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Asari (=Manila) clam, Ruditapes philippinarum , is the second bivalve mollusc in terms of production in the world and, in many coastal areas, can beget important socio-economic issues. In Europe, this species was introduced after 1973. In Arcachon Bay, after a decade of aquaculture attempt, Asari clam rapidly constituted neo-naturalized population which is now fished. However, recent studies emphasized the decline of population and individual performances. In the framework of a national project (REPAMEP), some elements of fitness, stressors and responses in Arcachon Bay were measured and compared to international data (41 publications, 9 countries). The condition index (CI = flesh weight/shell weight) was the lowest among all compared sites. Variation in average Chl a concentration explained 30% of variation of CI among different areas. Among potential diseases, perkinsosis was particularly prevalent in Arcachon Bay, with high abundance, and Asari clams underwent Brown Muscle Disease, a pathology strictly restricted to this lagoon. Overall element contamination was relatively low, although arsenic, cobalt, nickel and chromium displayed higher values than in other ecosystems where Asari clam is exploited. Finally, total hemocyte count (THC) of Asari clam in Arcachon Bay, related to the immune system activity, exhibited values that were also under what is generally observed elsewhere. In conclusion, this study, with all reserves due to heterogeneity of available data, suggest that the particularly low fitness of Asari clam in Arcachon Bay is due to poor trophic condition, high prevalence and intensity of a disease (perkinsosis), moderate inorganic contamination, and poor efficiency of the immune system. [ABSTRACT FROM AUTHOR]

The present study tested the ‘Enemy Release Hypothesis’ (ERH) which states that the success of an introduced species is related to the scarcity of natural enemies in the introduced range compared with the native range. Digeneans are dominant macroparasites of molluscs; therefore, the interaction between R. philippinarum and these parasites was ideal for investigation. A two-year monitoring in Arcachon Bay (SW France) was performed to estimate digenean loads in R. philippinarum and in three infaunal native bivalves ( R. decussatus, Paphia aurea, Cerastoderma edule). A laboratory experiment allowed comparison of infection success among these bivalves (except P. aurea) by generalist digenean larvae ( Himasthla elongata cercariae) . R. philippinarum digenean abundance in Arcachon Bay was much lower than in native bivalves, with values depending on species, sites and time. Similarly, mean digenean species richness per host individual was always lower in R. philippinarum than in sympatric bivalves. A comparison of digenean metacercariae abundance between R. decussatus and C. edule in Mundaka Estuary (Spain) showed that both species had similar digenean loads but that R. decussatus was depleted in digenean species encysting in host tissues (the non-gymnophallid species). Experimental infection confirmed that the two species of the genus Ruditapes (and not R. philippinarum only) were resistant to encysting digeneans, with an infection success 3–5 times lower than that of C. edule. The lack of infection that was observed in the field would therefore be the consequence of a tissue barrier, R. philippinarum epithelium being too tough for cercariae penetration. Concordantly, according to the literature, digenean infection in the native range of R. philippinarum is also low. Consequently, the ERH, as an explanation for R. philippinarum success in Europe, is not totally consistent in the case of digenean trematodes as enemies, R. philippinarum hosting low load of digeneans in its native as well as colonized range. [ABSTRACT FROM AUTHOR]

Some newly-settled bivalve molluscs can experience a second dispersal stage in the water column and colonize areas distant from initial settlement zone ('secondary settlement'). To document mechanisms involved in such a process, experiments were conducted in a recirculating 13-m flume, using juvenile cockles Cerastoderma edule (shell length between 0.8 and 5.7 mm). After 4 h and under current surface velocities of 10, 20 and 24 cm s-1, all juveniles left the plexiglass substratum (site of initial introduction) and 42.4, 58.6 and 76.2% of juveniles, respectively, were retrieved from a downstream sand area (which only represented 7.7% of the total flume surface). Naked-eye observations showed that smaller individuals were borne within the water column, whereas larger individuals tended to roll or slip on the substratum. Byssus threads produced by the juveniles were often seen, sometimes covered in sand grains. These observations were confirmed by finding larger juveniles in the upstream part of the sand area. When sand was replaced by mud, a reduced proportion of cockles was retrieved in the experimental substratum (18.4, 20.0 and 16.4%, respectively). Observations showed that juveniles rarely succeeded in anchoring themselves in mud. When initially introduced on a favourable substratum (medium sand), more than 87% of juveniles were retrieved from that sand array at all flow velocities. This study shows that secondary settlement occurs for juvenile cockles up to 5.7 mm in shell length and depends not only on flow velocity but on substratum type. [ABSTRACT FROM AUTHOR]

Presents information on a study conducted on two bivalves species, the cockle (Cerastoderma edule) and the Japanese clam (Ruditapes philippinarum), to determine juvenile resuspension. Factors contributing to a decrease in bivalves; Methodology used to conduct the study; Various fall velocity profiles; Conclusion of the study.

Abstract: Biotic indices based on soft-bottom macrozoobenthic communities are currently used throughout Europe to assess the ecological quality of coastal and transitional water bodies according to the European Water Framework Directive. However, the performance of the currently available biotic indices still has to be tested against a variety of different impact sources. In particular, physical perturbations have received much less attention than other kind of disturbances. This study consisted in testing the capacity of currently available uni- (BOPA, AMBI and BENTIX) and multivariate (M-AMBI) Biotic Indices to assess the ecological impact of the destruction of a Zostera noltii seagrass bed in Arcachon Bay (France) following sediment deposits. Changes of habitat after this physical perturbation were hardly assessed by any of these Biotic Indices whereas analysis of the benthic community showed drastic changes of structure following the perturbation and no recovery after 15 months. This study demonstrates that these Biotic Indices must be integrated into a multimetric approach which describes better the biological integrity of the benthic community by including a complementary set of metrics. A new multimetric approach, named MISS (Macrobenthic Index of Sheltered Systems) is proposed. MISS correctly highlighted the destruction of the seagrass beds, by using 16 metrics describing the biological integrity of the macrofauna. [Copyright &y& Elsevier]

Abstract Recently, we reported an emerging pathology named Brown Muscle Disease (BMD) affecting Asari clams inhabiting the most productive area for this species in France, the Arcachon Bay. The main macroscopic feature of the pathology relies on the atrophy of the posterior adductor muscle, affecting the ability of clams to burry. The research of the etiological agent of BMD privileged a viral infection. Contrary to healthy clams, infected animals are always found at the surface of the sediment and exhibit 30 nm virus-like particles in muscle, granulocytic and rectal cells. In order to get more insights on the etiology and impacts of the BMD on clams, we took advantage in the present study of next generation sequencing technologies. An RNA-Seq approach was used (i) to test whether viral RNA sequences can be specifically found in the transcriptome of diseased animals and (ii) to identify the genes that are differentially regulated between diseased and healthy clams. Contrary to healthy buried animals, in diseased clams one sequence showing extensive homologies with retroviridae-related genes was detected. Among the biological processes that were affected in diseased clams, the synaptic transmission process was the most represented. To deepen this result, a new sampling was carried out and the transcription level of genes involved in synaptic transmission was determined in healthy and diseased clams but also in clams with no visible sign of pathology but located at the surface of the sediment. Our findings suggest that muscle atrophy is a latter sign of the pathology and that nervous system could be instead a primary target of the BMD agent. Graphical abstract Image 1 Highlights • The etiology and impacts of the BMD on clams was studied by an RNA-Seq approach. • One sequence with extensive homologies with retroviridae-related genes was detected. • Synaptic transmission was the main biological process affected in diseased clams. • New findings support that nervous system could be a primary target of the BMD agent. [ABSTRACT FROM AUTHOR]

The Manila clam Ruditapes philippinarum (Adams and Reeve, 1850) is one of the mollusc species that, driven mainly by the shellfish market industry, has extended throughout the world, far beyond the limits of its original habitat. The Manila clam was introduced into France for aquaculture purposes, between 1972 and 1975. In France, this venerid culture became increasingly widespread and, since 1988, this species has colonised most of the embayments along the French Atlantic coast. In 2004, this development resulted in a fishery of ca. 520t in Arcachon Bay. Within this context, evaluation of the clam stock in Arcachon, as undertaken by the French Institute for the Marine Research (IFREMER) in 2000, 2003 and 2006, underlined: (a) the presence of disequilibrium in the population structure and (b) the need for an improved knowledge of the population dynamics. In order to meet these requirements, a research project was initiated. The main objective of this study was to provide an improved knowledge of the biotic and abiotic parameters which control the dynamics of the clam population, within Arcachon Bay (growth, mortality, reproduction, trophic sources, etc.). With these data, the aim of the present study is to develop a system dynamics model, capable of predicting clam population evolution within the embayment, in response to different management measures. Once the model was constructed, the effect of diverse management measures and environmental scenarios on the stock response, over time, was simulated: (i) sensitivity to exceptional environmental changes (sea water temperature); (ii) changes in the minimum legal size for capture; (iii) changes in season capture duration; (iv) an increase of the surface area of no-take zones. Considering all the assumptions made here, the best management decisions are those which are able to maximise, at the same time, the captures and the number of shellfishers; likewise, minimise stock losses, as a consequence of this exploitation. Taking into account this approach, the best management decisions are, in order of efficiency: (i) the minimum legal size for capture maintenance; (ii) a reduction in the capture season; (iii) an increase of no-fishing zones. [Copyright &y& Elsevier]

Recently, Manila clam, Ruditapes philippinarum, populations have suffered mortalities in Arcachon Bay (SW France). Mortality was associated with extensive lesions of the posterior adductor muscle, which become progressively brown and calcified. Ultrastructural observations by transmission electron microscopy revealed tissue degradation with necrotized muscle fibres and granulocytomas. Unenveloped virus-like particles (VLPs) were detected in muscle, granulocytic, epithelial and rectal cells. VLPs were abundant in the extracellular space, in the cytoplasm (free or enclosed in vesicles) and in the nucleoplasm of granulocytes. Nuclei and mitochondria of granulocytes displayed changes which suggested reactive oxygen species production and apoptosis induction. VLPs exhibited an icosahedral structure with a diameter of 25 to 35 nm. These observations suggest that the VLPs could belong to the family Picornaviridae or the Parvoviridae. [ABSTRACT FROM AUTHOR]

Abstract: This study deals with the application of macrozoobenthos-based biotic indices (BI) within the frame of the implementation of the European Water Framework Directive. More precisely, this study aimed at assessing the performance of five recently developed methodologies (BI) for the assessment of ecological quality status (EcoQ) in two semi-enclosed, sheltered coastal ecosystems and in one transitional water body situated along the Western French coast, namely Marennes-Oléron Bay, Arcachon Bay, and the Seine Estuary. This study showed that these five indices rarely agreed with each other, describing very different pictures of the overall EcoQ of the three study sites. This work also clearly underlined the limitations of these approaches, notably the dependency of most of these BI and the resulting EcoQ classifications on habitat characteristics, more particularly to natural levels of sediment silt–clay content and the location of stations in the subtidal or the intertidal. The implication of our observations concerning the use of these BI for implementation of the WFD is discussed in terms of definition of habitat-specific reference conditions and necessity to adjust thresholds to the particular habitat occurring in semi-enclosed ecosystems. Meanwhile, the unmodified use of these BI severely impaired accurate assessment of EcoQ and decision-making on the managers’ point of view. [Copyright &y& Elsevier]