence of significantly different allele frequencies was found, either between samples from putative spawning cohorts or geographical areas, indicating that L.gahi around the Falkland Islands comprise a single genetically homogeneous population. Age structure analysis of samples (from statolith growth increments) indicated substantial spread in hatching dates among individuals of similar size and maturity status, suggesting the potential for extensive interbreeding between seasonal cohorts. A sample of L.gahi from the SE Pacific (Peru) displayed distinctly different gene frequencies (and allele size distribution at one locus) from SW Atlantic samples, supporting the suggestion that SE Pacific and SW Atlantic populations may represent distinct subspecies.Contributed by J.P. Thorpe, Port Erin     

Sex in the tropics: reproduction of<Emphasis Type="Italic"> Chthamalus malayensis</Emphasis> Pilsbury (Class Cirripedia) at the equator

The reproduction of a key intertidal space occupier, the barnacle, Chthamalus malayensis, was examined over a 13-month period. The reproductive cycle of C. malayensis was studied at three shores (Singapore, east and west coast of Malaysia) near the centre of its geographical range. Testes, vesiculae seminales, ovaries and embryos were scored for their stages of development. Although, in general, C. malayensis contained gonads throughout the year, the timing and level of peak development varied between the three shores. Embryos were present throughout the year in west Malaysia but were absent for 4–5 months in east Malaysia (June–August and October) and Singapore (February, April and August). Variations in the reproductive cycles and brooding were not clearly linked to the timing of monsoons. The lack of seasonal peaks in breeding may be attributed to the relatively constant climatic conditions experienced near the equator.Communicated by M. S. Johnson, Crawley     

Population genetic structure of the neon damselfish (<Emphasis Type="Italic">Pomacentrus coelestis</Emphasis>) in the northwestern Pacific Ocean

The population genetic structure of the neon damselfish (Pomacentrus coelestis) in the northwestern Pacific Ocean was revealed by the hypervariable control region of the mitochondrial gene (343 bp). In total, 170 individuals were sampled from 8 localities distributed between Taiwan and Japan, and 71 haplotypes were obtained through sequence alignment. High haplotype diversity (h = 0.956 ± 0.008) with low nucleotide diversity (π = 0.010 ± 0.006) was observed, and the results of the mismatch distribution test suggested that a historical population expansion after a period of population bottleneck might have occurred among P. coelestis populations. Based on the results of the UPGMA tree and AMOVA (Φct = 0.193, P < 0.05) analyses, fish populations from eight localities could be divided into two groups: one includes populations from localities around mainland Japan, and the other includes those from Okinawa and southern Taiwan. A genetic break was found between populations from mainland Japan and Okinawa, and this break was congruent with the pattern of phenotypic variations documented in previous studies. This evidence supports the latitudinal variation of reproductive traits among P. coelestis populations likely being genetically based. It is suggested that the changes in sea level and sea surface temperatures during past glaciations might have resulted in population bottlenecks in P. coelestis and the modern populations in the northern West Pacific are likely the results of recolonization after such events. The Kuroshio Current acts not only as a vehicle for larval transport along its pathway (between populations in southern Taiwan and Okinawa) but also as a barrier for larval dispersal across the Kuroshio axis (between populations in mainland Japan and Okinawa). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genealogical relationships within and among shallow-water <Emphasis Type="Italic">Ciona</Emphasis> species (Ascidiacea)

In spite of historical and current interest in Ciona intestinalis and its congeners, little is known about evolutionary relationships among the members of the genus Ciona. Here 744-bp sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene are used to examine phylogenetic relationships among three described species (C. intestinalis, C. roulei, C. savignyi) sampled from multiple coastal sites in the Northeast Pacific (CA, USA), Northwest Atlantic (from New Hampshire to Connecticut, USA), Northeast Atlantic (Sweden and The Netherlands), and Mediterranean (Banyuls-sur-Mer, France). The samples were collected in June–October 2005. The COI sequences of Northeast Pacific/Mediterranean (Type A) and Northwest Atlantic (Type B) C. intestinalis differ by ∼12% and C. roulei is nested within Type B C. intestinalis. Ciona savignyi differs from all other haplotypes by 13–16%. A previously undescribed but morphologically distinct Ciona sp. found at the Banyuls-sur-Mer site was >10% divergent from all other haplotypes. Although these data arise from a single gene study, they indicate that further elucidation of species relationships within the genus and of the species’ distributions will be needed if continuing invasions and potential reproductive isolation are to be investigated.     

Burrow morphology of the ghost shrimp<Emphasis Type="Italic"> Nihonotrypaea petalura</Emphasis> (Decapoda: Thalassinidea: Callianassidae) from western Kyushu,Japan

The callianassid shrimp Nihonotrypaea petalura (Stimpson, 1860) is a common member on boulder beaches in Japanese waters. Its burrow morphology was investigated, based on 28 resin casts collected from a steeply sloping beach with dense boulders and 30 from a more gently sloping beach with less dense boulders in Ariake Sound, southern Japan. The structure and dimensions of the burrows from the two beaches were basically the same, and thus a combined mean value could be given to most of their constituent elements. In its entire dimensions, the burrow is greater in lateral extent than depth, with a mean maximum horizontal extension of 145 mm and a mean maximum depth of 119 mm for the shrimp with a mean total length of 31.8 mm. The burrow winds along boulders or cobbles and consists of, from top to bottom, a single surface opening with an ejected mound, a top shaft leading to the uppermost chamber at a mean depth of 48–56 mm, a passage with a regular cross section that is wider than that of the top shaft, and bulbous chambers (mean no.=4.7) with an irregular cross section associated with branches (mean no. per burrow=1.2). Bulbous chambers are much larger than the uppermost chamber and are usually connected by passages, with some directly attached to each other. The combined architecture of these features is unique and relatively simple among the burrows of all callianassid species, the majority of which inhabit bare soft sediments. The structure and function of the N. petalura burrow are discussed in relation to lifestyle.Communicated by T. Ikeda, Hakodate     

Luminous bacteria of a monocentrid fish (Monocentris japonicus) and two anomalopid fishes (Photoblepharon palpebratus and Kryptophanaron alfredi): population sizes and growth within the light organs,and rates of release into the seawater

The light organs of monocentrid and anomalopid fishes consist of bacteria-filled tubular invaginations of the epidermis which are connected to the surrounding seawater by ducts. We used the release of bacteria from the light organs to estimate bacterial rates of growth in the light organs. For one monocentrid fish (4 specimens of Monocentris japonicus collected at Jogashima, Japan in 1980) and for two anomalopid fishes (2 specimens each of Photoblepharon palpebratus collected at Sebu, Phillipines in 1981 and Grand Comore Island in 1975 and Kryptophanaron alfredi collected at Parguera, Puerto Rico in 1982) we measured rates of release of bacteria into the surrounding seawater and the bacterial population sizes in the light organs; from this information we calculated doubling times of bacteria in the light organs. In addition, we determined the luminescence of bacteria after their release into the seawater. For M. japonicus, two specimens released 1.1 to 6×10⁶ and 2×10⁷ bacteria h^–1, respectively; the light organs contained about 1.5×10⁸ bacteria. For P. palpebratus, one specimen released 2.2×10⁸ bacteria h^–1; a second specimen had light organs containing 5.2×10⁹ bacteria. For K. alfredi, one specimen released 7×10⁷ bacteria h^–1 and had light organs containing 5.6×10⁸ bacteria; a second specimen released 3.6×10⁷ bacteria h^–1 and had light organs containing 7.3×10⁸ bacteria. Bacterial doubling times in the light organs of these three fishes were variable and ranged from 7.5 to 135 h in M. japonicus and 8 to 23 h in the anomalopids. Bacteria released from M. japonicus into the seawater remained viable, but bacteria from all of the fishes soon ceased to emit light.     

Population genetic structure of the milkfish,<Emphasis Type="Italic"> Chanos chanos</Emphasis>, based on PCR-RFLP analysis of the mitochondrial control region

The milkfish, Chanos chanos (Forsskål, 1775) is a pelagic, monotypic gonorhynchiform widely distributed in the tropical Indo-Pacific. This study evaluates temporal variability of milkfish samples from the Philippine archipelago, and spatial variability at two geographic scales based on restriction fragment length polymorphism (RFLP) analysis of a portion of the mitochondrial control region. High levels of genetic diversity characterize the milkfish control region (mean h=0.908, =1.59%), with 74 haplotypes detected among the 367 fish analyzed. For temporal analysis of Philippine samples, milkfish were collected over 2 years from three sites (inter-annual variation), and sampled twice within a year during different seasons at four sites (intra-annual variation). No significant temporal variability was detected between or within years. Significant spatial differentiation among the Philippine samples was observed (F_ST=0.006, P<0.05), with two northeastern samples, Claveria and Dingalan, found to be genetically distinct. However, an hierarchical analysis of molecular variance (AMOVA), where samples were grouped into four geographic regions, revealed very low levels of genetic partitioning, with less than 1% of the total variation attributed to between-region differences, and lack of genetic structure. Nonetheless, the existence of putative northeastern Philippine populations is not discounted. Strong genetic structure across broad geographical scales was revealed by AMOVA, with 11% of the molecular variance based on haplotype frequencies allocated between three distinct groups: Indian Ocean, west Pacific (Philippines) and north central Pacific (Hawaii) The broad-scale genetic structure points to limited gene flow among disjunct Indo-Pacific populations.Communicated by T. Ikeda, Hakodate     

Geographical genetic structure and phylogeography of the <Emphasis Type="Italic">Sargassum horneri</Emphasis>/<Emphasis Type="Italic">filicinum</Emphasis> complex in Japan,based on the mitochondrial <Emphasis Type="Italic">cox3</Emphasis> haplotype

The genetic structure and phylogeography of the brown seaweed Sargassum horneri/filicinum complex in Japan were studied based on the mitochondrial cox3 haplotype. The cox3 haplotypes found were divided into three clades in a statistical parsimony network, among which there were large numbers of steps. Contrary to the reported large amount of drifting S. horneri along the Japanese coast, the three clades were dividedly distributed on the Japanese coast: the northern Pacific, the central Pacific, and western Japan. The western Japan S. horneri had haplotypes that were phylogenetically closer to those of S. filicinum than to the northern and central Pacific S. horneri populations. The S. filicinum populations were included within the western Japan clade and grouped together with the S. horneri samples from western Japan. Taken together with the unstable morphological diagnosis, this result suggests that S. filicinum should be reduced into a synonymy of S. horneri. The TMRCA analysis suggested that the divergence time of each clade may go back to the last interglacial period and a skyline plot suggested that the last glacial maximum had only a small effect on the population size of S. horneri. The geographic subdivision of the three groups, in spite of a large amount of drifting mats, suggests a limited contribution of drifting mats to gene flow on a large geographic scale. On a small geographic scale, a small number of haplotypes were shared between S. horneri-type and S. filicinum-type populations. This result suggests that populations of these two types are partially, though not completely, isolated from each other, possibly by selfing in S. filicinum-type populations or by a difference in peak reproduction.     

Mitochondrial cytochrome oxidase I gene sequences support an Asian origin for the Portuguese oyster Crassostrea angulata

The Portuguese oyster Crassostrea angulata (Lamarck, 1819) was long assumed to be native to the northeastern Atlantic, however, a number of lines of evidence now indicate that it is a close relative, or identical, to the Asian Pacific oyster C. gigas (Thunberg, 1793). Three hypotheses have been proposed to explain how this strikingly disjunct geographic distribution may have come about: ancient vicariance events, recent anthropogenic introduction to Asia and recent anthropogenic introduction to Europe. We have performed a molecular phylogenetic analysis of C. angulata based on mitochondrial DNA sequence data for a 579-nucleotide fragment of cytochrome oxidase I. Our results show that Portuguese oyster haplotypes cluster robustly within a clade of Asian congeners and are closely related, but not identical, to C. gigas from Japan. The mitochondrial data are the first to show that Portuguese oysters are genetically distinct from geographically representative samples of Japanese Pacific oysters. Our phylogenetic analyses are consistent with a recent introduction of C. angulata to Europe either from a non-Japanese Asian source population or from a subsequently displaced Japanese source population. Genetic characterization of Pacific oysters throughout their Asian range is necessary to fully reveal the phylogenetic relationships among Portuguese and Pacific oysters. Received: 15 April 1997 / Accepted: 12 February 1998     

Disjunct Sea of Cortez–Pacific Ocean Gillichthys mirabilis populations and the evolutionary origin of their Sea of Cortez endemic relative, Gillichthys seta

The shortjaw mudsucker, Gillichthys seta, an intertidal goby endemic to the Sea of Cortez, has been proposed to be the paedomorphic derivative of the longjaw mudsucker, Gillichthys mirabilis. G. mirabilis is a disjunct species, with populations found along the Pacific coast of central California to central Baja California, and with isolated populations found in the northern Sea of Cortez. Previous studies have suggested that the endemic paedomorph form speciated in sympatry with the Sea of Cortez population of G. mirabilis. Alternatively, this speciation event could have occurred before the separation of G. mirabilis populations into two disjunct entities. To test these alternative hypotheses, we collected adult individuals from both species throughout their ranges from December 1997 to November 1998. We amplified and sequenced 142 partial [527 base pairs (bp)] mitochondrial cytochrome b regions and 18 nuclear creatine kinase introns (140 bp). We found that Pacific populations of G. mirabilis separated into two distinct clades, possibly reflecting a phylogeographic break found in other fish species along the Baja California coast at Punta Eugenia. These two Pacific populations were well separated from Sea of Cortez populations. Furthermore, our results indicate that the split between Sea of Cortez and Pacific populations of G. mirabilis occurred well after the speciation event that separated G. mirabilis from its paedomorphic counterpart, G. seta. Received: 30 March 2000 / Accepted: 4 September 2000     

Lack of mtDNA and morphological differentiation between two acorn barnacles <Emphasis Type="Italic">Tetraclita japonica</Emphasis> and <Emphasis Type="Italic">T. formosana</Emphasis> differing in parietes colours and geographical distribution

Tetraclita japonica and T. formosana are common intertidal barnacles with similar morphology, which leads to uncertainty in their species status. In the present study, we try to elucidate the taxonomic status of the two taxa using morphology and mitochondrial control region and cytochrome c oxidase subunit I (COI) sequences of the barnacles in their distribution range. The two taxa were found to be morphologically similar; a diagnostic difference between them was only observed in the colour of the parietes and opercular plates. Little genetic differentiation was detected in the control region and COI (Φ_CT < 0.025 for both markers) between two taxa, but differentiation was found between the southern (Taiwan and Hong Kong) and northern (Japan) populations of T. japonica/T. formosana, which might be the result of isolation by distance and upwelling in summer. Our data suggest that the two presently recognized species probably represent two colour morphotypes of the same species exhibiting different geographical distribution. T. japonica is abundant in Japan and southeast coast of China, whereas T. formosana is only abundant in Taiwan. The heterogeneous environment might exert a divergent selection pressure leading to asymmetric distribution of the two colour morphotypes. The different colours might be a result of either phenotypic plasticity adaptive to environmental variables or genetic hitchhiking of local adaptive genotypes. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.