Gonad morphology, sexual development, and colony composition in the obligate coral-dwelling damselfish Dascyllus aruanus

Gonad morphology and colony composition support the existing supposition that the obligate coral-dwelling damselfish Dascyllus aruanus has a protogynous hermaphroditic sexual pattern. Adults had either an active ovary containing vitellogenic oocytes, an ovotestis, or a spermiated testis and were classified as adult female, hermaphrodite, or adult male, respectively. Among individuals having male function, the testis (or testis portion of the ovotestis) takes the form of an unrestricted spermatogonial lobular testis. Among hermaphrodites having an ovotestis, a small proportion of individuals had a gonad in which both the ovarian and testicular portions were inactive (inactive hermaphrodites), whereas the majority had a predominantly testicular ovotestis that contained spermatozoa (male-active hermaphrodites). The size range of individuals within gonadal classes indicates that all D. aruanus first develop an ovariform gonad. Some individuals then undergo ovarian maturation to become adult females while others develop testicular tissue to form an ovotestis and become male-active hermaphrodites. Subsequently, progressive loss of ovarian tissue results in the development of a secondary testis from an ovotestis with the retention of a residual, afunctional lumen among adult males. The wide size range of individuals having an ovotestis suggests that some hermaphrodites function as adult females before developing testicular tissue while other individuals do not pass through an adult female stage. If this is the case, D. aruanus exhibits a diandric protogynous hermaphroditic sexual pattern. The apparent prolonged retention of an ovotestis with both healthy oocytes and an ovarian-type lumen in a spermiated ovotestis, as well as a functional sex ratio of 1:1 for adult females:adult males plus male-active hermaphrodites also raises the possibility that D. aruanus may be capable of bidirectional sex change during the hermaphroditic stage. Such a capability would be highly adaptive for a species having limited mobility and unpredictable recruitment of new colony members resulting in unpredictable mating opportunities.     

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.     

Gonadal development and infrequent sex change in a population of the humbug damselfish,Dascyllus aruanus,in continuous coral-cover habitat

The expression of protogyny often differs among populations of a single species. The humbug damselfish, Dascyllus aruanus (Linnaeus), typically lives in spatially discrete groups of several individuals around live coral colonies and has been reported to be protogynous. The size and sexual composition of groups of this species, however, vary with the size of the coral patch or the degree of coral cover. The present study examined gonadal development and sexual pattern in a population of D. aruanus living in continuous coral-cover habitat. Fish were collected in Tumon Bay, Guam (13°31′N; 144°47′E), during a 2-week period in July and August 1996. Reflecting their distributions, small fish (n=54) were collected at depths of 1–2 m over continuous Porites spp. patches, and large fish (n=56) were collected at depths of 2–3 m over a continuous Acropora spp. patch. Gonadal development was examined histologically. Gonads of D. aruanus first developed an ovarian lumen and primary-growth stage oocytes. From this ovarian state or from more developed ovaries with cortical-alveolus stage oocytes, some gonads developed into testes through degeneration of oocytes and development of spermatogenic tissue. Developing spermatogenic tissue only occurred in gonads having pre-cortical alveolus stage oocytes, and the size of individuals with gonads of this type overlapped mainly with the size range of individuals with gonads containing only pre-vitellogenic oocytes (i.e. immature females). This suggested that functional female-to-male sex change was rare or absent in this population of D. aruanus. Moreover, the different habitat distributions of small and large fish suggest that sex determination in immature fish occurs in the absence of adults.     

Gonadal development in the coral reef damselfish<Emphasis Type="Italic"> Dascyllus flavicaudus</Emphasis> from Moorea,French Polynesia

The present study investigated the sexual pattern of the yellow-tailed dascyllus, Dascyllus flavicaudus (Randall and Allen), through histological examination of the process of gonadal differentiation and maturation and through male-removal experiments in the laboratory. The study was conducted in Moorea, French Polynesia (17°32S; 149°50W), from 31 December 1994 to 11 March 1995. For gonadal histology, two populations were sampled, one consisting of large aggregations (n=54) and the other consisting of small- to medium-sized groups around isolated corals (n=55). An additional small sample (n=21) was also collected from a population that consisted of small groups around isolated corals. After an initially undifferentiated state, gonads of D. flavicaudus developed an ovarian lumen, followed by oocytes in the primary growth stage. From this ovarian state or from more developed ovaries with cortical-alveolus stage oocytes, some gonads developed into testes through degeneration of oocytes and development of spermatogenic tissue. In all three populations, spermatogenic tissue developed only in gonads that contained pre-vitellogenic oocytes (termed as mixed stage 1–2 gonads). The two main populations did not differ in their expression of sex change despite differences in their social organization. In both populations, size of individuals with mixed stage 1–2 gonads overlapped mainly with the size range of immature females, which suggests that functional female-to-male sex change was rare. The hypothesis that D. flavicaudus is primarily gonochoristic is further supported by removal experiments in laboratory groups, in which removal of a dominant male(s) failed to induce sex change in any of the high-ranking females. Yet, in all three field populations, some fish with mixed stage 1–2 gonads were found within the size ranges of mature females, which indicates possible occurrences of functional sex change. These conflicting results indicate that it may be premature to draw any definitive conclusions about the sexual pattern of D. flavicaudus.Communicated by T. Ikeda, Hakodate     

Tuna food habits related to the micronekton distribution in French Polynesia

Stomach content analyses are commonly used to study both fish feeding behaviour and trophic conditions. However, the interpretation of such data depends on fish foraging behaviour for a given environment and how representative the stomach contents are to the prey distribution. Tuna feeding behaviour was studied within the context of a research programme conducted in French Polynesia. Tuna prey distribution was characterised using acoustic measurements and pelagic trawls; thereafter, this distribution was compared with the stomach contents of tuna caught using an instrumented longline. Acoustic, pelagic trawling and stomach content analyses give complementary elements to describe the pelagic trophic habitat and to better understand tuna-prey relationships. The classic concept of a reduced food availability for tunas in the tropical pelagic environment seems relative. Tunas able to dive enough during daytime to exploit the migrant micronektonic species secure a source of regular food. This is particularly true of bigeye tuna (Thunnus obesus), which have ecophysiological capacities for this purpose. The behaviour of albacore tuna (T. alalunga), which dive >400 m in depth, remains less clear, as little is known about their vertical behaviour. Lastly, yellowfin tuna (T. albacares), which are distributed in more superficial waters, can better exploit the biomass of juvenile fish and crustaceans exported from the reefs. Analysis of the stomach fullness of tuna caught by longline, a passive gear, generally showed an empty state. This result suggests that most tuna foraging on large prey aggregations present in the study area are quickly satiated and escape longline capture and sampling. A consequence is that studies of tuna feeding behaviour based on longlining may be biased, particularly when large aggregations of prey are present such as in convergence zones. Another potential consequence is that longline tuna catch rates could differ according to prey richness. Longline tuna catch rates may sometimes reflect the relative abundance of prey rather than relative tuna abundance. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0776-3.     

Antimicrobial activities of marine sponges from French Polynesia and Brittany

Extracts from marine sponges collected in French Polynesia and along the Brittany coast (France) have been tested for antibacterial and antifungal activity. The bioactity is concentrated in Dendrilla sp., Homaxinella trachys, Hyrtios eubamma, Verongia aerophoba and Phakellia ventilabrum. The compound responsible for the activity of H. eubamma has been identified as puupehenone, while that of P. ventilabrum is due to a new sesquiteprenoid isonitrile.This study forms part of the Thèse 3e cycle of Ph. Amade, Université de Bretagne Occidentale     

Natural and anthropogenic influences on the diversity structure of reef fish communities in the Tuamotu Archipelago (French Polynesia)

Despite the rapid rate of human-induced species losses, the relative influence of natural and anthropogenic factors on the functional diversity of species assemblages remains unknown for most ecosystems. A model was previously developed to predict the diversity structure of coral reef fish assemblages in 10 atolls of low human pressure and contrasting morphology of the Tuamotu Archipelago (French Polynesia). This existing model predicted smoothed histograms (spectra) of species richness according to size classes, diet classes and life-history classes of fish assemblages using a combination of environmental characteristics at different spatial scales. The present study applied the model to Tikehau, another atoll of the same archipelago where commercial fishing is practiced and where the same sampling strategy was reproduced. Significant differences appeared between predicted and observed species richness in several size, diet and life-history classes of fish assemblages in Tikehau. Two parameters which were not accounted for in the initial model, i.e. fishing pressure and atoll position within the archipelago, explained together 63% of variance in model residuals, >60% being explained by fishing pressure only. The respective effects of fishing and atoll position on the diversity of coral reef fish assemblages are discussed, with the potential of such modelling approach to assess the relative importance of factors affecting functional diversity within communities.     

Unprecedented bleaching-induced mortality in Porites spp. at Rangiroa Atoll, French Polynesia

In April-May 1998, mass coral bleaching was observed in the lagoon of Rangiroa Atoll, Tuamotu Archipelago, French Polynesia. Six months later, the extent of bleaching-induced coral mortality was assessed at three sites. Corals in the fast-growing genus Pocillopora had experienced >99% mortality. Many large colonies of the slow-growing genus Porites (mean horizontal cross-sectional area 5.8 m²) had also died - a phenomenon not previously observed in French Polynesia and virtually unprecedented world-wide. At one site, 25% of colonies, or 44% of the pre-bleaching cover of living Porites, experienced whole-colony mortality. At the two other sites, recently dead Porites accounted for 41% and 82% of the pre-bleaching live cover. Mortality in Porites was negatively correlated with depth between 1.5 and 5 m. Using a 50-year dataset of mean monthly sea surface temperature (SST), derived from ship- and satellite-borne instruments, we show that bleaching occurred during a period of exceptionally high summer SST. 1998 was the first year in which mean monthly SSTs exceeded the 1961-1990 upper 95% confidence limit (29.4°C) for a period of three consecutive months. We suggest that the sustained 3-month anomaly in local summer SST was a major cause of coral mortality, but do not discount the synergistic effect of solar radiation. Recovery of the size-frequency distribution of Porites colonies to pre-bleaching levels may take at least 100 years.     

Genetic structure in native and non-native populations of the direct-developing gastropod Crepidula convexa

In many marine invertebrate species, larval development plays an important role in population connectivity and gene flow: species with direct benthic development generally show more genetic structure than those with planktonic development. We used nuclear markers (microsatellites) to determine population genetic structure of the direct-developing snail Crepidula convexa (Gastropoda: Calyptraeidae) in seven populations with 15–85 individuals each within its native range of the northwest Atlantic and compared it to Crepidula fornicata, a congener with planktonic development. Our results are consistent with general expectations and previous work in these species with other markers: C. convexa had greater population structure and even at a regional scale shows significant isolation-by-distance, in contrast to C. fornicata. We also genotyped a single population of C. convexa introduced to the northeastern Pacific to investigate the prediction of reduced genetic diversity following introduction (founder effect). We did not find a reduction in genetic diversity, suggesting that this non-native population may be characterized by multiple introductions. This pattern is consistent with many other introduced populations of marine invertebrates, including C. fornicata.     

Genetic structure of dictyoceratid sponge populations on the western Coral Sea reefs

Allozyme variation at six polymorphic loci was examined in foliose dictyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamellosa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs formed genetic outliers to the P. lamellosa group, and may represent further taxa related to P. lamellosa. Gene frequencies in individual reef populations were largely in Hardy-Weinberg equilibrium, suggesting that random mating occurred in local populations of all four common species. Genetic variability was high and observed heterozygosities within populations ranged from 0.13 to 0.40. All four taxa showed significant genetic differentiation among populations (F _ST=0.05 to 0.36). Genetic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial Current (SEC) divergence was greater than expected on the basis of their geographical separation. The SEC divergence may form a partial barrier to gene flow among populations of these ecologically important sponges on the submerged Queensland Plateau. Levels of migration among populations of three of the species was less than those required to prevent divergence of the populations through genetic drift (Nm<1). Restricted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges whose specific identity is not clear, and may allow them additionally to develop partial reproduction isolation from other populations.     

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.     

Genetic structure of hybrid mussel populations in the west of Ireland: two hypotheses revisited

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, the blue mussel, Mytilus edulis, and the Mediterranean mussel, M. galloprovincialis. Results from an Irish study in the 1980s, using partially diagnostic allozyme markers, indicated that mussels higher up the shore were more galloprovincialis-like than those lower down. In this study we set out to test two hypotheses: (a) recruits arriving on the shore are composed of genetically distinct cohorts that settle preferentially at different levels on the shore, and maintain genetic distinctiveness into adulthood; (b) recruits are genetically homogeneous, but once settled they diverge genetically over time, due to within-habitat site specific-selection. The diagnostic Me 15/16 DNA marker was used to analyse the genetic composition of newly-settled spat recruiting to artificial substrates, which were placed at two-week intervals from May–October 2002, on the mid- and low shore areas of two exposed sites in Galway Bay. Adult mussels were also collected on each sampling date. Results did not support the preferential settlement hypothesis, i.e., the genetic composition of primary settlers (≤ 500 μm) was similar between tidal heights and shores. Neither was there evidence of post settlement selective mortality, as adults were genetically similar to settling spat. In spat and adults the frequency of the M. galloprovincialis allele was high (0.56–0.80), due to high frequencies of M. galloprovincialis (> 37%) and hybrid (> 33%) genotypes, and correspondingly low frequencies of the M. edulis genotype (< 11%). Adult mussels from a nearby sheltered estuarine site, while significantly different to exposed shore mussels, still had low frequencies of the M. edulis genotype (< 17%), indicating no apparent advantage for the genotype in this environment. There are indications that the genetic composition of mussels may be changing on the Atlantic coasts of Ireland.     

Genetic structure of giant clam (Tridacna derasa) populations from reefs in the Indo-Pacific

Large genetic differences were observed among the Great Barrier Reef (GBR), Fiji and Philippine populations of Tridacna derasa (Roding) sampled in 1989 and 1990 (Nei's unbiased genetic distance, D,=0.137 to 0.341). This result contrasted strongly with the low genetic distance (D=0.032) reported previously for the giant clam T. maxima over similar geographical scales. No significant genetic differentiation was observed among most populations from the GBR (mean D=0.007), consistent with the high gene exchange expected in this highly connected reef system. However, significant differentiation resulting from differences in the frequencies of less common alleles between the North-Central GBR and South GBR (Swain region) were observed. Historical isolation of blocks of the Central Indo-West Pacific from the GBR and present-day restrictions to gene exchange between the GBR, Fiji and the Philippines as a result of oceanographic current patterns, were thought to be responsible for the high degree of genetic differentiation of T. derasa populations. The relevance of these findings to clam mariculture and reef restocking are briefly discussed.Contribution No. 197 from the Marine Science Institute, University of the Philippines and Contribution No. 562 from the Australian Institute of Marine Science     

Genetic structure of deep-water redfish, Sebastes mentella, populations across the North Atlantic

Patterns of genetic diversity of the marine deep-water redfish, Sebastes mentella Travin, from 21 sampling locations (n=46 per sample, from 1995 to 1998) were examined throughout its range in the North Atlantic. Eight microsatellite loci were used in order to: (1) define the genetic structure and consider the possible influence of dispersal, geographic distance, oceanic currents and historical factors; and (2) relate the structure to present management units and practices. Three divergent population units (mean Š=0.012) associated with distinct geographic areas were defined: Eastern (Norway and Barents Sea), Panoceanic and Western (Gulf of St. Lawrence and offshore Newfoundland). The most important observation was the lack of genetic differences (mean Š=-0.0004) and lack of genetic isolation by geographic distance (r=0.318, P=0.112) among samples within the Panoceanic zone, from the Faroe Islands to the Grand Banks (6,000 km). A combination of vicariance, historical introgression with a closely related species, S. fasciatus, and contemporary patterns of oceanic circulation are likely to have shaped the observed population structure. These results only partially support current management units, and call for more integrated practices for regulating the exploitation of S. mentella throughout its extensive range.     

Spatial repartition and ontogenetic shifts in habitat use by coral reef fishes (Moorea,French Polynesia)

This study explores the extent to which ontogenetic habitat shifts modify spatial patterns of fish established at settlement in the Moorea Island lagoon (French Polynesia). The lagoon of Moorea Island was divided into 12 habitat zones (i.e. coral seascapes), which were distinct in terms of depth, wave exposure, and substratum composition. Eighty-two species of recently settled juveniles were recorded from March to June 2001. Visual censuses documented changes in the distribution of juveniles of each species over time among the 12 habitats. Two patterns of juvenile habitat use were found among species. Firstly, some species settled and remained in the same habitat until the adoption of the adult habitats (i.e. recruitment; e.g. Chaetodon citrinellus, Halichoeres hortulanus, Rhinecanthus aculeatus). Secondly, others settled to several habitats and then disappeared from some habitats through differential mortality and/or post-settlement movement (e.g. 65–70 mm size class for Ctenochaetus striatus, 40–45 mm size class for Epinephelus merra, 50–55 mm size class for Scarus sordidus). A comparison of the spatial distribution of juveniles to that of adults (61 species recorded at both stages) illustrated four patterns of subsequent recruitment in habitat use: (1) an increase in the number of habitats used during the adult stage (e.g. H. hortulanus, Mulloidichthys flavolineatus); (2) a decrease in the number of habitats adults used compared to recently settled juveniles (e.g. Chrysiptera leucopoma, Stethojulis bandanensis); (3) the use of different habitat types (e.g. Acanthurus triostegus, Caranx melampygus); and (4) no change in habitat use (e.g. Naso litturatus, Stegastes nigricans). Of the 20 most abundant species recorded in Moorea lagoon, 12 species modified the spatial patterns established at settlement by an ontogenetic habitat shift.Communicated by T. Ikeda, Hakodate     

Genetic structure of summer flounder (Paralichthys dentatus) populations north and south of Cape Hatteras

Genetic divergence among samples of summer flounder (Paralichthys dentatus, Linnaeus) was estimated to evaluate the effect of Cape Hatteras, a well-known zoogeographic barrier in the western Atlantic Ocean, on intraspecific gene flow. Previous studies based on meristic and morphometric variation and tagging data suggested that gene flow might be limited among summer flounder populations north and south of Cape Hatteras, although the high vagility of both juveniles and adults suggests otherwise. We analyzed genetic diversity revealed in the mitochondrial DNA (mtDNA) control region in samples of juveniles and adults collected in 1992 to 1996 from coastal sites from Buzzard's Bay, Massachusetts to Charleston, South Carolina. In contrast to previous morphological studies, analyses of mtDNA variation reveal no significant population subdivision centered around Cape Hatteras. Received: 17 September 1997 / Accepted: 8 September 1998     

Genetic structure of fissiparous populations of Holothuria (Halodeima) atra on the Great Barrier Reef

Allozyme variation at five polymorphic loci was surveyed in a total of 311 individuals of the sea cucumber Holothuria (Halodeima) atra (Jäger, 1833) collected from two nearshore and two midshelf populations in the Great Barrier Reef in November 1996. Strong deviations in genotype frequencies from those expected under Hardy–Weinberg equilibrium, particularly a large number of heterozygote excesses, confirmed the occurrence of asexual reproduction. Females and males differed significantly in genotype frequencies as a result of differences in either the amount of fission in each sex or in the mortality of asexually produced recruits. The estimated maximum sexual input (number of sexually produced individuals: sample size = N*:N_i) to the two nearshore reefs (38 to 67%) was low relative to that to the two midshelf reefs (74 to 87%). The three ratios and G _o :G _e , N _go :N _i , N*:N _i , (where G _o = observed genotypic diversity, G _e = expected genotypic diversity, N _go = number of genotypes) considered to be indicators of the extent of asexual reproduction, showed a consistent trend in the degree of asexual reproduction similar to that derived from the number of regenerating individuals observed in the populations for which data were available. F-statistic analyses of clonal gene frequencies demonstrated that all populations received sexual recruits from the same gene pool. There was evidence of restricted sexual recruitment to the Fantome population, suggesting that asexual reproduction was dominant only in areas where sexual recruitment was limited by other factors.     

Nutrient budget of the lagoonal waters in an open central South Pacific atoll (Tikehau,Tuamotu, French Polynesia)

Nutrient concentrations were measured in the lagoon and surrounding oceanic waters of Tikehau Atoll (Tuamotu Archipelago, French Polynesia) from 1984 to 1987. The atoll-mass effect alters the nutrient profiles: turbulent vertical mixing of the waters along the deeper slopes of the atoll induces nitrogen and phosphorus enrichment of the surface layer. Nutrient concentrations varied with year and month of sampling; except for ammonium, inorganic nutrient levels were lower inside the lagoon than in the surrounding oceanic waters. Nitrogen, phosphorus and silica budgets were calculated by mean differences in nutrient concentrations recorded between lagoon and oceanic surface waters and by the waterexchange rate through the passage linking the lagoon and oceanic waters and the reef-flat spillways. Particulate and dissolved organic nitrogen and ammonium are exported from the lagoon to the open ocean through the westward passage. The nitrogen budget is not balanced by the nitrate input from oceanic waters and the organic nitrogen and ammonium output from lagoonal waters. Nitrogen fixation would appear to constitute another source of nitrogen for lagoonal waters. The phosphorus budget is largely balanced by phosphate input from the oceanic waters and organic phosphorus output from the lagoon waters. The oceanic waters became impoverished in silicate during their crossing of the atoll reef edge and their residence in the lagoon. The atoll constitutes a source of nitrogen for the surrounding oceanic waters.