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Habitat engineers make strong and far-reaching imprints on ecosystem processes. In intertidal mudflats, the dominant primary producer, microphytobenthos (MPB), often forms high biomass patches around oyster reefs. We evaluate multiple hypotheses linking MPB with oyster reefs, including oyster biodeposition, meiofaunal grazing, and abiotic factors, aiming to help predict effects of reef removal or proliferation. We quantify spatial patterns of an Atlantic mudflat community and its environment around two large Crassostrea reefs before experimentally sacrificing one reef via burning. MPB biomass was enriched surrounding living oyster reefs although infaunal biomass and individual sizes were low. Structural equation modelling best supported the hypothesis that crab predation intensity, which decayed with distance from the reefs, locally freed MPB from grazing. Our results suggest that Crassostrea reef expansion may enrich local MPB patches and redirect trophic energy flows away from mudflat infauna, with potential implications for the sustainability of local fisheries and bird conservation.

Eight different experimental substrates were tested on Paracentrotus lividus competent larvae in order to evaluate their potential for inducing metamorphosis and enhance survival after recruitment. Two benthic diatoms species, Nitzschia laevis (NL) and Halamphora coffeaeformis (HC), were selected according to their capacity to adhere and to form strong biofilms. They were tested in monocultures and in a mixed biofilm (MIX) that was also tried in combination with Gamma-Aminobutyric Acid, involved in triggering some invertebrate metamorphosis (MIX + GABA). Histamine (HIS) was also used as a treatment according to the high metamorphosis rates that have been recorded for this compound on other sea urchin species. Finally, a natural microphytobenthic biofilm (NATURAL) and oyster shells particles colonized by epiphytic diatoms (SHELL) were sampled from the mud of a refining oyster pond. Batches of 21 days-old larvae were placed on each experimental substrate and their effect was compared to a negative control of filtered sea water (without any treatment; FSW). Metamorphosis rate was daily recorded in each treatment. The sea urchin larvae on substrates NL, NATURAL, GABA + MIX and SHELL showed significantly higher metamorphosis rates than larvae on the other treatments (P

Haslea ostrearia is a marine diatom that synthesizes and releases marennine, a water-soluble blue-green pigment responsible for the greening of the gills and labial palps of bivalves. The present study evaluated the effect of different marennine concentrations (0, 0.5, 1.0 and 2.0 mg l− 1) on the behavior (valve opening), physiology (clearance rates, oxygen consumption, assimilation efficiency and scope for growth) and biochemistry (fatty acid composition of neutral and polar lipids) of two commercially important bivalves, the blue mussel Mytilus edulis and the eastern oyster Crassostrea virginica. Under short-term (

Haslea ostrearia is a marine diatom known to produce marennine, a water-soluble blue-green pigment responsible for the greening of oysters in ponds along the French Atlantic coast. This phenomenon occurs seasonally when H. ostrearia blooms in oyster ponds, and it increases the economic value of cultured oysters. From an ecological perspective, H. ostrearia blooms are accompanied by a decrease in the abundance of other microalgae, suggesting that this diatom produces allelochemicals. Recent studies showed that purified marennine has other biological activities, for instance antioxidant, antibacterial, and antiviral activities, which could be used in aquaculture to promote this pigment as a natural antipathogen agent. One important issue regarding the possible use of H. ostrearia in aquaculture as a mixed algal diet, however, is the importance of marennine allelopathy. In this study, we investigated the allelopathic effect of H. ostrearia on the growth of five microalgal species relevant to aquaculture: Chaetoceros calcitrans, Skeletonema costatum, Phaeodactylum tricornutum, Tetraselmis suecica, and Tisochrysis lutea. Allelopathic tests were realized by co-culturing these microalgae with H. ostrearia in batch and in semi-continuous mode, based on initial biovolume ratios. Our findings showed that inhibition of the growth of microalgae due to the presence of H. ostrearia and marennine was species dependent. Skeletonema costatum, C. calcitrans, and T. lutea were significantly more sensitive, whereas T. suecica and P. tricornutum appeared to be more resistant. Growth irradiance significantly influenced the allelopathic effect against the sensitive species S. costatum, and the H. ostrearia production of marennine increases with irradiance. Data presented in this study partly support the hypothesis that marennine released into the culture medium possibly acts as an allelochemical compound, thus explaining the dominance of H. ostrearia and the loss of sensitive algae in oyster ponds, but also that some species are insensitive, which allows co-culturing and use in a mixed algal diet in aquaculture.

The suitability of two red algae species, the introduced Grateloupia turuturu and the native Palmaria palmata, as feed for the culture of the European abalone Haliotis tuberculata, was compared over a 5-month period. Three experimental diets were tested: (1) P. palmata, (2) G. turuturu, and (3) a mixed diet of P. palmata and G. turuturu (1:1). Biochemical composition (proteins, carbohydrates, lipids, ashes) of algae was measured. No mortality was observed during the time of the experiment in any of the treatments. Growth in length and weight was higher for abalone fed with P. palmata and the mixed diet. Abalone exhibited a preference for P. palmata and showed the highest weight gain with this macroalgae (107.8 ± 7.2 %). Grateloupia turuturu disintegrated faster in abalone rearing conditions and was not suitable for significant growth of H. tuberculata in a monospecific diet. However, when it is provided in a mixed diet, abalone seem to show a higher lipid content than when fed a P. palmata monospecific diet. The invasive G. turuturu could be an interesting food supplement for European abalone.

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. Age at competence was reached at 15 days post-fertilisation for all diets. Larval survival rate and growth in length and width were higher for larvae fed with Rhodomonas sp. These larvae also had the highest lipid content. Post-settlement survival was higher in larvae fed with the combined diet. Larvae in all treatments demonstrated normal growth and development, but Rhodomonas sp. presented nutritional advantages for sea urchin larvae: a more rapid development and a higher lipid content. The use of Rhodomonas sp., already widely used in oyster aquaculture, as a diet in Paracentrotus lividus culture could be a factor in enabling oyster farmers to diversify towards “echinoculture” and increase their revenue.

International audience; Metamorphosis induction cues (by chemical mediation or direct contact) were tested in Paracentrotus lividus using three different macroalgae treatments: Corallina sp., Palmaria palmata, and Laminaria digitata. Higher percentages of metamorphosis were reached in Paracentrotus lividus larvae by direct contact with a Palmaria palmata substrate.

Although water temperature is thought to be an important factor in the proliferation of the invasive slipper limpet Crepidula fornicata along the northern European Atlantic coast, evidence for this proposition is scarce. We used quantitative histology to compare the reproductive dynamics over 2 temporally-separated sexual cycles (2000−2001 and 2006−2007) from the same C. fornicata population in Bourneuf Bay, France. Water temperature and chlorophyll (chl) a data were also analyzed in relation to historical trends and to reproductive processes. Historical temperature data show a progressive warming trend, especially since 1995, when the slipper limpet problem began to be severe. Similarly, the incidence of high chl a peaks increased markedly from 1996 onwards. Significantly higher water temperatures and more chl a peaks were found for the 2006−2007 sampling period compared to 2000−2001, and corresponded to (1) an increase in duration of brood presence, most notably the appearance of broods earlier in the year, and (2) an increase in intensity of gametogenesis in 2006−2007. These results support the hypo thesis that increased Northern European water temperatures enhance the reproductive success of C. fornicata, both through more favourable gametogenic/brooding temperatures and through enhanced phytoplankton availability.

At least part of the invasive success of the slipper limpet, Crepidula fornicata, in European waters must be due to reproductive characteristics, yet the events underlying the easily-observed brooding and non-brooding periods have not yet been studied in this species. The reproductive system dynamics were therefore investigated using topological histology and quantitative histological techniques. Specimens were sampled twice monthly for 18 months from Bourgneuf Bay, France, a mid-latitudinal point in the European distribution of C. fornicata. Both the testicles and ovaries showed active and resting phases, corresponding to the brooding and non-brooding periods, respectively. Maximum spermatozoan production corresponds to the female brooding period (female incubation of oviposited eggs, mid-March to late August), and allows males to possess full spermatozoan stocks at the height of fresh mature oocyte availability. The year-round presence of mature oocytes in the female gonad is misleading, since the histological aspect reveals that they are vestigial oocytes which slowly degenerate during the brooding period, possibly providing metabolites for the developing oocytes that become increasingly abundant during this period. A complete scheme of the C. fornicata reproductive cycle is presented, showing the events in the major reproductive organs. The seminal vesicle shows high inter-month variability in sperm presence, suggesting year-round copulation and sperm storage in the seminal receptacle. The seminal receptacle shows a uniform covering of spermatozoa throughout the year, suggesting rapid renewal after fertilization, again in line with multiple copulation throughout the year. Given the limited available space on the seminal epithelium, against which all spermatozoa abut, as well as polyandrous copulation, it is postulated that sperm competition may take place.

Sabellaria alveolata is a tube-building gregarious polychaete that constructs large biogenic reefs. In macrotidal shellfish ecosystems, this species competes for food with cultivated suspension-feeders. The suspension-feeding activity and clearance rate of S. alveolata were investigated in response to changes in seston concentration. A flow-through system was designed to study 225 cm(2) reef blocks with more than 500 individuals. The experimental conditions were characterized by increasing concentrations of suspended particulate matter ranging from 6.5 to 153.8 mg L-1, while the organic content of the diet (microalgae Skeletonema costatum) decreased inversely from 49 to 9%, to mimic the dilution of organic matter by inorganic particles, characteristic of tidal resuspension. We showed that the clearance rate exponentially decreased in relation to an increase in SPM concentration. Clearance rate was estimated at 5.3 10(-3) Lh(-1) or 0.93 L h(-1) g(-1) (dry weight) for the lowest seston concentration (SPM = 6.5 mg L-1) and reached the asymptote at CR = 1.97 10(-3) L h(-1) or 0.35 L h(-1) g(-1) (dry weight) when SPM exceeded 45 mg L-1. Using picture analyses of polychaete movements, we showed that, paradoxically, an increase in SPM concentration did not adversely affect the feeding activity of S. alveolata since the number of filtering individuals remained stable from SPM = 6.5 to 55.5 mg L-1. These values were applied at the scale of the bay of Mont-Saint-Michel (France) to demonstrate that the filtration pressure of large populations of wild suspension-feeders should not be underestimated when the carrying capacity has to be assessed in the context of increasing bivalve cultures. (C) 2009 Elsevier B.V. All rights reserved.

Strategies for biomass control are actively sought in response to proliferation of the introduced, suspension-feeding gastropod Crepidula fornicata along European coasts. Among these, dredging, crushing and on-site rejection may only be envisaged if the risk of further dissemination through the release of gametes and of excapsulated embryos/larvae is found to be nil or extremely low. This study evaluates such risk using three approaches: (1) deter- mination of the periods of brood presence/absence, (2) external fertilizations of gametes obtained by gonad stripping, (3) cultures of mechanically-excapsulated embryos and larvae. A period of near- or total brood absence was observed between September 2006 and January 2007. The few zygotes (1.5% to 7.5%) obtained from external fertilizations did not proceed beyond the 4-cell stage. Excapsulated young embryos and mid-stages died in less than 4 days of culture. Despite their ability to swim, grow and feed, excapsulated veligers all died before metamorphosis. No juveniles were obtained from external fertilizations and excapsulated cultures, indicating that the release of crushed catches directly at sea could not present a dissemination risk from the standpoint of gamete or prematurely-excapsulated larval disper- sal. Furthermore, by carrying out the crushing operations during the period of brood absence, the dissemination risk becomes nil. Negative ecological impact of such operations could be offset by attraction of motile predators, as yet unaccustomed to this introduced species, and their subsequent generalization to live slipper limpets and invasive feral Crassostrea gigas. At the very least, this biomass reduction approach would concentrate bottom habitat unavailability to restricted zones, freeing up the rest of the subtidal for other, formerly displaced species, as well as reducing the pressure on the trophic carrying capacity of affected habitats.

To date, knowledge of the qualitative particle selection sites and conditions in the widely-distributed bivalve Crassostrea gigas is incomplete, having focussed either on heterogeneous particles, or on particles intentionally too large to enter the gill principal filament tracts. We used endoscope-directed sampling and the intact diatom-empty, cleaned frustule approach to unambiguously establish qualitative selection sites and the influence of seston quality (varying proportions of intact diatoms and empty, cleaned frustules) and quantity (particle loads) on the degree of qualitative selection. Normally-feeding oysters were presented test mixtures of the naturally-occurring Actinoptychus senarius (small enough to enter the gill principal filaments), and the potential selection sites (gill: dorsal and ventral collecting tracts; labial palps: anteriorly-deposited pseudofaeces), were sampled for comparison with the proportions and concentrations of the ambient medium. Qualitative selection was demonstrated at both the gills and labial palps. Gill selection efficiency was shown to be directly proportional to seston quality and quantity, using a technique independant of pseudofeces mucus content. The oyster gill is thus able to increase ingested food quality when environmental food quality is low and / or when seston concentrations are high, which is typical of oyster habitats. Palp selection efficiency was directly proportional to seston quality, but at the highest concentration tested, no qualitative selection was observed on the labial palps, probably due to overload on these smaller organs. The partial functional redundancy of these key processing organs in heterorhabdic species such as oysters and scallops may enhance their success in high-turbidity habitats.

Understanding the quality cues which govern the grazing of phytoplankton by suspension feeders is essential to understanding seston dynamics and benthic-pelagic coupling. We studied the effect of functioning cellular status on selection of two common coastal diatoms, Coscinodiscus perforatus var. pavillardii and Actinoptychus senarius, graced upon by the commercially farmed oyster, Crassostrea gigas Thunberg. Two contexts were investigated, corresponding to those naturally encountered by oysters and their prey: (i) diatoms (C. perforatus var. pavillardii) larger than the size allowing selection on the gill (hence subject to selection on the labial palps only) and (ii) diatoms (A. senarius) within the size range allowing selection on the gill (hence subject to selection on both the gills and labial palps). Both intact and naturally dead cells possessed a perifmstular envelope, but the naturally dead cells lacked intracellular organic content. Oysters were able to discriminate between intact and naturally dead (empty) cells, both in the absence of prior selection on the gill and subsequent to prior selection on the gill. In contrast to previous findings in scallops, functioning cellular status is thus an important determinant of diatom selection and ingestion by oysters. This fine degree of discrimination may have evolved as an adaptation to the high turbidity, low food-quality estuarine environment typically colonized by oysters. The reduced organic content of rejected diatom cells in oyster pseudofaeces may affect the organic content of biodeposits, and subsequently the dynamics of nutrient release to the water column during mixing and resuspension.

The plasticity and function of the pallial organs were studied in the Pacific oyster Crassostrea gigas from three sites of Bourgneuf Bay (French Atlantic coast, 46–47°N, 1–2°W) characterized by different turbidity conditions. Labial palp area was closely and positively related to the turbidity gradient. No clear pattern was established between the gill area and the gradient of suspended particulate matter (SPM). The functional responses induced by these morphological variations were investigated in the laboratory by means of ecophysiological experiments and endoscopic observations. Oysters with different pallial organ areas were supplied with mixed suspensions of heat-killed Tetraselmis suecica and living Skeletonema costatum added to different concentrations of kaolinite to simulate low (SPM = 8.5 ± 0.4 mg l − 1 ) and high (SPM = 48.3 ± 1.4 mg l − 1 ) turbidity conditions. At each SPM concentration, heat-killed T. suecica were preferentially rejected in pseudofaeces compared to S. costatum , indicating a preingestive particle selection. At low seston load, clearance rate (CR) was closely and positively related to gill area and particle selection occurred only on the gills, between the ventral grooves and dorsal tracts. At higher seston load, palps exhibited a particle-sorting capacity dependent on gill area. Indeed, with small gills, an increase in selection efficiency (SE) and CR was positively related to palp area. On the other hand, large gills processed the particles without an effect of palps but with a decrease in CR. The functional responses associated with pallial organ variations clearly showed that the preingestive particle processing in oysters is an integrated mechanism dependant on the gill and labial palp areas.

The high-speed videos provided by Riisgard & Nielsen (2006; this volume) allow new insights into Sabellaria alveolata feeding processes, but the interpretation of these video observations is compromised both by artifacts of sample preparation and premature dismissal of our findings in Dubois et al. (2005; Mar Ecol Prog Ser 301:159-171). S. alveolata appears clearly to be an active suspension-feeder, while not using grouped lateral cilia to catch particles directly from the water col- umn, but by using both the grouped lateral cilia and grouped frontal cilia to modify current patterns around tentacular filaments to entrain suspended particles. Evidence of inter-tentacle variability in particle processing and bi-directional transport of particles make the calculation of the clearance rate based on ciliary band inappropriate by clearly overestimating the actual clearance rate of S. alveolata.

We investigated the response of the suspension-feeding activity of the prosobranch gastropod Crepidula fornicata (L.) to changes in seston abundance and composition, characteristic of the natural particle variability experienced by suspension-feeding organisms as a consequence of tidal resuspension. The experimental conditions were characterized by seston concentrations (12.2 to 196.0 mg 1 -1 ) inversely related to the organic content of particulate matter (56 to 6%). The organic fraction was composed of 2 benthic diatoms, Entomoneis pseudoduplex (Smith) and Nitzschia aurariae (Cholnoky). Inorganic matter was composed of 2 to 3 μm diameter kaolinite particles. The functional unit used for determination of ecophysiological responses was a chain of 6 individuals with mean chain dry tissue weight of 1.05 g. The efficiency of particle retention as a function of particle size increased with increasing particle size from 2 to 4 μm (equivalent spherical diameter). Above 4 μm, all particles were cleared from suspension with 100% efficiency, but retention efficiencies of smaller particles significantly declined with increasing seston loads. A constant clearance rate (CR) was found throughout the seston range, with a mean of 0.76 ± 0.05 1 h -1 g -1 (±SE). Filtration rate increased linearly with seston concentration from a minimum value of 10 mg h -1 g -1 to a maximum of 42.8 mg h -1 g -1 at the highest concentration. The total biodeposition rate (sum of pseudofaeces and faeces) followed a similar linear trend, with an increase from 3.8 to 41.6 mg h -1 g -1 at the highest concentration. Absorption rate was very stable throughout the range of suspended particulate matter concentrations tested with a mean of 4.9 ± 0.5 mg h -1 g -1 (±SE). It was not possible to ascertain pre-ingestive selection, even though the organic content of biodeposits and diet was very similar. The low retention efficiency of small inorganic particles at high seston loads was the sole mechanism that was clearly identified in this study as counteracting food dilution by inorganic material, through the enrichment of the organic content of filtered matter. The absence of pre-ingestive selection in C. fornicata would increase the physiological significance of this process in turbid environments.

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 degrees 58'N), and wild C gigas began to colonize coastal areas such as our study site, Bourgneuf Bay (1 degrees-2 degrees W, 46 degrees-47 degrees 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 oyster-fanning 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 oyster-farming practices and to destroy wild oyster stocks in concession areas. (c) 2006 Elsevier B.V. All fights reserved.

Sabellaria alveolata, a ciliary suspension feeder, is a reef-building polychaete widely distributed in European waters, and is an important element of the trophic web of the ecosystems they have colonised. The lack of data concerning the feeding mechanisms of polychaete sabellariids is an impediment to understanding their roles in these systems. In this study, we combine a descrip- tive approach based on SEM observations and histological mapping of mucocytes with video endo- scopic recordings to provide a functional interpretation of particle capture and processing mecha- nisms. It is hypothesised that particle capture in S. alveolata is based on the beat of grouped cilia. The cilia types and distributions of tentacles are described, and both downstream and upstream collect- ing mechanisms are demonstrated in this species. Other feeding characteristics, such as the flicking of tentacles, are involved in particle capture. Feeding mechanisms suggest previously unsuspected similarities with lophophorids. The role of mucus in S. alveolata was characterised in the tentacles, grooves and buccal cavity, and the functional specialisation of organs in transport or rejection of par- ticles was confirmed with video recordings. Tentacles and grooves collect and transport particles towards the mouth, and only viscous acid mucopolysaccharide-secreting mucocytes occurred on these organs, protecting particles from being removed with currents. The presence of mixed muco- cytes on the cephalic floor, situated just posterior to the mouth, suggests that this area is involved in rejection of pseudofaeces. The implications of these results are discussed in terms of environmental adaptation and are compared to other families of ciliary suspension-feeding polychaetes.

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 Coscino- discus 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 con- trast, 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 oyster- dominated ecosystems.

L'affinage traditionnel des huitres creuses, pratique dans les claires ostreicoles de la facade Atlantique francaise, est soumis aux fluctuations des facteurs environnementaux. Un procede d'affinage controle en terme de matieres organique et inorganique particulaires (MOP et MIP) et de temperature a ete developpe afin de reduire la variabilite de croissance de la chair des huitres. Cet elevage intensif repose sur la production de la diatomee Skeletonema costatum, distribuee aux huitres avec une concentration moyenne de 4–5 mg de MOP·L–1. Un modele ecophysiologique de l'huitre Crassostrea gigas, simulant les croissances du soma et des reserves–gonades, a ete applique aux conditions d'affinage controle dans le but d'analyser les reponses du bivalve. Son elaboration a necessite un retour a l'experimentation. Deux fonctions d'alimentation ont ete etudiees au laboratoire : la filtration et la production de pseudofeces, intervenant dans la regulation de l'ingestion. Les resultats, pour une temperature de 14 °C, et pour une gamme de MOP et de MIP variant respectivement de 4 a 18 mg·L–1 et de 15 a 55 mg·L–1, montrent que l'ingestion est regulee par la production de pseudofeces, la filtration ne presentant pas de variation significative (moyenne de 2,09 ± 0,11 L·h–1·g–1, avec un taux d'activite de 59%). Cette production de pseudofeces, qui engendre une augmentation de la fraction organique ingeree par le mecanisme de selection pre-ingestive, permet de compenser la variabilite des concentrations en MIP. Des simulations ont permis d’analyser les effets negatifs de ce facteur environnemental sur la croissance du bivalve. Il apparait alors que la ration utilisee en affinage controle (4–5 mg de MOP.L–1) permet une croissance en chair seche malgre des concentrations simulees atteignant 50 mg·L–1 de MIP. Lors de l'elaboration du modele pour les conditions specifiques a l'affinage controle, la gametogenese est apparue comme un processus determinant pour la repartition de l'energie chez le bivalve.

Variations in gill, palp, and adductor muscle areas and in the length and spacing of eulatero-frontal cirri (EFC) were investigated in specimens of the Japanese oyster (Crassostrea gigas) within the same bay in two nearby sites (15 km) differing in turbidity. Significant variations occurred on a small geographical scale. In particular, individu - als from the higher turbidity zone had smaller gills and larger labial palps. For a standard oyster (1 g dry body weight), gill area was 25% larger in oysters from the low-turbidity zone, while palp area was 66% larger in oysters from the high-turbidity zone. No difference was observed for total adductor muscle area. Intersite differences in the mean length of EFC and in the distance between two adjacent cirri sampled on ordinary filaments in the midregion of the gill were studied by image analysis. Cirri tended to be longer and more evenly spaced in oysters from the higher turbidity zone, although differences were not significant at the 5% probability level. The geometric characteristics of the EFC were apparently not modified in response to high turbidity, although the wide individual variability observed may have prevented detection of ciliary adaptation. Resume : Les variations de surface de la branchie, des palpes labiaux et du muscle adducteur ainsi que la longueur et l'espacement des cirres eulatero-frontaux (CEF) ont ete etudies chez deux populations d'huitres japonaises ( Crassostrea gigas), prelevees dans deux sites proches (15 km) mais de turbidite differente, a l'interieur d'une meme baie. Des va- riations significatives ont ete observees a cette faible echelle geographique, les individus de la zone plus turbide ayant

Bivalves, by releasing inorganic nutrients, can stimulate the growth of the primary producers on which they feed. Dissolved excretion and mineralization of biodeposits (pseudofaeces and faeces), two indirect processes related to their feeding activity, have already been identified. This study investigated whether microalgal growth is also dependent on the mucus secreted by bivalves and rejected with their biodeposits. An experimental study based on algal growth tests was conducted to determine whether the mucus coating the pseudofaeces and faeces of the oyster Crassostrea gigas (Thunberg) could stimulate the growth of nine marine microalgae: a Prasinophyceae [Tetraselmis suecica (Butcher)] and eight Diatomophyceae [Amphora sp. (Ehrenberg), Chaetoceros sp. (Ehrenberg), Entomoneis alata (Ehrenberg), Haslea ostrearia (Simonsen), Nitzschia acicularis (Wm Smith), N. closterium (Wm Smith), N. gandersheimiensis (Krasske) and Skeletonema costatum (Cleve)]. Oysters were fed only inorganic particles at a concentration well above the threshold of pseudofaeces production. A fraction of pseudofaeces and faeces was then inoculated into glass tubes containing a single algal species. Algal growth tests showed that mucus stimulated the growth of four of the nine species studied: Chaetoceros sp., H. ostrearia, N. gandersheimiensis and T. suecica. The strongest responses were observed with H. ostrearia and Chaetoceros sp., for which mean algal biomass was significantly higher with mucus originating from pseudofaeces than faeces. Monitoring of the release of dissolved components from mucus showed significant enrichment in inorganic phosporus. The use of an antibiotic treatment suggested that this dissolved component is released by simple solubilization rather than by bacterial breakdown., Les bivalves, en relarguant des sels nutritifs, peuvent stimuler la croissance des microalgues dont ils se nourrissent. L'excrétion dissoute et la minéralisation de leurs biodépôts (pseudofèces et fèces), deux processus liés à leur alimentation, ont déjà été identifiés. Dans ce travail, il est envisagé que la croissance des microalgues puisse être aussi stimulée par le mucus sécrété par les bivalves et rejeté dans leurs biodépôts. Une étude expérimentale fondée sur des tests de croissance algale a été réalisée afin de déterminer si le mucus enrobant les pseudofèces et les fèces de l'huître Crassostrea gigas (Thunberg) pouvait stimuler la croissance de neuf microalgues marines : une Prasinophycée [Tetraselmis suecica (Butcher)] et huit Diatomophycées [Amphora sp. (Ehrenberg), Chaetoceros sp. (Ehrenberg), Entomoneis alata (Ehrenberg), Haslea ostrearia (Simonsen), Nitzschia acicularis (Wm Smith), N. closterium (Wm Smith), N. gandersheimiensis (Krasske) et Skeletonema costatum (Cleve)]. Les huîtres sont alimentées uniquement par des particules inorganiques à une concentration très supérieure au seuil de production des pseudofèces. Une fraction des pseudofèces et fèces est alors inoculée dans des tubes en verre contenant une espèce de microalgue. Les tests de croissance algale montrent que le mucus stimule la croissance de quatre des neuf espèces étudiées : Chaetoceros sp., H. ostrearia, N. gandersheimiensis et T. suecica. Les réponses les plus fortes sont obtenues avec Chaetoceros sp. et H. ostrearia, pour lesquelles la biomasse algale moyenne est significativement plus importante avec le mucus provenant des pseudofèces qu'avec celui provenant des fèces. Un suivi du relargage de composés dissous par le mucus montre un enrichissement significatif en phosphore inorganique. Un traitement antibiotique suggère que ce composé dissous est libéré par simple solubilisation du mucus plutôt que par dégradation bactérienne.

In estuarine ecosystems, microphytobenthos resuspended by tidal currents often represents a large part of the food supply available to bivalves which feed selectively. This study investigated the feeding behavior of the Pacific oysterCrassostrea gigas (Thunberg) relative to a natural microphytobenthic assemblage and the effect on the structure of this assemblage. Oysters were fed only benthic microalgae collected on the intertidal mud flats of Bourgneuf Bay (France) at a suspended particulate matter concentration above the threshold of pseudofeces production. All species in the assemblage were endemic diatoms characteritic of tidal mud flat environments. Four dominant ones, which were all solitary cells with spear-shaped frustules [Navicula ammophila (Grunow),Navicula rostellata (Kutzing),Plagiotropis lepidoptera (Kuntze), andStaurophora amphioxys (Mann)] represented more than 95% of the 16 species involved. Analysis of feeding processes showed that the retention of the four main diatoms was not significantly different, but that two species, the smallest (N. ammophila [22×4 μm]) and the largest (P. lepidoptera [60×15 μm]), were preferentially ingested. The study of post-ingestive selection revealed that these two species were also preferentially digeted, i.e., preferentially directed into the digestive diverticulum, when they passed through the gut ofC. gigas. Cell size and shape did not appear to account for pre-ingestive and post-ingestive selection. The composition of the assemblage was significantly modified by oyster filtration, although the retention rates of the four main species were not significantly different. The composition of microalgae in pseudofeces and feces as a result of pre-ingestive and post-ingestive selection differed from that in seawater.

The revival capacity of eight species of diatoms rejected in pseudofaeces and faeces of a filter-feeder, the Pacific oyster Crassostrea gigas Thunberg, was assessed by experimental culture tests. Isolated from oyster fattening ponds near Bourgneuf Bay on the French Atlantic coast (46–47°N, 1–2°W), the eight diatom species were: Amphora sp., Chaetoceros sp., Entomoneis alata, Haslea ostrearia, Nitzschia acicularis, Nitzschia closterium, Nitzschia gandersheimiensis and Skeletonema costatum. The results show that none of the diatoms were totally digested and that all underwent significant growth. Cell concentrations with pseudofaecal cultures were higher for all species excepted Haslea ostrearia for which the difference with faecal cultures was not significant. This result is consistent with the fact that diatoms rejected in faeces are processed through the bivalve gut, while pseudofaeces are simply formed on the gill and the palps. Nevertheless, the significant biomass obtained with faecal cultures compared...