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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

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Abstract. Satellite remote sensing (RS) is routinely used for the large-scale monitoring of microphytobenthos (MPB) biomass in intertidal mudflats and has greatly improved our knowledge of MPB spatio-temporal variability and its potential drivers. Processes operating on smaller scales however, such as the impact of benthic macrofauna on MPB development, to date remain underinvestigated. In this study, we analysed the influence of wild Crassostrea gigas oyster reefs on MPB biofilm development using multispectral RS. A 30-year time series combining high-resolution (30 m) Landsat and SPOT data was built in order to explore the relationship between C. gigas reefs and MPB spatial distribution and seasonal dynamics, using the normalized difference vegetation index (NDVI). Emphasis was placed on the analysis of a before-after control-impact (BACI) experiment designed to assess the effect of oyster killing on the surrounding MPB biofilms. Our RS data reveal that the presence of oyster reefs positively affects MPB biofilm development. Analysis of the historical time series first showed the presence of persistent, highly concentrated MPB patches around oyster reefs. This observation was supported by the BACI experiment which showed that killing the oysters (while leaving the physical reef structure, i.e. oyster shells, intact) negatively affected both MPB biofilm biomass and spatial stability around the reef. As such, our results are consistent with the hypothesis of nutrient input as an explanation for the MPB growth-promoting effect of oysters, whereby organic and inorganic matter released through oyster excretion and biodeposition stimulates MPB biomass accumulation.MPB also showed marked seasonal variations in biomass and patch shape, size and degree of aggregation around the oyster reefs. Seasonal variations in biomass, with higher NDVI during spring and autumn, were consistent with those observed on broader scales in other European mudflats. Our study provides the first multi-sensor RS satellite evidence of the promoting and structuring effect of oyster reefs on MPB biofilms.

The objective of this study was to optimise the rearing of the sea urchin Paracentrotus lividus (Lamarck, 1816) larvae through the choice of an optimal diet. The effects on larvae reared in oyster facilities and using different microalgae species were analysed. Four experimental diets were tested: 1) Isochrysis aff. galbana (Clone T-ISO), 2) Dunaliella tertiolecta, 3) Rhodomonas sp. and 4) a combined diet of these three species (1:1:1). The biometrics of larvae were carried out every two days. Post-larval survival was assessed when competence for settlement was achieved. Induction of settlement was carried out by contact between larvae and oyster shell particles. This method, adapted from oyster farming, was used for the first time in sea urchin culture. After 9 days post-settlement, metamorphosed juveniles were sampled and post-settlement survival was assessed. The biochemical composition (proteins, carbohydrates, lipids) of microalgae and larvae was measured.

Trophic ecology is the study of feeding interactions and food acquisition by organisms. It includes the causes and consequences of those behaviours at all levels of biological organisation. As a field of research, it crosses many disciplinary boundaries and provides knowledge that is pertinent to many other areas of ecology. Here we list and categorise the methods available to trophic ecologists whose toolbox has broadened considerably in recent years. They encompass empirical and numerical approaches with focus ranging from molecules to ecosystems. We further examine the relationship of each method to features such as the scale of observation (from microbes to largest organisms) and organisational level (from individuals to ecosystems) as well as the ecological question the method is capable of answering (from detecting predator-prey relationships to studying implications and consequences at different scales). Our survey reveals a very wide range of methodologies, each more-or-less appropriate for a particular line of research. It also identifies deficits, for example, trophic interactions at microscopic scales, for which empirical methods have hardly been used, as well as trophic models that have failed to consider fluxes at the ecosystem scale. Furthermore, we note that the combination of methodologies remains under-exploited despite great opportunities to solve complex ecological questions and to foster the emergence of new insights and hypotheses regarding organism, population and/or ecosystem properties.