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The impact of a microplastic (MP) mixture composed of polyethylene (PE) and polypropylene (PP) plastic particles, prepared from commercially available products, was evaluated in blue mussels Mytilus spp. exposed to three environmentally relevant concentrations: 0.008 µg L −1 (low), 10 µg L −1 (medium), and 100 µg L −1 (high). Organisms were exposed for 10 days followed by 10 days of depuration in clean seawater under controlled laboratory conditions. The evaluation of MP effects on mussel clearance rate, tissue structure, antioxidant defenses, immune and digestive parameters, and DNA integrity were investigated while the identification of plastic particles in mussel tissues (gills, digestive gland, and remaining tissues), and biodeposits (feces and pseudofaeces) was performed using infrared microscopy (µFT-IR). Results showed the presence of MPs only in the digestive gland of mussels exposed to the highest tested concentration of MPs with a mean of 0.75 particle/mussel (after the 10 days of exposure). In biodeposits, PE and PP particles were detected following exposure to all tested concentrations confirming the ingestion of MPs by the organisms. A differential response of antioxidant enzyme activities between digestive gland and gills was observed. Significant increases in superoxide dismutase (SOD) and catalase (CAT) activities were measured in the digestive gland of mussels exposed to the low (0.008 µg L −1) and medium (10 µg L −1) concentrations of MPs and in the gills from mussels exposed to the highest concentration (100 µg L −1) of MPs that could be indicative of a change in the redox balance. Moreover, an increase in acid phosphatase activity was measured in hemolymph of mussels exposed to 0.008 and 10 µg L −1 concentrations. No significant difference was observed in the clearance rate, and histopathological parameters between control and exposed mussels. This study brings new insights on the potential sublethal impacts of MPs at environmentally relevant concentrations in marine bivalves.

The mechanisms and sites of processing and selection of large natural algal particles were studied in the oyster Crassostrea gigas, which possesses a heterorhabdic pseudolamellibranch gill type. Endoscopic observations of processing 1000 cells ml -1 suspensions of the pennate diatoms Pleurosigma planctonicum and Rhizosolenia setigera were performed, as well as endoscope-directed sampling of 100 cells ml -1 mixed suspensions of intact (25%) and empty (75%) 150 × 200 µm Coscinodiscus perforatus. Video endoscopy was used to observe processing on the gill and to allow in vivo sampling of contents of the dorsal and ventral particle tracts, while pseudofaeces was collected from the individual flow-through chambers. Selection indices were calculated for empty C. perforatus at each processing site. Only those P. planctonicum orientated in a dorso-ventral position could enter the principal filaments (PF) for delivery to the dorsal acceptance tract. R. setigera almost never entered the PF, being prevented by its curved and twisted shape. Visual counts of intact versus empty C. perforatus were done on samples from the ambient medium, ventral and dorsal particle tracts, and pseudofaeces. They showed that the percentages of intact and empty cells in both the dorsal and ventral particle tracts were identical to those initially presented (Kruskal-Wallis test; p > 0.05). In contrast, the pseudofaeces contained over 98% empty cells (Kruskal-Wallis test; p ≤ 0.001, degree of selection ranged from + 29 to + 34%). These results show that (1) large natural particles within the size range naturally encountered by C. gigas may only be subjected to qualitative selection on the gills if their shape and dimension allow them to enter the PF, and (2) the site of selection for particles unable to enter the PF is not the gill, but rather the labial palps. Selection sites and mechanisms in heterorhabdic bivalves should therefore be studied over the entire range of naturally available particle sizes. Given the periodic dominance of large diatoms in coastal temperate waters, the labial palp processing site could be of considerable importance in the dynamics of seston flow in oysterdominated ecosystems.

The Pacific oyster, Crassostrea gigas, well known throughout the world because of its ability to adapt to a wide range of environmental conditions, was introduced for cultivation into France on a massive scale in the 1970s. With global warming, the reproductive population, confined at the beginning to the south of the French Atlantic coast, became established at more northern latitudes (above 45° 58′ N), and wild C. gigas began to colonize coastal areas such as our study site, Bourgneuf Bay (1°-2° W, 46°-47° N), an oyster-farming site. An original approach, based on orthophotograph analysis and in situ biomass sampling, revealed that, in the northern part of this bay, more than 70% of the total C. gigas biomass was composed of wild oysters (i.e. C. gigas not bred by oystermen). The analysis of the spatial distribution of wild oysters indicated that 75% of the stock consisted of wild oysters in natural beds (rocky areas) and on low retaining walls of former fisheries. Wild C. gigas also colonized oysterfarming structures with lower biomasses (21% of the stock composed of wild oysters), but locally they could reach densities of up to 55 kg.m− 1 i.e. 2.5 times the mean biomass of cultivated oysters. The economic and ecological consequences of this colonization by C. gigas of an oyster culture site are discussed. Wild oyster seems to be the principal trophic competitor of cultivated oysters in Bourgneuf Bay. This may partly explain the decrease in growth of cultivated oysters observed in this bay during the last decade. Moreover, the trophic and spatial competition exerted by wild oysters may also affect the native biota and, in particular, the honeycomb worm Sabellaria alveolata. The results obtained in this study have led oyster farmers and regional authorities to modify oysterfarming practices and to destroy wild oyster stocks in concession areas. Résumé