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International audience; Marinas are high-priority targets for marine non-indigenous species (NIS), where they compose a large portion of the biofouling communities. The practicality of water samples collection makes environmental DNA (eDNA) metabarcoding an interesting tool for routine NIS surveys. Here the effectiveness of water-eDNA-metabarcoding to identify biofouling NIS, in ten marinas from western France, was examined. Morphological identification of specimens collected in quadrats brought out 18 sessile benthic NIS beneath floating pontoons. Water-eDNA-metabarcoding detected two thirds of them, failing to detect important NIS. However, sampling and bioinformatics filtering steps can be optimized to identify more species. In addition, this method allowed the detection of additional NIS from neighboring micro-habitats. Caution should however be taken when reporting putative novel NIS, because of errors in species assignment. This work highlights that water-eDNAmetabarcoding is effective for active (targeted) NIS surveys, and could be significantly improved for its further use in marine NIS passive surveys.

The coastal oceans can be highly variable, especially near ocean fronts. The Ushant Front is the dominant oceanographic feature in the Iroise Sea (NE Atlantic) during summer, separating warm stratified offshore waters from cool vertically-mixed nearshore waters. Mesozooplankton community structure was investigated over an annual cycle to examine relationships with oceanographic conditions. DNA metabarcoding of COI and 18S genes was used in communities from six sites along two cross-shelf transects. Taxonomic assignments of 380 and 296 OTUs (COI and 18S, respectively) identified 21 classes across 13 phyla. Meroplankton relative abundances peaked in spring and summer, particularly for polychaete and decapod larvae, respectively, corresponding to the reproductive periods of these taxa. Meroplankton was most affected by season, while holoplankton varied most by shelf position. Copepods with a mixed feeding strategy were associated with the most offshore sites, especially in the presence of the front, while filter-feeding or carnivorous copepods were associated with nearshore sites. In sum, mesozooplankton communities in well-mixed coastal waters were distinct from those found in the Ushant Front (high thermal stratification and chlorophyll-a). Furthermore, the benthic compartment, through its partial life cycle in the water column, contributed to high heterogeneity in planktonic communities over short temporal and spatial scales.

The ability of marine invertebrate larvae to control their vertical position shapes their dispersal pattern. In species characterized by large variations in population density, like many echinoderm species, larval dispersal may contribute to outbreak and die-off phenomena. A proliferation of the ophiuroid Ophiocomina nigra was observed for several years in western Brittany (France), inducing drastic changes on the benthic communities. We here studied the larval vertical distribution in this species and two co-occurring ophiuroid species, Ophiothrix fragilis and Amphiura filiformis, in two contrasting hydrodynamic environments: stratified in the bay of Douarnenez and well-mixed in the bay of Brest. Larvae were collected at 3 depths during 25 h within each bay. In the bay of Brest, all larvae were evenly distributed in the water column due to the intense vertical mixing. Conversely, in the bay of Douarnenez, a diel vertical migration was observed for O. nigra, with a night ascent of young larvae, and ontogenetic differences. These different patterns in the two bays mediate the effects of tidal currents on larval fluxes. O. fragilis larvae were mainly distributed above the thermocline which may favour larval retention within the bay, while A. filiformis larvae, mostly concentrated near the bottom, were preferentially exported. This study highlighted the complex interactions between coastal hydrodynamics and specific larval traits, e.g. larval morphology, in the control of larval vertical distribution and larval dispersal.

International audience; The evolution of strong reproductive isolation (RI) is fundamental to the origins and maintenance of biological diversity, especially in situations where geographical distributions of taxa broadly overlap. But what is the history behind strong barriers currently acting in sympatry? Using whole-genome sequencing and single nucleotide polymorphism genotyping, we inferred (i) the evolutionary relationships, (ii) the strength of RI, and (iii) the demographic history of divergence between two broadly sympatric taxa of intertidal snail. Despite being cryptic, based on external morphology, Littorina arcana and Littorina saxatilis differ in their mode of female reproduction (egg-laying versus brooding), which may generate a strong post-zygotic barrier. We show that egg-laying and brooding snails are closely related, but genetically distinct. Genotyping of 3092 snails from three locations failed to recover any recent hybrid or backcrossed individuals, confirming that RI is strong. There was, however, evidence for a very low level of asymmetrical introgression, suggesting that isolation remains incomplete. The presence of strong, asymmetrical RI was further supported by demographic analysis of these populations. Although the taxa are currently broadly sympatric, demographic modelling suggests that they initially diverged during a short period of geographical separation involving very low gene flow. Our study suggests that some geographical separation may kick-start the evolution of strong RI, facilitating subsequent coexistence of taxa in sympatry. The strength of RI needed to achieve sympatry and the subsequent effect of sympatry on RI remain open questions. This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

International audience; This study described the occurrence of abnormalities in bivalve larvae from the Puck Bay. Analyses of plankton samples collected in 2012–2013 showed that larval Mytilus trossulus, Mya arenaria, and Cerastoderma glaucum exhibited abnormalities that could indicate adverse environmental impacts. The deformities were mainly in shells, but missing soft tissue fragments and protruding vela were also noted. In addition to larval studies, we analyzed benthic postlarvae of Mytilus trossulus. Interestingly, grooves and notches at different locations of the prodissoconch, dissoconch, and shell margin were observed. Some of these deformations were reminiscent of the indentations found on the shell edge of larvae. Comparing the proportion of abnormal postlarvae to larvae with shell abnormalities suggested that the survival of larvae with shell abnormalities was low. Overall, our results suggested that the ratio of abnormal bivalve larvae could be used as an indicator of the biological effects of hazardous substances in the pelagic environment.

International audience; Many marine gastropod species brood their embryos in thin-walled capsules to protect them during development. Despite its beneficial effects, encapsulation has two major constraints, nutrition and oxygen supply, which affect embryo development and larval survival. Developing embryos usually rely on intracapsular food sources provided by the mother, in the form of yolk, nurse eggs and intracapsular fluid. However, it is still not clear if they are able to feed on extracapsular sources that may cross the capsule wall. We investigated this possibility in the calyptraeid species Crepidula fornicata. In this species, the internal capsule wall thickness sharply decreases during embryonic development, which might change wall permeability to small organic molecules, thus providing embryos with external dissolved organic matter. To test this hypothesis, encapsulated and excapsulated embryos of C. fornicata were placed for 48 h in a C-13-enriched amino acid (L-alanine) solution. Excapsulated embryos were enriched in C-13 (+5.75%), which suggested that they were able to assimilate the labelled amino acid. In contrast, encapsulated embryos were weakly enriched (+0.75%), suggesting that encapsulation greatly reduces the potential for the use of extracapsular amino acids and that encapsulated embryos mainly rely on maternal food.

Marine invertebrate larvae represent a transitory, but nonetheless important component of planktonic communities. Assessing their contribution to plankton diversity has been hindered by numerous methodological difficulties, notably at the identification step. For many sessile invertebrates, planktonic larvae also play a crucial role, as they are their sole dispersal vectors. Understanding connectivity patterns among marine populations is fundamental for managing coastal ecosystems and their associated resources. Indirect approaches, relying on population genetics models, have widely contributed to elucidate population structure and gene flow patterns, but show, in some cases, conflicting results with larval dispersal potential. In an attempt to facilitate surveys of larval distributions and abundances, an in situ hybridization on whole larvae method was tested over a range of marine invertebrate larvae collected from environmental plankton samples. Ribosomal RNA-targeted oligonucleotide probes were used for hybridization, followed by a colorimetric reaction allowing signal detection at the light microscope. Promising results were obtained, showing an unambiguous positive hybridization signal with a eukaryotic (positive) probe, but no signal with a negative probe. Using species-specific probes, the method could be applied to resolve key current questions in marine ecology, addressing both wide and fine scales.

Accurate and efficient identification to the species level of early larval stages has long been a problematic step in the study of marine invertebrates, due to the extremely small size of the larvae and their lack of diagnostic morphological characters. It is nonetheless, a prerequisite for any ecological study. As a consequence, a number of molecular approaches, mostly based on the PCR technique, have been developed over the last decade. We developed a method relying on specific rRNA-targeted oligonucleotide probes for in situ molecular hybridization using whole larvae. A col- orimetric reaction following the hybridization allows signal detection with a light microscope. A total of 9 probes have been developed, targeting the species that are major components of benthic com- munities in several European bays. The method can be applied to roughly sorted wild plankton sam- ples, and some steps could be automated. It is relatively inexpensive to implement and does not require costly equipment or expensive staff training. The overall larval morphology is preserved, allowing visual inspection. The quantitative aspect of the approach is another asset. Field-based studies of larval distribution and behaviour are possible applications of the method, as large numbers of samples can be screened to meet the requirements of adequate spatial and temporal coverage. It is also relevant for routine monitoring of target species, such as species of commercial interest, bio-indicators or invasive species.

International audience; Biological invasions are a major threat to the world's biodiversity with consequences on ecosystem structure and functioning, species evolution, and human well-being (through ecosystem services). Conservation of biological diversity and management of biological resources require multi-level management strategies on non-native species, in order to (1) prevent biological introductions, (2) detect non-native species at an early stage of the introduction, and (3) eradicate or maintain at a low level of population density non-native species that were successfully introduced. A pre-requisite to any control measures on non-native species is the ability to rapidly and accurately identify the putative threatening alien species. DNA barcoding, and its recent extension, DNA metabarcoding are complementary tools that have proved their value in the identification of living beings. Here we review their use in the identification of non-native species at several steps of the introduction processes, and how they can be applied in the control and management of biological introductions. Through examples covering various taxa and ecosystems (terrestrial, freshwater, marine), we highlight the strengths and weaknesses of approaches that we foresee as crucial in the implementation of early warning strategies.

Philippart, Catharina J.M. et al., Reproductive cycles of marine invertebrates with complex life histories are considered to be synchronized by water temperature and feeding conditions, which vary with season and latitude. This study analyses seasonal variation in the occurrence of oyster (Crassostrea gigas) and mussel (Mytilus edulis/. galloprovincialis) larvae across European coastal waters at a synoptic scale (1000s of km) using standardised methods for sampling and molecular analyses.We tested a series of hypotheses to explain the observed seasonal patterns of occurrence of bivalve larvae at 12 European stations (located between 37°N and 60°N and 27°W and 18°E). These hypotheses included a model that stated that there was no synchronisation in seasonality of larval presence at all between the locations (null hypothesis), a model that assumed that there was one common seasonality pattern for all stations within Europe, and various models that supposed that the variation in seasonality could be grouped according to specific spatial scales (i.e., latitude, large marine ecosystems and ecoregions), taxonomic groups, or several combinations of these factors.For oysters, the best models explaining the presence/absence of larvae in European coastal waters were (1) the model that assumed one common seasonal pattern, and (2) the one that, in addition to this common pattern, assumed an enhanced probability of occurrence from south to north. The third best model for oysters, with less empirical support than the first two, stated that oysters reproduced later in the south than in the north. For mussels, the best models explaining the seasonality in occurrence of larvae were (1) the model that assumed four underlying trends related to large marine ecosystems, and (2) the one that assumed one common seasonal pattern for larvae occurrence throughout Europe.Such synchronies in larval occurrences suggest that environmental conditions relevant to bivalve larval survival are more or less similar at large spatial scales from 100s to 1000s of km. To unravel the underlying mechanisms for this synchronisation is of particular interest in the light of changing environmental conditions as the result of global climate change and the possible consequences for marine food webs and ecosystem services. © 2012 Elsevier Ltd.

Near the Azores Triple Junction as the Azores Plateau is approached, the ridge axis becomes shallower; its depth decreases from ca. 2400 m in the Rainbow vent field (36°13′N) to ca. 850 m in the Menez Gwen vent field (37°35′N). In this area, extensive mussel beds of the mytilid Bathymodiolus azoricus dominate the hydrothermal vent fauna, along with populations of three shrimps (Rimicaris exoculata, Mirocaris fortunata and Chorocaris chacei). The main physical and chemical characteristics of the vent habitat were studied by discrete sampling, in situ analysis and sediment trap moorings. The vent fauna is distributed along a variable band where the vent fluids and seawater mix, with R. exoculata living in the most concentrated areas and Bathymodiolus azoricus in the most diluted zones. Various non-endemic species live at the border of the vent field. The variations observed in structure and composition of the communities along the depth gradient are most likely due to changes in vent fluid toxicity (metallic and sulphide content) and suspended mineral particles, which render the fluids harsher for species living there. The main faunal differences observed between Lucky Strike and Menez Gwen hydrothermal fields are due to an impoverishment in the hydrothermal endemic species and to the penetration of bathyal species. The comparison of the three studied vent fields suggests the existence of a succession of several biogeographic islands rather than a single province.

To provide information about the export and the distribution of hydrothermal material to the deep ocean, two sediment traps with current-meters were moored on the Lucky Strike segment of the Azores Triple Junction (Mid-Atlantic Ridge). The results of a 25-day experiment with a single trap deployed 1.5 m from a chimney (2 m above the bottom) are used as a reference for the composition of particles produced by the vent. A 392-day experiment with a time-series sediment trap positioned 500 m from the Lucky Strike vents (17 m a.b.) showed seasonal variations in the particulate flux and in its composition. Particles sampled by the trap close to the chimney (264.3 mg m −2 day −1 ) were characterised by a high concentration of sulphur (10.8%) and barium (3.19%), and a low C/N ratio (5.5). From the current data recorded at the same time, we estimate that this small trap was under vent influence for about 10% of the experiment duration. The calculated mean particle flux obtained with the multisample sediment trap 500 m away (7.7 mg m −2 day −1 ) was among the lowest recorded in the Atlantic Ocean. This situation may be explained by a very low primary production in the Azores region. No significant influence of the Lucky Strike vent particles was recorded in this trap although near bottom currents were regularly favourable to transport particles towards it. No major effect was observed on the trap efficiency by variations of the hydrodynamic conditions during this experiment. Particles in the trap included bivalve larval shells. A comparison between their hinge structure and that of postlarvae of the mytilid species found on these vents suggests a non-hydrothermal origin for the larvae in the trap.

The polychaete family Polynoidae (scale-worms) is well-represented at deep sea hydrothermal vents. Most species are free-living in a wide range of habitats: from high-temperature hydrothermal `chimney' walls to diffuse venting areas. Conversely, species of the genus Branchipolynoe live inside the mantle cavity of vent and seep mytilids. Specimens, morphologically close to Branchipolynoe seepensis, have been reported from all the known vent areas on the Mid-Atlantic Ridge (MAR), with varying infestation rates (0–6 individuals per host). Reproductive tract, gametogenesis and population structures were examined for specimens from the Lucky Strike vent field (MAR) in order to test whether this species displays dwarf males, protandric hermaphroditism or differential mortality between males and females. Observations of histological sections reveal the presence of fully developed ovaries in females which originate ventrally in segments 7–9 and of an unusual genital tract in which both sperm and mature oocytes are stored. Oogenesis is intraovarian and quasi-continuous. The vitellogenic oocytes are only free in the coelom at their terminal growing stage and are then transferred into an ovisac through spermathecae. The species displays an external sexual dimorphism in the number of genital papillae and the shape of the pygidial appendages. Sex ratios showed significant deviations from a 1:1 expected ratio, in favour of females. The modal decompositions of size–frequency histograms show the occurrence of three modes in females and only two modes in males, indicating discrete breeding periods. The two first modes were not significantly different between males and females. These results indicate that B. seepensis forms heterosexual pairs and uses internal fertilization to reproduce during discrete spawning periods. Differential mortality between males and females is likely to shape size-histograms as observed by preventing males from reaching the female proportions. Such an observation could be a result of either cannibalism on larger males, small sizes facilitating the male escape, or natural predation when males move from one bivalve to another to breed.

Population structure and recruitment pattern of undescribed mytilid bivalves that dominate the animal communities associated with vents of the Lucky Strike and Menez Gwen hydrothermal vent fields (37 degrees 17' N and 37 degrees 50' N on the Mid-Atlantic Ridge) were studied from size-frequency analyses. Intra-site comparisons reveal that length-frequency distributions differ significantly from one location to the other. This spatial segregation of the sizes could be due to spatio-temporal variability in environmental conditions and to intraspecific competition processes, especially adult-larval interactions (competition for space and food resources, intraspecific larviphagy) that could prevent larvae from settling on surfaces covered by adults. A modal decomposition analysis was conducted after pooling samples collected from different locations at the same site. Polymodal structure of length-frequency distributions indicates that recruitment is likely discontinuous in time. The presence of postlarvae in each population of the Lucky Strike vent field suggests that a recruitment event took place just before sampling in June 1994, and that mytilid recruitment is synchronized between sites, which is confirmed by the comparison of modal values. Length-frequency distribution of the Menez Gwen mytilid population displays similar modal values, especially for the smallest mode. This suggests a synchronization of the recruitment between the 2 vent fields, which are separated by a distance of 60 km, No seasonal pattern in recruitment could be identified although newly recruited individuals were observed in June 1993 on Lucky Strike sites.

Newly discovered hydrothermal vent communities at Lucky Strike on the Mid-Atlantic Ridge (37°18'N, 32°16'W) are comprised of an invertebrate fauna sufficiently different from known vent faunas of TAG and Snake Pit to consider Lucky Strike part of a new biogeographic province. The dominant component of the fauna is a new species of mussel, and the most unusual feature of the fauna is an echinoid echinoderm, Echinus sp. An abundance of small mussels (< 5 mm) indicates a recent recruitment event at Lucky Strike, and modal analysis of length-freque ncy data indicate a discontinuous recruitment process in space and time. Copyright © 1996 Elsevier Science Ltd

International audience; In Europe, the gastropod Crepidula fornicata is an invasive species characterized by a long reproductive period (from February to November). Thus, its larvae are exposed to variations in available food sources (in terms of quantity and quality). We aimed to investigate if bacteria could contribute to larval food both in presence or absence of phytoplankton, and to compare these results to seasonal variations of bacteria and phytoplankton abundances at a coastal site in the English Channel. First, ingestion of fluorescent beads of 0.5 to 2 um diameter, showed that larvae were able to ingest particles of typical bacterial size. Then we used a dual stable isotope labeling approach which consisted in labeling a bacterial pelagic community with N-15 and a diatom (Chaetoceros gracilis) culture with C-13, and supplying larvae with N-15-labeled bacteria, C-13-labeled diatoms, and both labeled sources. This technique has, to our knowledge, never been applied to invertebrate larvae. After 24 h of experiment, larvae were significantly enriched in all treatments: +21.5 parts per thousand (Delta delta C-13) when supplied with diatoms, +1364 parts per thousand (Delta delta N-15) when supplied with bacteria, and +24 parts per thousand (Delta delta C-13) and +135 parts per thousand (Delta delta N-15) when supplied with the two mixed sources. These results indicated that bacteria can contribute to the larval nutrition in C. fornicata, even in the presence of phytoplankton. Our results however suggested that larvae of C. fornicata preferentially used diatoms and showed that the supply of free bacteria did not alter the uptake of diatoms. Considering the seasonal variations of bacteria and phytoplankton abundances at the study site, these results suggested that bacteria may constitute a complementary resource for the larvae of C. fornicata when phytoplankton is abundant and may become a substitute resource when phytoplankton is less available. This approach offers promising perspectives to trace food sources and assess nitrogen and carbon fluxes between planktotrophic larvae and their preys. (C) 2012 Elsevier B.V. All rights reserved.

International audience; The relationship between meroplankton distribution and spatio-temporal variabil- ity of coastal mesoscale hydrological structure was investigated in the northern Bay of Biscay, North-East Atlantic. For the three coastal polychaetes studied, i.e. Pectinaria koreni, Owenia fusiformis and Sabellaria alveolata, the highest larval abun- dances were sampled in low-salinity, low-density and high-temperature river plume waters. For two species (P. koreni and O. fusiformis), maximal abundances were observed in the surface and thermocline layers due to ontogenic migrations. Variance partitioning based on multiple regression and redundancy analyses was used to assess the relative roles played by the hydrological environment alone, the geographical space alone and their interactions, i.e. the spatial structure of the hydrological environment. These analyses demonstrate the key role played by the hydrological spatial structure in the distribution of larval abundances. The hydrological environment alone was insignificant, whereas geographical space alone explained a significant part of the variability in meroplankton distribution, probably in conjunction with ecological processes. For species whose benthic populations are spatially structured, the distribution and the size of adult popu- lations and the timing of spawning events can significantly affect larval distribution and dispersal.

International audience; Human-mediated transport and aquaculture have promoted the establishment of non-indigenous species in many estuaries around the world over the last century. This phenomenon has been demonstrated as a major cause of biodiversity alterations, which has prompted scientists to provide explanations for the success or failure of biological invasions. Crepidula fornicata is a gastropod native from the East coast of North America which has successfully invaded many European bays and estuaries since the 19th century, with some noticeable exceptions. Its spread over Europe has been explained by a combination of human-mediated transport and natural dispersal through its long-lived planktonic larva. We here investigated whether larval supply may explain the failure in the proliferation of this species within a particular bay, the Bay of Morlaix (France). Patterns of larval distribution and larval size structure were analysed over ten sites sampled three times (20 July, 4 August and 21 August 2006), regarding characteristics of the adult population and environmental features. Our results evidenced a strong spatial structure in both larval abundance and size at the bay scale, even if larval abundances were low. In this scheme, the location of spawning adults played a critical role, with high numbers of early larvae above the main spawning location. The larval size structure further showed that settlement-stage larvae were rare, which suggested that released larvae might have been exported out of the bay. The use of an analytical model aimed to study the effect of tidal currents on the potential for larval exportation confirmed that larval retention within the bay might be low. The limitation in larval supply resulting from the interactions between spawning location and local hydrodynamics may thus impede the proliferation of this species which is well established for more than 50 years. This study provided an example of factors which may explain the failure of the transition between two major steps of biological invasions, i.e. sustainable establishment and proliferation.

International audience; In many marine benthic invertebrates, the larval stage is the main vector of dispersal. Selective metamorphosis of larvae into an appropriate habitat is then essential to the survival of the benthic adult, with profound implications for population dynamics. Within this context, the identification of causal external factors triggering larval metamorphosis is of primary interest. In the widely invasive marine species Crepidula fornicata (L.), adult conditioned seawater is an effective inducer but the inducer has not yet been chemically characterized. In the present study, we experimentally tested the inductive ability of dibromomethane (DBM), a halogenated compound naturally-produced by red algae of the family Corallinaceae that co-occur with C. fornicata in the field. Through a series of three experiments, we demonstrated that DBM effectively induces metamorphosis of C. fomicata larvae, with a strong dose response effect. Positive responses were observed for the four tested concentrations (5, 50, 500 and 5000 ppm), 5000 ppm being the most effective at inducing rapid metamorphosis. At this concentration, larvae became responsive at least five days before they became responsive to excess KCl, a chemical commonly used to test for competence in C fornicata. Whether DBM could be an effective natural inducer would require further studies to quantify DBM and other halogenated compounds in the vicinity of C. fornicata beds, where they could influence larval behaviour even in the absence of other settlement cues. (C) 2010 Elsevier B.V. All rights reserved.

International audience; In diverse invertebrate species characterized by a biphasic life cycle, metamorphosis represents a fundamental biological transition which determines the fate of benthic population dynamics through settlement and recruitment. Within this context, nitric oxide (NO) is thought to act as an endogenous inhibitor of metamorphosis. While attention has been focused on the mechanisms of this inhibitory pathway with pharmacological agents and immunohistochemistry tools, relatively few studies have investigated transcriptional process at the origin of NO synthesis. In this paper, we report the isolation of a 218-bp cDNA fragment of an ortholog of the neuronal nitric oxide synthase (nNOS) gene in the invasive marine species Crepidula fornicata. By the use of quantitative real-time PCR, we examine the transcription of this gene throughout larval development and in response to two inducers [K+ excess (20 mM), dibromomethane (DBM) (1000 ppm)] that are known to potentiate metamorphosis in invertebrate species. The level of transcription increased constantly during the larval development, suggesting an increased repressive effect over metamorphosis as larvae aged. Interestingly, maximum values were reached 6 h post-treatment, before declining within 20 h for all the tested conditions. Overall, our results are in agreement with the involvement, at a molecular level, of the NO signalling pathway in metamorphosis. The decrease in nNOS gene transcription post-induction could support the inhibitory effect of NO upon the onset of metamorphosis in competent larvae, although further studies are needed to fully describe the pathways triggered by K+ ions and DBM induction. Furthermore, results indicate that metamorphosis could occur after termination of endogenous inhibition, bringing support to the hypothesis of spontaneous metamorphosis. (C) 2009 Elsevier B.V. All rights reserved.

The gregarious polychaete Sabellaria alveolata is an important foundation species whose reef structures add topographic complexity and high levels of biodiversity in sandflats. In the Bay of Mont-Saint-Michel, France, S. alveolata reefs are considered the largest reef formations in Europe and are increasingly endangered by anthropogenic pressure. In order to understand the role of larval transport on the resilience capacity of these biogenic reefs, multiple complementary approaches were carried out within the bay in 2002 to study horizontal and vertical distribution of S. alveolata larvae and assess larval dispersal processes. Plankton samples collected twice a month from March to October showed an extended period of larval occurrence, from the end of April to October, with 2 peaks of high abundance. The main spawning period occurred early in May, and a smaller peak occurred in September. Mean planktonic lifetime has been calculated to be between 4 and 10 wk. At large scale, larval distribution was patchy and the maximal abundance could reach about 28 000 larvae m -2 . Larval densities were strongly correlated with physico-chemical parameters so that larvae were more abundant in a warmer and less saline coastal water mass characterised by a higher concentration in chlorophyll a. The presence of a large anticyclonic gyre in the bay seemed to play a significant role in controlling larval dispersal by concentrating larvae within the bay. At 1 station in the bay, short-term variations of larval abundances in the water column appeared to be regulated by the tidal cycle, as larvae were more abundant at the end of ebb tide, confirming the very coastal distribution of larvae. Furthermore, S. alveolata larvae tended to migrate closer to the surface during flood and near the bottom during ebb, promoting a net landward transport of larvae. Despite a long larval lifespan, our results indicate that tidal residual currents, especially a coastal gyre, and tidal vertical migration may favour larval retention within the bay.

Steinhausia mytilovuln is a globally distributed microsporidian parasite which infects the oocytes of the blue mussels Mytilus edulis and M. galloprovincialis. Despite the intensive monitoring effort made on mussel populations, the parasite has not previously been reported in France. We report herein on the occurrence of S. mytilovum in Mytilus sp. from 1 cultured and 2 natural populations on the northern coast of France, thus extending the parasite's known distribution northwards. We also report on the observation in 1989 of S. mytilovum in M, galloprovincialis from the Golfe de Fos area in the Mediterranean Sea (South of France), S. mytilovum was observed in the European hybrid zone between M. edulis and M. galloprovincialis, which therefore renders the exact taxonomic status of the infected hosts unknown, The prevalence of the parasite was low, which suggests that its effect on mussel populations was probably limited.

We report the occurrence of the microsporidian parasite Steinhausia sp. in the oocytes of the common cockle Cerastoderma edule in a natural population in France, where high mortalities occurred. Steinhausia sp. appeared primarily as sporocysts containing many small spores, and putative earlier developmental stages were also observed. Both its prevalence and infection intensity were low, and no host defence reaction was recognized, suggesting that Steinhausia sp. had no detrimental effect on C. edule. Its prevalence was higher in cockles lying on the sediment surface, but the significance of this observation could not be explained given the poor knowledge of the Steinhausia life cycle. The present data did not allow specific identification of the parasite, and further studies are required to determine whether Steinhausia sp. in the cockle is a new species, or a microsporidian infecting multiple host species.

Until 1985, seven vent fields were described from the Mid-Atlantic Ridge (MAR). An eighth field, Mount Saldanha (36° N), discovered in 1998, showed unusual geological and biological settings. Vent sites on the MAR exhibit varied environmental conditions, resulting from depth variation of the axis and associated physical parameters, and different source rocks. These could be considered as first order (i.e. most dominant) factors affecting the composition of vent communities on the MAR, in contrast to the East Pacific Rise (EPR) where geographical isolation appears to be a major determinant of faunal differences. In this paper, the geological setting and vent fluid composition of the fields are considered together with their community composition to tentatively ascertain the order of a hierarchy between dispersal and environmental control. The deepest fields (>3000 m) are rather stable systems. The shallower fields, especially Rainbow and Menez Gwen, present some evidence of instability in time and space. The variability in fluid composition is related to phase separation processes (boiling/distillation of subsurface vent fluids) and to the nature of the basement rocks. Depending on depth, phase separation produces gas-enriched and metal-depleted fluids (Menez Gwen, Lucky Strike) or metal-enriched brines (Rainbow, TAG). In addition, high methane content characterises the fluids formed in ultramafic rocks (Rainbow, Logatchev) compared to basaltic rocks. The discrepancy in mineral particulate fluxes at Lucky Strike and Menez Gwen, on one hand, and TAG and Rainbow, on the other, is correlated to the predominance of the vapour or brine phase. The semi-quantitative description of the faunal composition of the different vent fields displays a continuum from Rimicaris-dominated to Bathymodiolus-dominated assemblages. Rather than geographic or bathymetric zonation, this gradation appears to be related to the metal content of the fluids. In addition, the penetration of non vent species into the vent environment increases with decreasing hydrostatic pressure and/or metal content in the fluids. Similarity analysis between vent communities shows that similarity is strongest between Menez Gwen and Lucky Strike (the shallowest fields), less significant between these sites and Rainbow, and weakest for Snake Pit. The inverse relationship between filter feeding organisms and metal concentration in vent fluids could result from a hindrance of mussel bed development by particulate or toxic metal fluxes, and has to be further investigated. Conversely, high metal and particulate content would less affect the more mobile Rimicaris populations. Considering specific similarities of endemic fauna between the four best known hydrothermal vents, the distance between vent fields appears to be a first order parameter. Nevertheless, within the proximity of the Azores Triple Junction area, and in the absence of geographical discontinuity, the similarity between fields stays rather low suggesting faunal islands that have distinct composition and habitat requirements.

on its mussel host. Several papers have reporteda strong hemocyte infiltration in the gonadal tissue(Gonzalez et al. 1987, Bower & Figueras 1989, Figueraset al. 1991, Villalba et al. 1997, Sagrista et al. 1998), butno conclusive evidence has been reported regardingthe effect on the whole organism. In their recent paper, Rayyan & Chintiroglou (2003)were the first to report a physiological impact of

Histological sections of the gonad of Bathymodiolus azoricus from the Lucky Strike and Menez Gwen hydrothermal vent fields (Mid-Atlantic Ridge) demonstrated a spawning event in this species in May 1994, synchronous between the two vent fields located 60 km apart, and a sexual pause in gametogenesis. As reported for other vent mytilid species, B. azoricus from Lucky Strike and Menez Gwen could exhibit hermaphroditism.

International audience; High-throughput sequencing (HTS) technologies offer new promise to support surveillance programs targeting marine non-indigenous species (NIS). Metabarcoding might surpass traditional monitoring methods, for example through its ability to detect rare species, a key feature in early detection of NIS. Another interest of this approach is the identification of organisms difficult to identify based on morphology only (e.g., early developmental stages), making it relevant in the context of management programs. Because many marine benthic NIS have a biphasic bentho-pelagic life cycle, targeting their pelagic larval stages in zooplankton may allow early detection and assessment of their establishment and potential spread. We illustrate this approach with an analysis of bulk-DNA retrieved from a time-series of zooplankton samples collected over 22 months in one bay in Brittany (France). Using HTS of amplicons obtained with two markers (COI and 18S) and a metabarcoding approach, 12 NIS were identified and their temporal larval dynamics were monitored. Importantly, we chose to focus on a closed list of species, from four metazoan classes encompassing 52 NIS reported within the study area or nearby seas, with molecular references available or obtained locally for 42 of them. The use of a custom-designed database allowed the detection of three NIS otherwise not detected when using public databases. Interestingly, NIS known to have a short-lived larval stage were detected (e.g., the bryozoan Bugula neritina or the tunicate Corella eumyota). For two molluscs Ruditapes philippinarum and Crepidula fornicata, metabarcoding results were compared to those obtained using traditional methods (i.e., barcoding of individual larvae and morphology, respectively) to show the reliability of the approach in detecting and assessing the extent of their reproductive periods. Our results also revealed that the Pacific oyster Crassostrea gigas, a notorious invasive species, failed to reproduce in the study bay, showing that metabarcoding on larval stages also provides information regarding the establishment success (or failure) of NIS. While metabarcoding has its limitations and biases, this study demonstrates its effectiveness for surveillance of targeted NIS, notably to support management strategies like the European Marine Strategy Framework Directive (MSFD).

Dispersal is major process which controls the dynamics and evolution of marine population. For bentho-pelagic invertebrates, larva is the main vector of dispersal. In the case of introduced species, along with humanactivities, the natural dispersal capacities conferred by the pelagic lavae are essential for their spread. The main objective of thesis was to study the spatio-temporel dynamics of the larvae of the invasive gastropod Crepidula fornicata and its potential effects on the benthic population dynamics in the Bay of Morlaix (France). The study of the spatial distribution of C. fornicata larvae at the scale of the Bay of Morlaix, showed that local recruitment could be limited by the interactions between the location of benthic adults and the local hydrodynamics, thus explaining the reduced expansion of this population. Besides, a 3-years survey of the larval pool dynamics in this bay showed that: (1) small larvae dominated early in the year when recruitment is very low and (2) larger larvae occured later during the annual peak of recruitment. This suggested a strong larval-adult coupling during the summer. This study also showed the occurence of larvae in the field over a wide range of temperatures (between 9 and 17°C) which led us to consider the temperature as a key parameter in the understanding of the larval pool dynamics. Laboratory experiments emphasized the strong influence of temperatures as recorded in situ on C. fonicata larval development. Considering the larval duration (PLD) as a proxy for the potential dispersal distance (i.e. larval transportation), a numerical approach allowed us to compute the PLD variations based on the temperature variations recorded during the breeding season of C. fornicata. These computaions showed the existence of a strong seasonal component in the dispersal capacities of C. fornicata larvae. this suggested the occurence of one season which favours dispersal (long PLD) and one which favours local recruitment (short PLD). Based on a similar approach applied to other European colonized sites, we showed that very low temperatures encountered in the northern sites could jeopardize the existence of a dispersal season. Hence, we suggested that climate warming in northern sites might increase the dispersal capacities of C. fornicata, significantly enhancing its potential for spred.; La dispersion est un processus majeur dans la dynamique et l'évolution des populations d'espèces marines. Pour les invertébrés à cycle bentho-pélagique, la larve est généralement le principal vecteur de dispersion. Dans le cas particulier des espèces introduites, aux côtés des activités humaines, les capacités de dispersion naturelles conférées par les larves pélagiques sont un élément essentiel pour leur expansion. L'objectif principal de cette thèse était d'étudier la dynamique spatio-temporelle des larves du gastéropode invasif Crepidula fornicata et son rôle sur la dynamique des populations benthiques adultes en baie Morlaix. Par une étude de la distribution spatiale du pool larvaire de C. fonicata à l'echelle de la baie Morlaix, nous avons montré que le recrutement local pourrait être limité par l'interaction entre la position des adultes benthiques et l'hydrodynamisme très actif, expliquant ainsi le caractère peu expansif de cette population. L'analyse de la dynamique temporelle du pool larvaire pendant trois années consécutive a montré (1) la présence majoritaire de petites larves durant les mois de début d'année, lorsque que le recrutement est très faible et (2) la présence plus tardive dans l'année de larves de plus grandes tailles durant le pic de recrutement, ce qui suggère l'existance d'un fort couplage larves-adultes durant les mois d'été Au cours de cette même étude, l'observation de larves crépidules dans des conditions de température très différentes (entre 9 et 17°C) nous a amené à considérer la température comme un paramètre clé de la dynamique du pool larvaire. Un série d'expériences en laboratoire a permis de montrer en autre le fort impact des températures rencontrées in situ sur le développement larvaire. Considérant la durée de vie larvaire (PLD) comme un indicateur des distances potentielles de dispersion (i.e. transport larvaire), une estimation des variations de PLD en fonction des variation de températures durant la période de reproduction a mis en évidence l'existence d'une forte saisonnalité dans les capacités dispertives de l'espèce. Ces résultats suggèrent l'existence d'une saison plus propice à la dispersion (longue PLD) et d'une saison plus propice au recrutement local (PLD courte). L'application de cette approche à d'autres site colonisés en Europe a montré que les très basses température hivernales rencontrées dans les sites situés au nord pouvaient compromettre l'existence d'une saison propice à la dispersion. Nos résultats suggèrent finalement que dans la perspective d'un réchauffement climatique dans les sites nordiques, les capacités dispertives de l'espèce pourraient être accrues, augmentant ses potentialités d'expansion.

Dispersal is major process which controls the dynamics and evolution of marine population. For bentho-pelagic invertebrates, larva is the main vector of dispersal. In the case of introduced species, along with humanactivities, the natural dispersal capacities conferred by the pelagic lavae are essential for their spread. The main objective of thesis was to study the spatio-temporel dynamics of the larvae of the invasive gastropod Crepidula fornicata and its potential effects on the benthic population dynamics in the Bay of Morlaix (France). The study of the spatial distribution of C. fornicata larvae at the scale of the Bay of Morlaix, showed that local recruitment could be limited by the interactions between the location of benthic adults and the local hydrodynamics, thus explaining the reduced expansion of this population. Besides, a 3-years survey of the larval pool dynamics in this bay showed that: (1) small larvae dominated early in the year when recruitment is very low and (2) larger larvae occured later during the annual peak of recruitment. This suggested a strong larval-adult coupling during the summer. This study also showed the occurence of larvae in the field over a wide range of temperatures (between 9 and 17°C) which led us to consider the temperature as a key parameter in the understanding of the larval pool dynamics. Laboratory experiments emphasized the strong influence of temperatures as recorded in situ on C. fonicata larval development. Considering the larval duration (PLD) as a proxy for the potential dispersal distance (i.e. larval transportation), a numerical approach allowed us to compute the PLD variations based on the temperature variations recorded during the breeding season of C. fornicata. These computaions showed the existence of a strong seasonal component in the dispersal capacities of C. fornicata larvae. this suggested the occurence of one season which favours dispersal (long PLD) and one which favours local recruitment (short PLD). Based on a similar approach applied to other European colonized sites, we showed that very low temperatures encountered in the northern sites could jeopardize the existence of a dispersal season. Hence, we suggested that climate warming in northern sites might increase the dispersal capacities of C. fornicata, significantly enhancing its potential for spred.; La dispersion est un processus majeur dans la dynamique et l'évolution des populations d'espèces marines. Pour les invertébrés à cycle bentho-pélagique, la larve est généralement le principal vecteur de dispersion. Dans le cas particulier des espèces introduites, aux côtés des activités humaines, les capacités de dispersion naturelles conférées par les larves pélagiques sont un élément essentiel pour leur expansion. L'objectif principal de cette thèse était d'étudier la dynamique spatio-temporelle des larves du gastéropode invasif Crepidula fornicata et son rôle sur la dynamique des populations benthiques adultes en baie Morlaix. Par une étude de la distribution spatiale du pool larvaire de C. fonicata à l'echelle de la baie Morlaix, nous avons montré que le recrutement local pourrait être limité par l'interaction entre la position des adultes benthiques et l'hydrodynamisme très actif, expliquant ainsi le caractère peu expansif de cette population. L'analyse de la dynamique temporelle du pool larvaire pendant trois années consécutive a montré (1) la présence majoritaire de petites larves durant les mois de début d'année, lorsque que le recrutement est très faible et (2) la présence plus tardive dans l'année de larves de plus grandes tailles durant le pic de recrutement, ce qui suggère l'existance d'un fort couplage larves-adultes durant les mois d'été Au cours de cette même étude, l'observation de larves crépidules dans des conditions de température très différentes (entre 9 et 17°C) nous a amené à considérer la température comme un paramètre clé de la dynamique du pool larvaire. Un série d'expériences en laboratoire a permis de montrer en autre le fort impact des températures rencontrées in situ sur le développement larvaire. Considérant la durée de vie larvaire (PLD) comme un indicateur des distances potentielles de dispersion (i.e. transport larvaire), une estimation des variations de PLD en fonction des variation de températures durant la période de reproduction a mis en évidence l'existence d'une forte saisonnalité dans les capacités dispertives de l'espèce. Ces résultats suggèrent l'existence d'une saison plus propice à la dispersion (longue PLD) et d'une saison plus propice au recrutement local (PLD courte). L'application de cette approche à d'autres site colonisés en Europe a montré que les très basses température hivernales rencontrées dans les sites situés au nord pouvaient compromettre l'existence d'une saison propice à la dispersion. Nos résultats suggèrent finalement que dans la perspective d'un réchauffement climatique dans les sites nordiques, les capacités dispertives de l'espèce pourraient être accrues, augmentant ses potentialités d'expansion.

Early detection of non-indigenous species (NIS) at their points of introduction (e.g. ports) is crucial for effective management of these species. High-throughput-sequencing (HTS) techniques are promising for reducing costs and processing time. In this thesis, we focused on the sessile marine organisms of port biofouling. We first studied the taxonomic and genetic diversity of Botrylloides colonies using HTS; all six bioinformatics pipelines tested provided a relevant assessment of biodiversity, but with variable results for haplotype detection. The study of environmental DNA obtained in ten ports over two seasons then revealed that port communities are variable in time and space. The HTS data also detected many NIS without, however, identifying all the NIS observed by a traditional method. Finally, we studied the dispersion of sessile benthic NIS in natural habitats by means of experimental plates deployed inside and outside a port; the species were identified by metabarcoding and using morphological criteria. Port assemblages were, as expected, different from natural habitats, but most NIS were found in both types of habitats. Zooplankton metabarcoding of a two-year time series confirmed the presence of NIS at the larval stage, and thus their reproduction. Altogether, although with limitations, HTS techniques were found to be complementary to traditional methods, and relevant for detecting and studying NIS, notably in ports and nearby natural habitats.; La détection précoce des espèces non-indigènes (ENI) à leurs points d'introduction (ex. ports) est cruciale pour une gestion efficace de ces espèces. Réduisant le coût et le temps de traitement, les techniques de séquençage à haut débit (HTS) sont prometteuses. Dans cette thèse, nous nous sommes intéressés aux organismes marins sessiles du biofouling des ports. Nous avons d'abord étudié par HTS la diversité taxonomique et génétique de colonies du genre Botrylloides; les six pipelines bioinformatiques testés ont tous fourni une évaluation pertinente de la biodiversité, mais avec des résultats variables pour la détection des haplotypes. L'étude de l'ADN environnemental obtenu dans dix ports en deux saisons a ensuite révélé que les communautés portuaires sont variables dans le temps et dans l'espace. Les données HTS détectent par ailleurs de nombreuses ENI sans toutefois identifier toutes les ENI observées par une méthode traditionnelle. Enfin, nous avons étudié la propagation des ENI benthiques sessiles en milieu naturel grâce au déploiement de plaques expérimentales au sein et hors d'un port; les espèces étant identifiées par metabarcoding et en utilisant des critères morphologiques. Les assemblages portuaires sont comme attendu différents des habitats naturels mais la plupart des ENI sont communes aux deux types d'habitats. Le metabarcoding de zooplancton d'une série chronologique de deux ans a confirmé la présence d'ENI en phase larvaire, et leur reproduction. Bien qu'avec des limites, les techniques HTS sont ainsi complémentaires des méthodes traditionnelles et pertinentes pour détecter et étudier les ENI, notamment dans les ports et milieux naturels voisins.