Molecular phylogenies help resolve taxonomic confusion with Asian <Emphasis Type="Italic">Crassostrea</Emphasis> oyster species

Identification of oyster species is still largely based on phenotypic characters that are highly plastic. Prompted by the proposed introduction of the Asian oyster species Crassostrea ariakensis into the Chesapeake Bay region of the U.S.A., this study uses molecular genetic information to understand the taxonomic framework surrounding C. ariakensis and to confidently distinguish among various sympatric oyster species. Putative samples of C. ariakensis and other species of cupped oysters from across Asia were collected and phylogenetic analyses were conducted on DNA sequences of both nuclear (ITS-1) and mitochondrial (COI) regions. Trees generated based on the two independent molecular datasets were highly congruent, and indicate that many oysters collected for this study as C. ariakensis were originally misidentified. Results also indicate that C. ariakensis, C. hongkongensis and C. nippona are distinct, but closely related species. There is strong support in both analyses for a close relationship between C. gigas and C. sikamea, as well as between C. belcheri and C. gryphoides, and between C. iredalei and C. madrasensis. The parsimony analyses based on these DNA markers, however, did not provide evidence to support C. angulata as a distinct species from C. gigas. Overall, the results emphasize the need for rigorous species identification, and additional extensive and intensive sampling to more accurately determine relationships among Crassostrea species, define their geographic distributions, and establish existing sympatry patterns.     

Gene flow among giant clam (Tridacna gigas) populations in Pacific does not parallel ocean circulation

Tridacna gigas (L.) populations were sampled from 13 locations throughout the west Pacific in 1990–1991 and screened for allozyme variation at seven polymorphic loci. Significant genetic differences were found between the few large stocks of T. gigas remaining in the world. Cluster analysis identified two major regional groups of populations: a Central Pacific group consisting of Kiribati and the Marshall Islands, and a West Pacific group comprising all other populations. Within the latter group, the Great Barrier Reef populations were significantly differentiated from those in the Solomon Islands. The main routes of gene flow did not run parallel with known major surface currents as might be expected, but crossed them. Gene exchange was limited east-west between the Central and West Pacific groups and the GBR and the Solomon Islands. The lack of correspondence between the major surface currents and the routes of gene flow suggests that the genetic structure of T. gigas reflects historical patterns of migration that no longer occur, rather than presentday dispersal. These findings raise fundamental questions concerning the origin and maintenance of genetic diversity in Pacific faunas, and have important implications for the mariculture and conservation of T. gigas.     

Feeding in dominant Antarctic copepods—does the morphology of the mandibular gnathobases relate to diet?

This study should clarify the importance of morphology and stability of the mandibular gnathobases for the diet of Antarctic copepod species. The gnathobase morphology of the dominant copepod species Calanoides acutus, Calanus propinquus, Ctenocalanus citer, Rhincalanus gigas, Metridia gerlachei, Stephos longipes, Microcalanus pygmaeus and Paraeuchaeta antarctica from the Southern Ocean was investigated in detail by means of a scanning electron microscope. The mandibular gnathobases of C. acutus, C. propinquus and C. citer have relatively short and compact teeth. These species feed mainly on diatoms and are able to crack the silicious diatom frustules with their mandibular gnathobases by directed pressure. In contrast the teeth of the mandibular gnathobases of P. antarctica are very long and pointed. The nutrition of this species consists predominantly of other smaller copepod species. The motile prey can be held by skewering, using the gnathobases, and then eventually minced. The mandibular gnathobases of P. antarctica have notably more small bristles than those of the other investigated copepod species. These bristles are probably associated with receptors and could serve to locate the prey. The morphology of the gnathobases of R. gigas and M. gerlachei is between that of P. antarctica on the one side and that of C. acutus, C. propinquus and C. citer on the other. Based on the morphology of its gnathobases the copepod species S. longipes, which has to date been found to feed primarily on phytoplankton, mainly ice algae, must also be considered a zooplankton feeder. The investigation showed that M. pygmaeus has gnathobases with surprisingly long and pointed teeth, indicating that this species very probably feeds both on phyto- and on zooplankton organisms. While the mandibular gnathobases of the males of C. propinquus, R. gigas, M. gerlachei and S. longipes have the same morphology as the females of the respective species, in the other four investigated copepod species the males have reduced (C. acutus, C. citer and M. pygmaeus) or completely missing mandibular gnathobases (P. antarctica). The teeth of the gnathobases of all studied species with the exception of M. gerlachei consist of a different material than the remaining parts of the gnathobases. This material seems to be silicate, which probably enhances the stability of the gnathobase teeth.Communicated by O. Kinne, Oldendorf/Luhe     

Vertical distribution,population structure and life cycles of four oncaeid copepods in the Oyashio region,western subarctic Pacific

Vertical distribution and population structure of four dominant oncaeid copepods (Triconia borealis, Triconia canadensis, Oncaea grossa and Oncaea parila) were investigated in the Oyashio region, western subarctic Pacific. Seasonal samples were collected with 0.06 mm mesh nets from five discrete layers between the surface and 2,000 m depth at seven occasions (March, May, June, August and October 2002, December 2003 and February 2004). The depth of occurrence of major populations of each species differed by species; the surface–250 m for T. borealis, 250–1,000 m for T. canadensis, 250–500 m for O. grossa and 500–1,000 m for O. parila. The ontogenetic vertical migration characterized by deeper occurrence of early and late copepodid stages, and shallower occurrence of middle copepodid stages was observed in T. canadensis and O. parila. Of the four oncaeid copepods, almost all copepodid stages occurred throughout the study period, suggesting that their reproduction continues throughout the year in the region. Nevertheless, a clear developmental sequence of stage-to-stage was traced for T. canadensis and O. grossa copepodids, implying their generation time to be 1 year. For T. borealis and O. parila copepodids, no clear seasonal succession was observed thus estimation of their generation time was uncertain. The present comprehensive results of vertical distribution and life cycle features for T. borealis, T. canadensis, O. grossa and O. parila are compared with the few published data on oncaeid species distributing in high latitude seas.     

Uptake of dissolved inorganic nitrogen by the symbiotic clam Tridacna gigas and the coral Acropora sp.

Dissolved inorganic nitrogen flux was studied in the giant clam Tridacna gigas and the corals Acropora sp. and Tubastrea micrantha from the tropical reefs of Belau, Micronesia in 1983. T. micrantha, a nonsymbiotic coral, excreted ammonium. However, Tridacna gigas and Acropora sp., which contain symbiotic dinoflagellates (zooxanthellae) were able to take up both ammonium and nitrate. The requirement for a previous light exposure to sustain uptake by T. gigas is reported. The uptake kinetics of these symbioses are described and include the capacity of the zooxanthellae for surge uptake when given nutrient spikes.Contribution No. 417 of the Allan Hancock Foundation     

Population genetics of the family ostreidae. I. Intraspecific studies of Crassostrea gigas and Saccostrea commercialis

The levels of genetic variation and estimates of genetic similarity and distance were determined for populations of Crassostrea gigas (Thunberg) and Saccostrea commercialis (Iredale and Roughley). The proportion of polymorphic loci in Japanese populations of C. gigas ranged from 0.58 to 0.63, while the individual heterozygosities ranged from 0.20 to 0.22. The proportion of polymorphic loci in Australian populations of S. commercialis ranged from 0.43 to 0.46, the individual heterozygosities from 0.17 to 0.19. The genetic similarity between different geographical populations of both species was approximately 99%. The genetic distance and similarity data derived from this investigation has resulted in some provisional reclassifications. The Portuguese oyster C. angulata (Lamarck) appears to be more appropriately regarded as a recent colonized isolate of C. gigas. The New Zealand oyster S. glomerata (Gould) is considered a subspecies of the eastern Australian rock oyster S. commercialis. The Japanese Kumamoto population of C. gigas warrants reclassification as the non-sibling species C. sikamea.     

Impact of starvation and of feeding algal and artificial diets on the lipid content and composition of juvenile oysters (Crassostrea gigas) and clams (Tapes philippinarum)

The present study evaluated the effect of starvation and of feeding various algal and lipid-supplemented diets on the lipid content and lipid class distribution in the polar and neutral lipids of early juvenile oysters (Crassostrea gigas) and clams (Tapes philippinarum L.). T. philippinarum was starved, fed a mixed algal diet [Tetraselmis suecica and Isochrysis galbana (clone T-Iso)] or solely T. suecica at three different feeding rations, either supplemented or not supplemented with lipid emulsions. C. gigas was fed T. suecica with and without the supplementation of lipid emulsions or liposomes. When T. philippinarum and C. gigas were fed solely T. suecica, no qualitative and only minor quantitative differences were observed between the lipid class profile of both species. The major neutral lipids were triglycerides (TAG) and free sterols plus diglycerides (FS+DAG), whereas the polar lipids were dominated by phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol plus ceramideaminoethanol phosphonate (PI+CAEP). An increase of the algal feeding ration resulted in an increase of the lipid content of spat which was associated with a significant augmentation of the TAG level (as percentage of the total lipids and as percentage of the neutral lipids). Lipid supplementation evoked a similar though more pronounced effect. Starvation resulted in a significant decline of the lipid content and a complete depletion of the TAG reserve. Contrary to the neutral lipids (NL), the relative proportion (percentage of total polar lipids, PL) of the individual PL classes was hardly affected by the diet. The importance of lipid and TAG reserves in early juveniles is discussed. Received: 15 October 1999 / Accepted: 15 February 2000     

Relationships between RNA content and egg production rate in Acartia bifilosa (Copepoda, Calanoida) of different spatial and temporal origin

To evaluate factors regulating RNA content (RNA, μg RNA female⁻¹) – egg production rate (EPR, eggs female⁻¹ day⁻¹) relationship and to develop a model for in situ egg production rate estimates for Acartia bifilosa, we (1) measured EPR and RNA in females sampled at geographically distant areas at varying temperature (T, °C), (2) determined environmental (station, season, and T), endogenous (prosome length (PL), mm and EPR) variables that influence RNA levels, and (3) explored a set of multiple regression models to predict EPR from RNA, PL, station, season, and T. Egg production experiments were carried out in spring and summer 2005 in the Gulf of Finland and in the western part of the northern Baltic proper. We found that up to 88% of the RNA variation could be explained by variations in PL, EPR, and season/T. In explaining the RNA variability, PL played a major role followed in order of importance by EPR and season/T. Further, PL, RNA, and season/T explained up to 53% of the variation in EPR, nearly half of which is explained by RNA alone. The effect of spatial origin was never significant, suggesting that the derived relationships are general for A. bifilosa inhabiting northern Baltic proper. An erratum to this article is available at .     

Toxic dinoflagellates (Alexandrium fundyense and A. catenella) have minimal apparent effects on oyster hemocytes

The possible effect of Alexandrium spp. containing paralytic shellfish poisoning (PSP) toxins on the hemocytes of oysters was tested experimentally. In one trial, eastern oysters, Crassostrea virginica Gmelin, were exposed to bloom concentrations of the sympatric dinoflagellate, Alexandrium fundyense Balech, alone and in a mixture with a non-toxic diatom, Thalassiosira weissflogii (Grun) Fryxell et Hasle. Subsequently, another experiment exposed Pacific oysters, Crassostrea gigas Thunberg, to a mixed suspension of the sympatric, toxic species Alexandrium catenella (Whedon et Kofoid) Balech, with T. weissflogii. Measurements of numbers of oyster hemocytes, percentages of different cell types, and functions (phagocytosis, reactive oxygen species (ROS) production, and mortality) were made using flow-cytometry. During and after exposure, almost no significant effects of Alexandrium spp. upon hemocyte numbers, morphology, or functions were detected, despite observations of adductor-muscle paralysis in C. virginica and measured toxin accumulation in C. gigas. The only significant correlation found was between toxin accumulation at one temperature and higher numbers of circulating live and dead hemocytes in C. gigas. The PSP toxins are known to interfere specifically with sodium-channel function; therefore, the finding that the toxins had no effect on measured hemocyte functions suggests that sodium-channel physiology is not important in these hemocyte functions. Finally, because oysters were exposed to the living algae, not purified toxins, there was no evidence of bioactive compounds other than PSP toxins affecting hemocytes in the two species of Alexandrium studied.     

Comparing the effect of elevated pCO2 and temperature on the fertilization and early development of two species of oysters

This study compared the synergistic effects of elevated pCO₂ and temperature on the early life history stages of two ecologically and economically important oysters: the Sydney rock oyster, Saccostrea glomerata and the Pacific oyster, Crassostrea gigas. Gametes, embryos, larvae and spat were exposed to four pCO₂ (375, 600, 750, 1,000 μatm) and four temperature (18, 22, 26, 30°C) levels. At elevated pCO₂ and suboptimal temperatures, there was a reduction in the fertilization success of gametes, a reduction in the development of embryos and size of larvae and spat and an increase in abnormal morphology of larvae. These effects varied between species and fertilization treatments with S. glomerata having greater sensitivity than C. gigas. In the absence of adaptation, C. gigas may become the more dominant species along the south-eastern coast of Australia, recruiting into estuaries currently dominated by the native S. glomerata.     

Occurrence of the Kumamoto oyster Crassostrea sikamea in the Ariake Sea, Japan

The Kumamoto oyster Crassostrea sikamea is distinguished from the closely related Pacific oyster C. gigas by concordant differences in 16S rDNA, allozymes, and a one-way gametic incompatibility. After repeated failures to find this oyster in its native habitat, we speculated in 1994 that “the Kumamoto oyster may be extinct in Japan”. In September 1996, we sampled small, deep-cupped oysters from the Ariake Sea and typed these for 16S rDNA and ITS-1 DNA markers previously shown to be diagnostic for the three most common oysters in the Ariake Sea, C. gigas, C. sikamea and  C. ariakensis. Our earlier suggestion of the demise of  C. sikamea proved incorrect. Of the 256 oysters sampled, 181 (71%) were  C. gigas, 53 (21%) were C. sikamea, and 22 (9%) were  C. ariakensis; no interspecific hybrids were observed. The distributions of C. sikamea and  C. ariakensis are clumped in the Ariake Sea:  C. sikamea occurs on the eastern and northern shores,  C. ariakensis occurs only in the northern part. These results emphasize the value of molecular markers for discriminating these morphologically plastic species both in the field and in aquaculture. Received 6 July 1998 / Accepted 8 October 1998     

Persistence of atrazine impact on aneuploidy in Pacific oysters,<Emphasis Type="Italic"> Crassostrea gigas</Emphasis>

Widespread use of the herbicide atrazine has incited much research on its toxicity in aquatic systems, where it is routinely detected due to runoff from cultivated fields. Moreover, the determination of the genotoxic effect of such pollutants in the marine environment has become a major requirement for ecosystem protection. In the Pacific oyster, Crassostrea gigas, hypodiploid aneuploid cells have regularly been reported. There is a negative correlation between this phenomenon and growth, as well as evidence for a genetic basis. A positive relationship between atrazine and aneuploidy has previously been demonstrated in C. gigas adults and juveniles. To evaluate the persistence of this impact, our study examined the offspring of the same adult population previously treated with different atrazine doses (10 µg l^–1, representing a peak value found in a polluted environment and 100 µg l^–1), and a seawater control. We observed that these offspring exhibited significantly higher aneuploidy levels when their parents had been exposed to atrazine (14.9–16.9% in comparison with the control where the levels ranged from 11.4% to 12.8%). In addition, the present study examined the aneuploidy level of a sample of juveniles, previously exposed for 3.5 months to the same doses of atrazine, then transferred to non-polluted conditions for an additional period of 2.5 months; this aneuploidy level remained significantly different between the treatments applied. These results demonstrate the persistence of an atrazine impact on Pacific oyster aneuploidy in time, within and between generations, indicating that this widely used compound may represent an important factor causing at least medium-term damage to genetic material.Communicated by S.A. Poulet, Roscoff     

Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas, Mytilus edulis and Cerastoderma edule, in relation to food competition

Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.     

Phylogeographic study of the dwarf oyster, Ostreola stentina, from Morocco, Portugal and Tunisia: evidence of a geographic disjunction with the closely related taxa, Ostrea aupouria and Ostreola equestris

Despite the economic importance of oysters due to the high aquaculture production of several species, the current knowledge of oyster phylogeny and systematics is still fragmentary. In Europe, Ostrea edulis, the European flat oyster, and Ostreola stentina, the Provence oyster or dwarf oyster, are both present along the European and African, Atlantic and Mediterranean, coasts. In order to document the relationship not only between O. stentina and O. edulis, but also with the other Ostrea and Ostreola species, we performed a sequence analysis of the 16S mitochondrial fragment (16S rDNA: the large subunit rRNA-coding gene) and the COI fragment (COI: cytochrome oxidase subunit I). Oysters were sampled from populations in Portugal (two populations), Tunisia (two populations) and Morocco (one population), identified as O. stentina on the basis of shell morphological characters. Our data supported a high degree of differentiation between O. stentina and O. edulis and a close relationship between O. stentina and both Ostrea aupouria (from New Zealand) and Ostreola equestris (from Mexico Gulf/Atlantic). The status of this geographic disjunction between these closely related species is discussed. Furthermore, although identified in a separate genus Ostreola by Harry (Veliger 28:121–158, 1985), our molecular data on O. stentina, together with those available for the other two putative congeneric species, O. equestris and Ostreola conchaphila, would favour incorporation of Ostreola in Ostrea. Finally, a PCR-RFLP approach allowed the rapid identification of O. edulis and O. stentina.     

Pheromone-based aggregation behaviour of the firebrat, <Emphasis Type="Italic">Thermobia domestica</Emphasis> (Packard) (Thysanura: Lepismatidae)

Summary. We tested the hypothesis that aggregation behaviour of the firebrat, Thermobia domestica (Packard) (Thysanura: Lepismatidae), an inhabitant of enclosed microhabitats, is mediated, at least in part, by a pheromone. Individual insects were released into the central chamber of a 3-chambered olfactometer and test stimuli were placed in lateral chambers. Paper discs previously exposed for 3 days to 10 female, male, or juvenile T. domestica were all preferred by female, male, or juvenile T. domestica over unexposed paper discs, indicating the presence of an aggregation/arrestment pheromone. In additional experiments, frass and scales from female T. domestica, tested singly and in combination, proved not to be the source of the pheromone. Physical contact was required for pheromone recognition, indicating that the pheromone arrests rather than attracts conspecifics. Arrestment by the long-tailed silverfish, Ctenolepisma longicaudata Escherich (Thysanura: Lepismatidae), but not by the common silverfish, Lepisma saccharina L. (Thysanura: Lepismatidae), to T. domestica exposed paper discs suggests closer phylogenetic relatedness between C. longicaudata and T. domestica, than between C. longicaudata and L. saccharina. Whether C. longicaudata or L. saccharina produce an aggregation signal, and whether T. domestica respond to this signal is unknown. Received 10 June 2002; accepted 30 September 2002.     

Gametic incompatibility and genetic divergence of Pacific and Kumamoto oysters,Crassostrea gigas and C. sikamea

The biological and taxonomic separation of the Pacific oyster Crassostrea gigas (Thunberg, 1793) from the Kumamoto oyster C. sikamea (Amemiya, 1928) is affirmed by three concordant lines of evidence: (1) fixed differences in 2% of a mtDNA sequence coding for large subunit rRNA; (2) a genetic distance of 0.440 based on 19 allozyme loci, including 5 diagnostic loci (Aat-1, Idh-1, Idh-2, Mpi-1, Mdh-2); (3) one-way gametic incompatibility resulting in partial reproductive isolation in interspecific crosses. C. gigas sperm x C. sikamea egg fertilizations form viable hybrid offspring, but C. sikamea sperm do not fertilize C. gigas eggs. Divergence between these two species is mediated by differing peaks in the periods for gamete release and by one-way sperm/egg incompatibility. Two attempts to recover C. sikamea from its place of origin in southern Japan have yielded only individuals with the mitochondrial haplotype that characterizes C. gigas. We thus identify a crucial need for careful screening, management, and conservation of the cultivated populations of C. sikamea on the US west coast.     

Genetic variation in Symbiodinium isolates from giant clams based on random-amplified-polymorphic DNA (RAPD) patterns

We have compared the random-amplified-polymorphic DNA (RAPD) patterns of Symbiodinium isolates from seven species of giant clams to investigate the large genetic variation that we previously reported for this group of dinoflagellate symbionts using allozyme analysis. Comparisons of 163 RAPD characters by unweighted pair-group arithmetic-average cluster analysis (UPGMA) corroborate our previous findings that giant clams associate with a large number of genetically distinguishable algal symbionts, and that the isolates from a single Tridacna gigas individual form a group of closely related algae. However, the overall topology of the UPGMA tree constructed from RAPD data differs from that of the previous allozyme data, indicating that the combined data we have collected to date are insufficient to accurately infer phylogenetic affiliations between the isolates studied. Comparisons of our data set with those published for strains of Gymnodinium catenatum, a toxic dinoflagellate with a sexual life stage, shows that our isolates are even more diverse. Algal isolates from giant clams have a level of RAPD variation comparable to organisms that are able to undergo sexual recombination. This study demonstrates the sensitivity of the RAPD technique in detecting genetic diversity in this group of algae, and highlights the need for more comparative data for the major clades of Symbiodinium. Received: 24 August 1999 / Accepted: 8 March 2000     

Diel vertical migration and feeding of copepods at an oceanic site near South Georgia

Twelve Longhurst Hardy Plankton Recorder (LHPR) profiles were taken over a 16 h period in January 1990, in order to study feeding of four copepod species at an Antarctic oceanic site near South Georgia. Vertical distributions of their life stages, as well as those of dominant competitors and predators, are described in relation to the feeding cycles of Calanoides acutus CV, Calanus simillimus CV, Calanus propinquus CV and Rhincalanus gigas CIII, CV and CVI. Comparisons with vertical ring-net catches, which were used for concomitant gutevacuation experiments, demonstrated the suitability of the LHPR for these fine-scale studies. Planktonic predators, with the exception of the diel migrant Themisto gaudichaudii, resided deeper than the herbivores. During the day and around midnight, when feeding rates were low, species and stages reached their maximum vertical separation. At these times, new generation copepodites of the four species lived progressively deeper and the overwintered generation (i.e., R. gigas Stages CIV, CV, CVI) were progressively shallower. During the afternoon or evening (depending on species), all stages older than CII, as well as Euphausia frigida and T. gaudichaudii, migrated upwards, to amass in the surface mixed layer. Feeding was restricted to darkness, although R. gigas commenced several hours before dusk. In detail their migration and feeding differed widely, with combinations of unimodal and apparent bimodal cycles. As a whole, the results suggest that (1) feeding could occur during sinking as well as during upward migrations, (2) upward migrations were not always associated with feeding increases, and (3) individuals appeared to descend after filling their guts.     

Evidence for two genetically and chemically defined host races of Tyria jacobaeae (Arctiidae, Lepidoptera)

Summary. A widely distributed host race of Tyria jacobaeae lives on Senecio jacobaea and related species and accumulates pyrrolizidine alkaloids (“PA race”), another race, which is restricted to the Alps and found on Petasites paradoxus, sequesters sesquiterpenes, such as petasol and isopetasol. Nucleotide sequences of the mitochondrial 16S rDNA gene show 1% sequence divergence, indicating that genetical differences exist between the PA exploiting and the terpene-sequestering host races of T. jacobaeae. This finding suggests that both host races of T. jacobaeae must have been separated for some time already, possibly since the Pleistocene. Received 2 May 2001; accepted 1 June 2001.     

Evolution of coral reef fish<Emphasis Type="Italic"> Thalassoma</Emphasis> spp. (Labridae). 2. Evolution of the eastern Atlantic species

The genetic relationships within and among congeneric species of marine fish from the Atlantic and the Mediterranean are poorly known. Relationships among all five species of the wrasse genus Thalassoma present in the Atlantic and the Mediterranean were examined using sequence data from the mitochondrial control region. Sampling was focused on the mid-Atlantic T. sanctaehelenae (Valenciennes, 1839) and T. ascensionis (Quoy & Gaimard, 1834), the eastern Atlantic T. newtoni (Osório, 1891) from Sao Tome, and the eastern Atlantic/Mediterranean T. pavo (Linnaeus, 1758). Two western Atlantic species T. bifasciatum (Bloch, 1791) from the Caribbean and T. noronhanum (Boulenger, 1890) from Brazil served as outgroups. Tissues from a total of 132 individuals were sequenced. T. newtoni from Sao Tome preferentially grouped with the central Atlantic T. sanctaehelenae and T. ascensionis. T. pavo exhibits two distinct coloration patterns, one in the Cape Verde Islands and one in the eastern Atlantic Islands and Mediterranean. However, no genetic discontinuities between the Cape Verde Islands and the remaining samples or between Atlantic and Mediterranean individuals were found. Within Mediterranean populations of T. pavo, our data suggested the presence of a genetic break between eastern and western regions.Communicated by J.P. Grassle, New Brunswick