Growth of the mussel, Mytilus edulis, was compared for the first time under three culture regimes, longline located in open sea, pole and on-bottom both situated on an intertidal flat, in the Pertuis Breton, on the Atlantic coastline of France. Mussel sampling was performed on a monthly basis over 1 year while monitoring of hydrobiological parameters was conducted on a biweekly basis. Fluctuation of environmental parameters showed a similar pattern on both sea and shore locations with generally higher concentration levels for the intertidal area compared with the open sea, i.e. 4.07 and 3.17 mg l−1 for particulate organic matter, 3.16 and 2 μg l−1 for chlorophyll-a, 58.43 and 38.72 mg l−1 for inorganic-N, respectively. A clear seasonal growth pattern was observed, being similar for all three cultural conditions. A gradient of length and weight growth appeared as a function of the culture type. Longline mussels exhibited the highest performance while Bottom-type culture showed the lowest. An emersion time of approximately 26% was estimated from the temperature record of the Pole station during the period of maximal growth. This could partly explain the reduced growth in length on Pole compared to Longline. While growth was faster in Longline culture and condition index were better for Pole culture, further data on the carrying capacity of the area are needed for the establishment of a mussel culture extension policy.

Clearance rate (CR) was measured in blue mussels Mytilus edulis L. from Aiguillon Bay and the Oosterschelde using 3 different methods: the flow-through method, the bio-deposition method and the indirect or clearance method. CR differed significantly as a function of the method used and of the origin of the mussels. CR measured with the bio-deposition method were significantly lower than rates measured with the other methods. Results for the flow-through method depended, however, on how CR was calculated. CR using the flow-through and indirect methods was on average 10.0 l g(-1) h(-1) in mussels from Aiguillon Bay and 5.3 l g(-1) h(-1) in mussels from the Oosterschelde. The significantly lower CR of mussels from Oosterschelde was related to condition index and gill area, but could not entirely be explained by these factors.

Pacific oysters and king scallops placed individually in a recirculating flume were fed for 2 weeks with a constant concentration (120 cell ml(-1)) of a toxic strain of Alexandrium minutum. Fluorescence at the outlet of each experimental unit was measured continuously, and biodeposits were recovered twice daily to evaluate feeding time activity (FTA) and rates of organic filtration (OFR), ingestion (OIR) and organic absorption (OAR). Ion-pairing high performance liquid chromatography (IP-HPLC) was performed concurrently to quantify paralytic phycotoxins both (i) individually in shellfish at the end of the contamination period and (ii) in A. minutum cultures to estimate cellular toxin concentration. These data allowed comparison of the actual tissue toxin concentration (TOX) with the theoretical toxin accumulation rate (TAR) calculated from the linear relations between OAR, cell number, fluorescence and cell toxicity. The results show high FTA/TAR and FTA/TOX correlations (R-2 = 0.78) for both oysters and scallops. The TAR/FOX relation, though not yet clearly defined, suggests the minimum quantity of absorbed toxin at which the accumulation process is induced. (C) 2003 Editions scientifiques et medicales Elsevier SAS and Ifremer/IRD/Inra/Cemagref. All rights reserved.

An increasing number of hypotheses are being proposed to explain the faster growth potential of triploids in molluscs, including their partial sterility or their higher heterozygosity compared to diploids. Triploid advantage however, remains controversial for poorer sites, because of a potential trade-off with survival. These questions were addressed in Crassostrea gigas by deploying meiosis II triploids and their diploid siblings from a single mass spawning of three males and seven females, in two contrasting locations for their trophic resources. One hundred and fifty individuals were sampled at each site after nine months, measured for weight and biochemical composition, and genotyped using three microsatellite and seven allozyme loci. Higher performance was observed at the fast-growing site for all traits except shell weight, and triploids had greater weights and biochemical contents than diploids at harvest. Triploids also grew faster at the poorer site, and showed similar survival rates to diploids at both sites. Triploids had significantly higher average allozyme and microsatellite diversity. However, they performed better for a wide range of individual heterozygosity values, arguing for an advantage of the triploid state per se, that could be due to positive effects on growth of both sterility of triploids with subsequent resource re-allocation and possible faster transcription with three copies of each gene. Despite evidence of very low or no inbreeding in the diploid sample, positive associations between individual allozyme diversity and growth were detected, which explained little but significant amounts of phenotypic variation. These associations were interpreted as direct effects of allozymes, either alone or including epistatic interactions with other loci. In addition, measures of individual distance (mean-d2) specific to microsatellites, were negatively correlated with growth in diploids, indicating possible effects of outbreeding depression between more distant genomes of parents from distinct populations.

Triploid oysters were induced using cytochalasin B upon retention of either the first (meiosis I triploids) or the second (meiosis II triploids) polar body in embryos from a single cohort derived from mixed parentage. Allozyme and microsatellite assays enabled the confirmation of both parentage and triploidy status in each oyster. Comparison of meiosis I triploids, meiosis II triploids and diploid siblings established that improved physiological performance in triploids was associated with increased allelic variation, rather than with the quantitative dosage effects of ploidy status. An unidentified maternal influence also interacted with genotype. Among full sibs, allelic variation measured as multi-locus enzyme heterozygosity accounted for up to 42% of the variance in physiological performance; significant positive influences were identified upon feeding rate, absorption efficiency, net energy balance and growth efficiency (= net energy balance divided by energy absorbed). Whilst allelic variation was greater in both meiosis I and meiosis II triploids than in diploid siblings, both allelic variation and net energy balance were highest in triploids induced at meiosis I. This suggests that it may be preferable to induce triploidy by blocking meiosis I, rather than meiosis II as has traditionally been undertaken during commercial breeding programmes.

We have studied feeding behaviour in the cockle Cerastoderma edule (L.), the oyster Crassostrea gigas (Thunberg) and the mussel Mytilus edulis (L.) for input to a model predicting the environmental capacity of the bay of Marennes-Oléron, France for shellfish culture. We present a common set of equations that summarize functional interrelations observed between component processes of feeding physiology, whilst allowing predictions of feeding behaviour and net organic absorption rate on the basis of seston abundance and seston organic content alone. Selective processes, and the consequences of those processes, are an important feature of those interrelations. Each species was able selectively to enrich the organic content of ingested matter relative to filtered matter, preferentially rejecting inorganic matter prior to ingestion as pseudofaeces. When feeding upon natural seston, the efficiency of that selection varied positively with both the mass of seston filtered h−1 and the organic content of filtered matter. At the highest food availabilities, when the mass of seston filtered h−1 was greatest, more than 60% of the organic matter ingested h−1 by each species resulted from selective processes. Physiological consequences of that selection were amplified by positive exponential relations between the net absorption efficiency for ingested organics and the organic content of ingested matter. We show that our common set of equations satisfactorily predict net organic absorption rate measured directly in all three species feeding throughout the same natural tidal variations of food. Collective findings therefore establish that similar functional interrelations control feeding responses in each species, and identify key relations affecting selection and absorption for use in the future modelling of growth and environmental relations. By fitting a common set of equations to responses measured directly under the same natural conditions of seston availability, we have standardised the comparison of environmental influences upon rates and efficiencies of feeding behaviour for each species. The mass of seston filtered h−1 increased in similar positive relations with seston abundance in each species. However, there were significant behavioural differences in the processing of filtered particles. Compared with the epifaunal species C. gigas and M. edulis, C. edule is normally infaunal, and demonstrated a lower capacity to selectively ingest organic matter. Alternatively, compared with M. edulis, C. gigas was not as efficient either in the net selection of organic matter or in digesting and/or assimilating ingested organics, resulting in lower rates of net energy gain. These differences are discussed bearing in mind present experimental conditions, as well as the natural habitat and comparative morphology of each studied species.

Clearance and oxygen consumption rates of Ostrea edulis were measured at different temperatures (10-30 degrees C) in individual animals of a total weight ranging from 5 to 120 g (0.1 to 2.7 g dry tissue weight), outside the period of gametogenesis. For each temperature, the allometric relationship between the physiological response and dry tissue weight of the animal (DW) was estimated using the formula: Y = a * DWb. A statistical model is proposed for the variation in clearance rate or oxygen consumption rate (Y) with temperature (T) and DW: Y = [a + (b * c(T))] * DWd. The calculated value for d was 0.62 for clearance rate and 0.74 for oxygen consumption rate. The rates of both physiological functions increased as the temperature increased and reached maximum levels at 30 degrees C (clearance rate:2.83 +/- 0.32 1 h(-1) g(-1) oxygen consumption rate: 1.78 +/- 0.23 mg O-2 h(-1) g(-1)) when the flat oysters were fed a Skeletonema costatum microalgal grown in underground seawater. (C) 1998 Elsevier Science B.V. All rights reserved.

A continuous monitoring system has been developed to measure the respiratory time activity (RTA) of the oyster Crassostrea gigas (Thunberg). This activity was recorded over 24 h using this technique, at constant temperatures of 15 and 21°C in the laboratory. At 21°C, respiratory activity was measured before and after a period of starvation, during which spawning occurred. In the field, RTA was measured during two successive tidal cycles of a day. The respiratory time activity was defined as the percentage of time spent active by the oyster relative to the total experimental time. Important individual variability was noticed, percentage of time spent active ranged from 0 (RTA=0%) to 100% (RTA=100%). RTA measured on the whole experimental population, taking into account active and non-active oysters, varied from 52 to 86% under laboratory conditions and, according to the season, from 44 to 82% in the field. Considering only the active oysters (RTA>0%) which represented, according to the different experiments, from 75 to 100% of the studied population, RTA varied from 54 to 86% in the laboratory and from 59 to 94% in the field experiments. The influence of RTA on growth predictions is discussed.

Results are presented from laboratory-based experiments in which we investigated short-term responses of Mytilus edulis L. to experimental changes in the amount and composition of suspended seston. Working with large quantities of cultured algae, we have studied feeding behaviour over ranges of food availability and quality that extend well beyond earlier limits. Findings confirm the ability of mussels to selectively reject inorganic particles as pseudofaeces prior to ingestion, thereby enriching the organic content of ingested matter by 30% more than the organic content of natural filtered seston. Our findings also establish acclimation both of selective and absorptive processes, indicating that the extent to which growth of M. edulis can be stimulated in the short-term will depend upon prior nutritional history. Present maximal growth of 14.8% dry soft tissue d(-1) in a standard M. edulis of 1 g dry soft tissue was much higher than has previously been documented for any adult mussel. Maximal growth was achieved when natural seston that had been enriched to more than about 60% organic content with a mixture of algal monocultures was available at concentrations above about 11.5 mg total particulates l(-1). Neither ingestion rate nor net energy balance were improved with further increases in food availability, associated with regulatory reductions in the rate of water filtration (clearance rate) that maintained organic ingestion rate independent of increases in both the amount and organic content of available seston. These findings suggest that digestive processes had become saturated when organic ingestion reached about 6.5 mg organics g(-1) dry soft tissue h(-1) in a standard M. edulis of Ig dry soft tissue, representing as much as 20.6% of all soft tissue organic mass mussel(-1) d(-1). Such saturation of organic ingestion is consistent with previous conclusions based on comparative allometries showing that limitations to growth in M. edulis and other bivalves are associated with the rate of food processing.

The potential impact of selective grazing by filter-feeding bivalves was studied on the relative composition of both planktonic and benthic algae that are commonly suspended in coastal areas. Different feeding behaviour was observed in the oyster Crassostrea gigas and the mussel Mytilus edulis. C. gigas preferentially filtered and rejected (as pseudofaeces prior to ingestion) diatom species relative to flagellates. These differences appear to depend upon differences in algal shape and flexibility. Findings also suggest that ratios of rejection to filtration for flagellate species were influenced by the planktonic or benthic origin of the other available algal species. Future studies of trophic flux and resource utilisation should therefore consider the extent to which different filter-feeding species may preferentially filter and/or ingest separate algal species that are simultaneously available in the surrounding seston.

La diversification des especes elevees en conchyliculture est indispensable pour elargir le marche et pour minimiser les risques pathologiques qui peuvent mettre en peril l'activite professionnelle des cultures marines, orientees traditionnellement vers la monoculture. Le Laboratoire Genetique Aquaculture et Pathologique (GAP) de l'IFREMER, dans le cadre du REGEMO (Reseau Genetique Mollusques) effectue depuis 1989 un programme de selection d'huitres plates (Ostrea edulis) resistantes ou tolerantes a Bonamia ostreae pour relancer la culture de cette espece a l'echelle nationale.

An experimental flow-through system allowing determination of the feeding behaviour of individual molluscs was used to study oysters exposed to mixed diets composed of varying proportions of the diatom Thalassiosira weissflogii and two strains (toxic and nontoxic) of the dinoflagellate Alexandrium tamarense. Our results show that, when compared to a T. weissflogii unialgal diet, even a diatom/toxic dinoflagellate ratio as low in biomass as 90/10 reduced clearance rates and biodeposit production by oysters. Consumption was slightly but significantly decreased for a 50/50 diatom/toxic dinoflagellate mixture. For the toxic dinoflagellate unialgal diet, ingestion, absorption and consumption were completely inhibited. Thus, the inclusion of low amounts of toxic A. tamarense in a diet composed of T. weissflogii significantly altered pseudofaeces production and probably oyster filtering capacity, whereas no significant effect was observed with the non-toxic dinoflagellate.

Physiological processes controlling food acquisition by the filter feeding bivalve Cerastoderma edule (L.) were quantified under a broad range of seston concentrations and compositions. Experimental diets consisted of suspensions elaborated by adding variable amounts of microalgal cells of different species (or sediment particles in one case) to natural sea-water. Clearance rates exponentially decreased with seston concentration, but the rate of reduction was higher with suspensions of high organic content. Pseudofaeces production appeared as a positive function of the rate of particle filtration; however, for a given filtration rate, more pseudofaeces were rejected when filtered matter had a low organic content. As a consequence, after an initial elevation, ingestion rate remained almost constant across particle concentrations. Pre-ingestive food selection enhanced the rate of particulate organic matter ingestion and this organic enrichment of ingested matter became more pronounced for diets of low food value, where most filtered matter was being rejected as pseudofaeces. Selection of particles at the pre-ingestive level was more efficient in terms of chlorophyll, revealing preferential ingestion of algal particles compared with the whole organic matter. Stronger selection for algae, however, was not evident in terms of preferential nitrogen ingestion, as compared with carbon, which was probably due to similar low values of the CN ratios in all experimental conditions. Absorption efficiency depended on the organic content of ingested matter according to an exponential, saturating function. In general, feeding processes of cockles appear well adapted to cope with elevations in particle concentration and simultaneous reductions in the food value of available seston that occur when resuspended bottom sediments constitute a significant fraction of particulate materials of the water column. Under these conditions, high rates of seston filtration and pseudofaeces production, together with preferential organic ingestion act to compensate for the dilution of organic matter in suspension and its detrimental effect on the rate of food absorption. However, this compensatory behaviour is not so efficient as to make absorption rate independent of the organic value of available particles. The organic content of resuspensible sediments may thus become the main determinant of food acquisition in cockles.

Le programme de recherche presente ci-dessous est le fruit d'une reflexion conjointe entre geneticiens et physiologistes afin de mettre sur pied un ensemble d'experiences permettant d'etudier les bases genetiques de la variabilite des caracteres impliques dans la croissance chez l'huitre creuse, Crassostrea gigas.

Clearance and oxygen consumption rates of Crassostrea gigas were investigated with animals of 5-200 g total wet weight (0.1-3 g dry tissue weight), and at different temperatures (5-32 degrees C) after 10 days acclimation. During this period significant mortalities were observed at 32 degrees C, which may be close to the upper thermal limit for this species. For each temperature, allometric relationships between physiological rates and the dry weight (DW, g) of the animal were estimated. Clearance rate (CR, 1 . h(-1)) was maximal at 19 degrees C; oxygen consumption rate (VO2, mg O-2 . h(-1)) increased over the range of experimental temperatures (T,degrees C). Two statistical models are proposed: CR = [a-(b*(T-c)(2))]*DWd and VO2 = [a+(b*c(T))]*DWd. However, neither model is appropriate during the reproductive period.

The influence on the feeding physiology (clearance, filtration, ingestion, absorption) of the Japanese oyster Crassostrea gigas of low concentrations of suspended particulate matter (SPM ranging from 2 to 20 mg.1(-1)) and high food quality (mean organic fraction = 54 %) was studied in the laboratory. Experimental diets were made up with three strains of the ''blue diatom'' Haslea ostrearia, which is responsible of the greening of oyster-ponds. The strains were characterized by different sizes (48, 65 and 98 mu m respectively) but possessed the same density and biochemical composition. Clearance rate was found constant at 2.2 1.h(-1).g(-1) dry weight and therefore was not influenced by increasing seston load. Pseudofaeces production was observed for SPM concentration adjusted at 2.1 and 2.3 mg.1(-1), which suggests a decrease of the threshold of pseudofaeces production for diets having a high food quality. The effect of particle size on particle selection during pseudofaeces production was not demonstrated. Digestibility fluctuated from 60 to 90 %, but no differences between the three sizes were recorded. Total ingestion rate, which was regulated by pseudofaeces production, levelled off at 6.51 mg.h(-1).g(-1). Organic ingestion and organic faeces production models suggest that physiological processes in the Japanese oyster lead to a constant organic absorption estimated at 1.22 mg.h(-1).g(-1) in this study.

The aquaculture industry is often faced with the problem of toxic microalgal blooms that can cause human poisoning after contamination of cultivated shellfish. The oyster Crassostrea gigas, because of its particular sensitivity to dinoflagellates that produce paralytic toxins, was chosen as a model for study of the absorption of different monospecific algal diets of varying toxicity. A continuous, flow-through system was used to expose batches of shellfish successively to nontoxic and toxic diets of Alexandrium tamarense (7 200 ng STX eq. per 10(6) cells). The same experiment was repeated with other batches for cultures of Scrippsiella trochoidea (nontoxic) and Alexandrium minutum (500 ng eq. STX per 10(6) cells). The results indicate that the clearance rate decreases in the order Scrippsiella trochoidea > A. minutum > nontoxic A. tamarense > toxic A. tamarense. It was difficult to determine the exact nature of the physiological process enabling oysters to perform selective feeding since particle size probably interfered with toxin effects. However, C. gigas would seem to prefer very toxic Alexandrium cells during absorption in the digestive tube, perhaps because of higher nutritive value or easier digestibility.

Ecophysiological study was conducted on the japanese oyster (Crassostrea gigas) reared in Marennes-Oléron bay (France), during May 1992. Most of the somatic growth of the oyster occured during that month. The measurements were achieved from a removable laboratory stated in Le Chapus for 3 days at different tide level (coefficient of 49, 57, and 88). The food conditions were monitored through a computer with a continuous record ofthe turbidity during aIl the tidal cycle. In spite of an important variation between the food conditions in mean seston load, from 13,5 ta 54,6 mg/!, the available organic matter ranged from 3 ta 4,9 mg/!. The rate of organic matter in the water ranged 22 % ta 9 %. The organic selection efficiency was significantly higher for the condition 3 compare to the others conditions. This indicated an improvement in the sorting of the oyster at lower organic content (58 %), compare to 30 % and 42 % for the condition 1 and 2. Furthermore, the best absorption rate of the food condition 2 and 3 (coef. 57 and 88), suggested a difference in food quality, probably associated to resuspension of the organic material from the bottom. So, the scope for growth was significantly different for the three conditions in a ratio of 1, 4 and 10, respectively for the conditions 1, 2 and 3 (coeff. 49, 57 and 88). These values ranged from 50 ta 500 jouleslh/g for three different days in may 1992.

The flat oyster Ostrea edulis was originally reared in several regions of the European waters. The tidal saline ponds on the Atlantic coasts of France were highly productive before being hit by two consecutive epizooties. Parasitic diseases were described on this species, both of them leading 10 economic losses, as heavy mortalities have usually been recorded before the oysters reach the commercial size. Repeated experiments showed that the two diseases, Marteilia refringens and Bonamia ostreae are still active in this environment. Recently, some evidences were given that haemocytoblasts of Crassostrea gigas had the ability to lyze the Bonamia cells. Therefore, in an experiment of contiguous culture of the two species of oysters, the growth, mortality and prevalence of the two diseases were recorded of two populations of the fiat oyster. The prevalence of Marteilia was higher then the one of Bonamia (49.8 % against 7.4 %). The oysters originated from the area exhibited a better growth and survival then the ones coming from mediterranean waters. However, the Mediterranean origin was significantly less sensitive then the Atlantic one to Bonamia (respective prevalence of 5.6 % and 9.2 %). The rearing in different proportions with Crassostrpa gigas, in the same oyster bags, resulted in an absence of significant difference between these proportions, thus leading to the conclusion that the mixed rearing of the two species did not reduce significantly the prevalence of Bonamia parasites on the fiat oyster. The very hi level of the prevalence for Marteilia refringens after 8 months of cultivation, should be emphasized. For this disease, a significant effect of the mixed rearing was observed which may be due 10 the pond variability. These results were discussed in terms of culture management.

The incidence of aneuploidy was studied in different size-class full-sib juveniles of two pair matings in Crassostrea gigas. In all cases, slow-growing juveniles showed the highest percentage of aneuploid cells. The relationship between aneuploidy and growth rate is discussed in the light of previous genetic studies on heterozygosity-growth rate correlation and heterozygosity-viability correlation.