Oyster Transfers Research Articles

Genetic diversity and connectivity of the Ostreid herpesvirus 1 populations in France: A first attempt to phylogeographic inference for a marine mollusc disease.

The genetic diversity of viral populations is a key driver of the spatial and temporal diffusion of viruses; yet, studying the diversity of whole genomes from natural populations still remains a challenge. Phylodynamic approaches are commonly used for RNA viruses harboring small genomes but have only rarely been applied to DNA viruses with larger genomes. Here, we used the Pacific oyster mortality syndrome (a disease that affects oyster farms around the world) as a model to study the genetic diversity of its causative agent, the Ostreid herpesvirus 1 (OsHV-1) in the three main French oyster-farming areas. Using ultra-deep sequencing on individual moribund oysters and an innovative combination of bioinformatics tools, we de novo assembled twenty-one OsHV-1 new genomes. Combining quantification of major and minor genetic variations, phylogenetic analysis, and ancestral state reconstruction of discrete traits approaches, we assessed the connectivity of OsHV-1 viral populations between the three oyster-farming areas. Our results suggest that the Marennes-Oléron Bay represents the main source of OsHV-1 diversity, from where the virus has dispersed to other farming areas, a scenario consistent with current practices of oyster transfers in France. We demonstrate that phylodynamic approaches can be applied to aquatic DNA viruses to determine how epidemiological, immunological, and evolutionary processes act and potentially interact to shape their diversity patterns.

Conservation aquaculture as a tool for imperiled marine species: Evaluation of opportunities and risks for Olympia oysters, Ostrea lurida.

Conservation aquaculture is becoming an important tool to support the recovery of declining marine species and meet human needs. However, this tool comes with risks as well as rewards, which must be assessed to guide aquaculture activities and recovery efforts. Olympia oysters (Ostrea lurida) provide key ecosystem functions and services along the west coast of North America, but populations have declined to the point of local extinction in some estuaries. Here, we present a species-level, range-wide approach to strategically planning the use of aquaculture to promote recovery of Olympia oysters. We identified 12 benefits of culturing Olympia oysters, including identifying climate-resilient phenotypes that add diversity to growers' portfolios. We also identified 11 key risks, including potential negative ecological and genetic consequences associated with the transfer of hatchery-raised oysters into wild populations. Informed by these trade-offs, we identified ten priority estuaries where aquaculture is most likely to benefit Olympia oyster recovery. The two highest scoring estuaries have isolated populations with extreme recruitment limitation-issues that can be addressed via aquaculture if hatchery capacity is expanded in priority areas. By integrating social criteria, we evaluated which project types would likely meet the goals of local stakeholders in each estuary. Community restoration was most broadly suited to the priority areas, with limited commercial aquaculture and no current community harvest of the species, although this is a future stakeholder goal. The framework we developed to evaluate aquaculture as a tool to support species recovery is transferable to other systems and species globally; we provide a guide to prioritizing local knowledge and developing recommendations for implementation by using transparent criteria. Our collaborative process engaging diverse stakeholders including managers, scientists, Indigenous Tribal representatives, and shellfish growers can be used elsewhere to seek win-win opportunities to expand conservation aquaculture where benefits are maximized for both people and imperiled species.

Multiple genetic marker analysis challenges the introduction history of Ulva australis (Ulvales, Chlorophyta) on French coasts

ABSTRACT The green seaweeds Ulva australis and U. pertusa were described from southern Australia and Japan, respectively. They are conspecific and U. australis, the currently accepted taxon, is native to temperate marine waters in north-eastern Asia, and known to be introduced overseas into Australasia, the Americas and Europe. Although the genetics of U. australis have been investigated elsewhere, along French coasts the origins and history of the introduction of this species need to be clarified. We used mitochondrial, plastid and nuclear markers to differentiate introduced populations of U. australis along the French Atlantic coasts. The plastid tufA gene used as a barcoding marker revealed a well-defined species with a higher haplotype diversity in native vs. introduced areas. The ITS2 region (nuclear) and rbcL (plastid) were used to compare French specimens with the lectotype of U. australis. Putative geographic origins of the genetically determined U. australis were examined using genetic markers with better resolution, the plastid atpI-H combined with the mitochondrial trnA-N. Origin(s) and introduction history of French specimens were inferred from the comparison between their haplotypes and those previously described in native and non-native temperate areas worldwide. Our results indicate that the presence of U. australis along the French Atlantic and Mediterranean coasts is the result of multiple introductions and independent pathways, and suggest that historical oyster transfers from Japan and British Columbia can only partially explain the observed patterns in genetic markers. Alternative hypotheses for the timing and pathways of introductions are proposed in the light of the historic background of maritime transport networks and trade between north-eastern Asia and Europe.

The pearl oyster (Pinctada margaritifera) aquaculture in French Polynesia and the indirect impact of long-distance transfers and collection-culture site combinations on pearl quality traits

In French Polynesia, the P. margaritifera pearl aquaculture industry is spread over a vast area, as large as Europe. All the oysters for this the highly economically important activity are supplied from just a few collection lagoons, but they are grown in numerous sites across three archipelagos (Gambier, Society and Tuamotu). Many oyster transfers thus indirectly bring about grafting combinations mixing different geographic origins and production sites. This study aims to examine the impact of such graft combinations on cultured pearl quality traits. For this, six homogeneous and standardised experimental graft combinations (N = 6197) were conducted at commercial scale in the two growing locations the most frequently used in French Polynesia: Arutua atoll (Tuamotu) and Mangareva island (Gambier), using oysters supplied from by the top three collection sites: Ahe, Takapoto and Mangareva lagoons. At harvest, four main pearl quality traits: nacre weight deposition speed, pearl colour components (darkness level and green overtone), grade and shape categories were recorded by a professional sorter from the Tahiti auction and compared. Results revealed effects of the combinations of oyster origin and grow-out location, with: 1) significant origin × site interaction for nacre weight deposition speed; 2) colour variation at intra- and inter-site scales, with Ahe origin producing the most dark pearls and Gambier highest rate of the attractive green coloured pearls; and 3) higher grade categories for the Gambier origin and rearing location. These oyster-site combination effects highlight the benefit for the Polynesian pearl industry of switching from a mono-site/ company production system to a new multi-site production strategy to maximize overall cultured pearl quality expression.

Genetic signature analysis of Perkinsus marinus in Mexico suggests possible translocation from the Atlantic Ocean to the Pacific coast of Mexico

BackgroundThe protozoan Perkinsus marinus (Mackin, Owen & Collier) Levine, 1978 causes perkinsosis in the American oyster Crassostrea virginica Gmelin, 1791. This pathogen is present in cultured C. virginica from the Gulf of Mexico and has been reported recently in Saccostrea palmula (Carpenter, 1857), Crassostrea corteziensis (Hertlein, 1951) and Crassostrea gigas (Thunberg, 1793) from the Mexican Pacific coast. Transportation of fresh oysters for human consumption and repopulation could be implicated in the transmission and dissemination of this parasite across the Mexican Pacific coast. The aim of this study was two-fold. First, we evaluated the P. marinus infection parameters by PCR and RFTM (Ray’s fluid thioglycollate medium) in C. virginica from four major lagoons (Términos Lagoon, Campeche; Carmen-Pajonal-Machona Lagoon complex, Tabasco; Mandinga Lagoon, Veracruz; and La Pesca Lagoon, Tamaulipas) from the Gulf of Mexico. Secondly, we used DNA sequence analyses of the ribosomal non-transcribed spacer (rNTS) region of P. marinus to determine the possible translocation of this species from the Gulf of Mexico to the Mexican Pacific coast.ResultsPerkinsus marinus prevalence by PCR was 57.7% (338 out of 586 oysters) and 38.2% (224 out of 586 oysters) by RFTM. The highest prevalence was observed in the Carmen-Pajonal-Machona Lagoon complex in the state of Tabasco (73% by PCR and 58% by RFTM) and the estimated weighted prevalence (WP) was less than 1.0 in the four lagoons. Ten unique rDNA-NTS sequences of P. marinus [termed herein the “P. marinus (Pm) haplotype”] were identified in the Gulf of Mexico sample. They shared 96–100% similarity with 18 rDNA-NTS sequences from the GenBank database which were derived from 16 Mexican Pacific coast infections and two sequences from the USA. The phylogenetic tree and the haplotype network showed that the P. marinus rDNA-NTS sequences from Mexico were distant from the rDNA-NTS sequences of P. marinus reported from the USA. The ten rDNA-NTS sequences described herein were restricted to specific locations displaying different geographical connections within the Gulf of Mexico; the Carmen-Pajonal-Machona Pm1 haplotype from the state of Tabasco shared a cluster with P. marinus isolates reported from the Mexican Pacific coast.ConclusionsThe rDNA-NTS sequences of P. marinus from the state of Tabasco shared high similarity with the reference rDNA-NTS sequences from the Mexican Pacific coast. The high similarity suggests a transfer of oysters infected with P. marinus from the Mexican part of the Gulf of Mexico into the Mexican Pacific coast.

Discovery of the non-indigenous bryozoan Smittoidea prolifica Osburn, 1952 near Helgoland: first record in 2011 for the German North Sea

Since 2011, the non-indigenous Pacific bryozoan Smittoidea prolifica Osburn, 1952 was observed repeatedly on shell gravel in the trench Tiefe Rinne during annual long-term monitoring assessments of subtidal sites around the island Helgoland. It is the earliest discovery for the German North Sea and three years earlier than its first detection at commercial harbour sites on the German Wadden Sea coast. Introduction to the North-East Atlantic is assumed to have occurred before 1995 with oyster transfers to The Netherlands for aquaculture purposes. The presence of S. prolifica in the natural habitat of the Tiefe Rinne, relatively distant from the main shipping routes and commercial ports in the German North Sea and without nearby mussel cultures, requires the consideration of introduction vectors other than intentional shellfish transport and international shipping traffic.

How network analysis of oyster movements can improve surveillance and control programs of infectious diseases?

Animal movements are one of the main ways to introduce and spread pathogens. In French oyster farming, many stakeholders and premises are heterogeneously divided over the country and a highly dynamic flow of oysters exists among them. In the context of animal disease surveillance and control, analysis of the animal movement’ network can provide useful information to build adapted surveillance strategies or to develop risk management. Movement network analysis has been widely used in terrestrial production, to evaluate the vulnerability of animal movement network to the spread of a specific disease. In France, since 2008, Pacific oyster spat (Crassostrea gigas) has been affected by massive mortality outbreaks associated with the detection of a newly reported variant of ostreid herpesvirus type 1 (OsHV-1). These mortality events have a direct economic impact causing considerable concern to oyster farmers. A previous epidemiological study has highlighted the potential role of oyster transfers in the spread of these outbreaks. However, neither mandatory database nor reliable data is publicly available concerning oyster transfers in France. In this context, a field study was carried out in the main oyster production area in France, Charente-Maritime bay, to map oyster movements and to characterize the corresponding network structure related to potential disease spread. Seventy-five oyster farmers were randomly selected in Charente-Maritime bay between July and September 2010, according to a stratified sampling design based on farm category regarding production type and location (i.e. spat producers, local farmers, beyond farmers, local farmer-senders, beyond farmer-senders). Data related to the farm characteristics and activities, routine rearing scheme and potential changes in husbandry practices were collected during a face-to-face interview of the oyster farmer, using a standardized questionnaire and a land register. Movement data were spatialized and analyzed using social network analysis. A 85.3% participation rate to the study was obtained. Results showed a seasonal variation of oyster transfers in all the sampling strata, with peaks of transfers in spring and autumn, and different geographical patterns of oyster transfers between strata. Growing sites were shared by farms belonging to different farm category which have different behaviour related to oyster movements in time. These results show heterogeneity of contacts between strata both in time and space. Network measures allowed identifying the growing sites having a high risk of becoming infected, of transmitting diseases to other sites or controlling the flow of oysters from one part of the network to another part. The growing sites at high risk of becoming infected could be targeted for surveillance activities (Figure 1). The growing sites at high risk of transmitting diseases to other sites could be the first ones where implementation of early or enhanced control measures (e.g. quarantine, depopulation) could be focused. The growing sites controlling the flow of oysters could be considered as key players in the initial spread of a disease. If these sites would be removed from the network by early movement restriction, this may contribute to contain the disease spread in a smaller part of the area, providing that the propagation by the marine currents and the effect of environmental factors are considered all together. As a conclusion, movement network analysis may provide valuable information to policy makers in preparedness (surveillance) and response (control) to an epidemic.

Three non-indigenous species ofAoroides(Crustacea: Amphipoda: Aoridae) from the French Atlantic coast

The amphipod genusAoroidesis reported for the first time in European marine waters. Specimens ofAoroides semicurvatusandAoroides curvipeswere collected in oyster reefs in Arcachon Bay between 2009 and 2014 and in Hossegor Lake in 2014 (SW France). Specimens ofAoroides longimeruswere collected in 2013 and 2014 in subtidal slipper limpet beds andZostera marinameadows in Arcachon Bay and in 2014 on floating pontoons in the Bay of Brest (W France). These species, native to Asia, may have been accidentally introduced in Arcachon Bay and Hossegor Lake with oyster transfers and in the Bay of Brest through both oyster transfers and shipping.

Stress response ofCrassostrea virginica(Gmelin, 1791) oysters following a reciprocal transfer between upriver and downriver sites

This study was designed to assess the physiological impact of transferring oysters between upriver and downriver aquaculture sites, a common practice in North America that is primarily aimed at reducing disease infections and predation on cultured stocks. In May 2009, oysters (Crassostrea virginica) were reciprocally transferred between an upriver and a downriver site in the Richibucto estuary in eastern Canada. Mortality, tissue and cellular stress responses were subsequently evaluated in August and October 2009. Overall, oyster mortality remained low (~5%) throughout the 5-month study period with no significant difference between sites or oyster sources. However, by October oysters reared at the upriver site, regardless of their origin, had significantly higher levels of lysosomal membrane destabilization (63.6%, SE = 1.9) and digestive tubule atrophy (33.3–42.4%, SE = 3.6) than oysters reared at the downriver site (47.5%, SE = 1.8; 15.6–19.1%, SE = 3.8 respectively). They also exhibited a greater salinity differential between their mantle/haemolymph fluids and the ambient seawater, possibly indicating more restricted exchange with the environment. In general, the transfer of upriver oysters to a downriver site had a positive impact, i.e. lower levels of lysosomal destabilization and tubule atrophy, whereas transfer of downriver oysters upriver had the opposite effect. These results suggest that upriver environmental conditions negatively impact cellular and tissue integrity in oysters without leading to mortality during the summer–autumn period.

Recent discovery of Paranthura japonica Richardson, 1909 (Crustacea: Isopoda: Paranthuridae) in European marine waters (Arcachon Bay, Bay of Biscay

The Asiatic isopod Paranthura japonica Richardson, 1909 was collected in 2007 in Arcachon Bay (SW France), where the species occurs in a variety of habitats, both in the intertidal and at shallow depths. This species, native to the Sea of Japan, may have been accidentally introduced in Arcachon Bay with oyster transfers or as fouling on ship hulls.

Sargassum muticum (Yendo) Fensholt (Fucales, Phaeophyta) in Morocco, an invasive marine species new to the Atlantic coast of Africa

The Japanese brown seaweed Sargassum muticum (Yendo) Fensholt (Fucales, Phaeophyta) is reported for the first time in Atlantic Morocco, along the shoreline of Doukkala (S of Casablanca). This record, the first in the African continent, represents a remarkable range expansion of this invasive marine species. Indeed, it is the southern-most occurrence of the species in the Atlantic. Preliminary phenological and ecological data of this non-native species were provided from the Doukkala coast. The major effects on benthic structure and composition of this invasive brown macroalga were also evaluated from published data. Oyster transfers from the French coast were the most likely primary

The potential role of an unregulated coastal anthropogenic activity in facilitating the spread of a non-indigenous biofoulant

Despite an exponential rise in anthropogenically-mediated transfers of non-indigenous species during the last 150 years, several coastal anthropogenic activities remain unregulated under current legislation frameworks. This study investigates the potential role of commercial periwinkle (Littorina littorea) harvesting as an unregulated facilitator of both small- and large-scale geographic range expansion of an invasive oyster epibiont (Ostrea chilensis) within the Menai Strait (North Wales, UK) and beyond. The frequency of oyster-fouled periwinkles was greatest in areas of high adult oyster abundance and restricted to large, market-sized periwinkles (>20 mm) inhabiting the low shore. Active efforts by commercial collectors to reject oyster-fouled periwinkles were found to be inadequate, with oysters of all sizes observed within collected hauls. Whilst the survival of fouled and unfouled periwinkles was comparable under post-collection refrigerated conditions, a significant decrease in both mobility and flesh content was associated with the presence of oyster epibionts. Survival of all but the smallest oyster epibionts under post-collection refrigerated conditions enhances the possibility of accidental non-indigenous oyster transfers. Better interventions during both initial visual inspection and post-griddling stages are recommended, as well as the development of techniques that kill off all non-indigenous epibionts, whilst leaving the freshness and marketability of the periwinkles uncompromised.

Experimental assessment of oyster transfers as a vector for macroalgal introductions

Introduction of non-indigenous species can alter marine communities and ecosystems. In shellfish farming, transfer of livestock, especially oysters, is a common practice and potentially constitutes a pathway for non-indigenous introductions. Many species of seaweeds are believed to have been accidentally introduced in association with these transfers, but there is little direct evidence. We experimentally simulated the transfer of oysters from the Thau Lagoon (France). These transfers involved increasing periods of aerial emersion and additional brine and hot water treatments. The brine and hot water treatments were evaluated as a means of reducing the probability of algal introductions with oyster transfers. Shells were cultured for 40 days in experimental tanks to identify the macroalgae likely to be introduced with any oyster transfer. A total of 57 macroalgal taxa, including 16 taxa not indigenous to the Thau Lagoon, were recorded across all treatments and experiments. The abundance of some species increased in several cases following aerial emersion. Elimination treatments (immersion in brine or hot water) significantly reduced algal diversity, with hot water treatments resulting in no species or only tubular Ulva spp. present. The results support the hypothesis that oyster transfers are effective as primary and secondary vectors of macroalgal introductions. Relatively simple changes to the transfer practice (particularly hot water treatments) are suggested as a means of reducing the risk of non-indigenous algal introductions.

Differentiating successful and failed molluscan invaders in estuarine ecosystems

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 332:41-51 (2007) - doi:10.3354/meps332041 Differentiating successful and failed molluscan invaders in estuarine ecosystems A. Whitman Miller1,*, Gregory M. Ruiz1, Mark S. Minton1, Richard F. Ambrose2 1Smithsonian Environmental Research Center, PO Box 28, Edgewater, Maryland 21037, USA 2Environmental Science and Engineering Program, Box 951772, University of California, Los Angeles, California 90095, USA *Email: millerw@si.edu ABSTRACT: Despite mounting evidence of invasive species’ impacts on the environment and society, our ability to predict invasion establishment, spread, and impact are inadequate. Efforts to explain and predict invasion outcomes have been limited primarily to terrestrial and freshwater ecosystems. Invasions are also common in coastal marine ecosystems, yet to date predictive marine invasion models are absent. Here we present a model based on biological attributes associated with invasion success (establishment) of marine molluscs that compares successful and failed invasions from a group of 93 species introduced to San Francisco Bay (SFB) in association with commercial oyster transfers from eastern North America (ca. 1869 to 1940). A multiple logistic regression model correctly classified 83% of successful and 80% of failed invaders according to their source region abundance at the time of oyster transfers, tolerance of low salinity, and developmental mode. We tested the generality of the SFB invasion model by applying it to 3 coastal locations (2 in North America and 1 in Europe) that received oyster transfers from the same source and during the same time as SFB. The model correctly predicted 100, 75, and 86% of successful invaders in these locations, indicating that abundance, environmental tolerance (ability to withstand low salinity), and developmental mode not only explain patterns of invasion success in SFB, but more importantly, predict invasion success in geographically disparate marine ecosystems. Finally, we demonstrate that the proportion of marine molluscs that succeeded in the latter stages of invasion (i.e. that establish self-sustaining populations, spread and become pests) is much greater than has been previously predicted or shown for other animals and plants. KEY WORDS: Invasion · Bivalve · Gastropod · Mollusc · Marine · Oyster · Vector · Risk assessment Full text in pdf format Supplementary appendices: Appendix 1, Appendix 2 PreviousNextExport citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 332. Online publication date: March 05, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Invasion of the eastern Bay of Biscay by the nassariid gastropod Cyclope neritea: origin and effects on resident fauna

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 276:147-159 (2004) - doi:10.3354/meps276147 Invasion of the eastern Bay of Biscay by the nassariid gastropod Cyclope neritea: origin and effects on resident fauna Guy Bachelet1,*, Benoît Simon-Bouhet2,3, Céline Desclaux1, Pascale Garcia-Meunier2, Guillaume Mairesse1, Xavier de Montaudouin1, Hélène Raigné1, Karine Randriambao1, Pierre-Guy Sauriau4, Frédérique Viard3 1Laboratoire d¹Océanographie Biologique, UMR 5805 CNRS-Université Bordeaux 1, Station Marine d¹Arcachon, 2 rue du Professeur Jolyet, 33120 Arcachon, France 2Laboratoire de Biologie et Environnement Marins, EA 3168, Université de La Rochelle, 17000 La Rochelle, France 3Evolution et Génétique des Populations Marines, UMR 7127 CNRS, Station Biologique de Roscoff, BP 74, 29682 Roscoff cedex, France 4Centre de Recherche sur les Ecosystèmes Marins et Aquacoles, UMR 10 CNRS-IFREMER, Place du Séminaire, BP 5, 17137 L¹Houmeau, France *Email: g.bachelet@epoc.u-bordeaux1.fr Abstract: The distribution area of the nassariid gastropod Cyclope neritea (L.) includes the Mediterranean and the Black Sea, as well as the Atlantic coasts of the southern Iberian Peninsula. The species has spread north to the eastern Bay of Biscay (Arcachon Bay in 1976, Marennes-Oléron area and Morbihan Gulf in 1983-84). This spread might be explained either by (1) a natural spread favoured by environmental changes (e.g. an increase of temperature) or (2) a sudden range expansion due to the introduction of individuals from distant native populations. Molecular genetic analyses based on mitochondrial markers suggest that the present C. neritea population in Arcachon Bay has been introduced, probably unintentionally with oyster transfers, from several source populations, genetically similar to the populations analysed in this study, i.e. those in the western Mediterranean and in south Portugal. Within its new distribution area, C. neritea could potentially compete with the autochthonous nassariid Nassarius reticulatus, both species being scavengers. Although C. neritea tends to occur mainly in relatively clean sands in the intertidal and N. reticulatus in subtidal, organic rich sediments, the habitats of both species partially overlap. Laboratory experiments showed that in still water conditions C. neritea was more active and reached carrion faster than N. reticulatus, thereby having a competitive advantage over the latter; flow conditions ( ~1 cm s-1) appeared to stimulate the activity of N. reticulatus. Analysis of parasite load in both species in Arcachon Bay indicated that N. reticulatus was more heavily parasitized by digenean trematodes than C. neritea. This suggests that the spread and population dynamics of C. neritea along the French Atlantic coast has been favoured by the lack of parasites. Altogether, recurrent introduction, competitive ability and lack of heavy parasitic load might explain the successful settlement of C. neritea along the French Atlantic coast. KEY WORDS: Introduced species · Scavenging gastropods · Population genetics · Interspecific competition · Behaviour · Parasites Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 276. Online publication date: August 02, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe

A checklist of macroalgae in Thau Lagoon (Hérault, France), based on literature records and new collections, is given. Using present-day taxonomy, 196 taxa and stadia of macroalgae have been identified to date. Sixty of these were no longer found in the samples, 36 were new to the lagoon, and the taxonomic identity of 15 species was amended. Twenty-five new introduced taxa were identified, giving a total of 45 introduced macroalgae for the flora (23% of total). Among these, 17 taxa and several genera ( Acrothrix, Chondrus, Dasysiphonia (?), Prionitis) are new to the Mediterranean Sea and 12 are new to Europe. The majority (43 taxa) may originate from the Pacific region, having arrived either directly or via other aquaculture sites. A highly probable vector of macroalgae introductions is the transfer of oysters, which appears, at present, to be the main vector of macrophyte introductions in the Mediterranean Sea, surpassing the Suez Canal. Thau Lagoon is revealed to be one of the major hot spots of marine species introduction in the Mediterranean Sea, Europe, but also in the world. This result is worrying as Thau is also an important exportation centre of living molluscs towards other French aquaculture sites and abroad. Therefore, the numerous exotic macroalgae, which have been acclimatized in the lagoon, have a high probability of being diffused throughout Europe and other Mediterranean countries. At present, the hard substrates of the Thau Lagoon are clearly dominated by the introduced species to the detriment of indigenous flora. The fate of these species in the Mediterranean Sea and Europe is unpredictable. It is probably not very realistic to hope that the accidental introductions linked to aquaculture activities will stop. A set of recommendations exists, however, that might contribute to limit the phenomenon.

Relations milieu-ressources: impact de la conchyliculture sur un environnement lagunaire méditerranéen (Thau)

Shellfish farming leaves its mark on the environment in which it has developed, and the men who depend upon it. These changes have altogether balanced the lagoon cycle and have caused disastrous episodic events. Increased water clarity caused by the uptake of particulate material by shellfish farming allows seagrass to grow in deeper areas of the lagoon (down to five metres). Shellfish farming nutrient transformations increase ecosystem productivity, even if the filtration pressure keeps phytoplankton biomass at a low level. Storage of phosphorus and nitrogen in animal tissue limits eutrophication in this ecosystem. Transfer of oysters from growout facilities increases animal and vegetal specific diversity. The presence of large amounts of shellfish allows for the development of a masive benthos, while organic enrichment from biodeposition changes the specific composition of soft-bottom benthos. In the deeper areas, (less than six metres), where summer thermoclines limit oxygen transfer from surface water, the organically enriched substrate induces oxygen depletion and ammonium and nitrogen sulfide accumulation in the water column. This ecosystem dysfunction kills benthic populations, and sometimes reaches pelagic populations and affects the shellfish farming economy.

PETROLEUM HYDROCARBONS AND OYSTER RESOURCES OF GALVESTON BAY, TEXAS

ABSTRACT Field and laboratory studies of petroleum hydrocarbons in the tissues of the American oyster, Crassostrea virginica, a primary shellfish resource in the Galveston Bay system, were conducted from 1971–1974. Initial ultraviolet spectrophotometric and gas chromatographic analyses of tissues revealed significant amounts of oil-derived petroleum hydrocarbons from oysters collected at Morgan's Point Reef at the lower end of the Houston Ship Channel. Lower values, when detectable, were found in oyster meats collected at natural and artificial reefs scattered throughout the lower bay system. Laboratory and field studies were conducted to determine uptake and depuration of petroleum hydrocarbons by oysters. Rapid depuration of petroleum hydrocarbons accumulated in field and experimental exposures was found. In oil-free seawater, oysters released saturated chains and most aromatic fractions rapidly with depuration to below detectable levels (0.1 ppm) taking place within 52 days. Transfer of oysters for depuration purposes shows promise of improving the overall quality of this shellfish resource.