Laurent Barillé

Effects of high natural seston concentrations on the feeding, selection, and absorption of the oyster Crassostrea gigas (Thunberg)

Feeding, selection and absorption were determined for the Pacific oyster Crassostrea gigas cultivated in the Bay of Marennes-Oleron, over a spring/neap tidal cycle. Physiological determinations were related to the highly variable food environment with continuous recordings of turbidity and fluorescence. In this bay, resuspension processes have a major influence on food availability and quality. Seston characteristics experienced by oysters can be summarized by high turbidity levels from 20 to 350 mg·l−1 and a predominance of the detritic fraction among the organic fraction (mean C/N ratio=16.57). Food is diluted by the fine resuspended sediment, and organic content of particulate matter in the water column decreases from 30% to 10% with increasing seston loads. Significant differences (Ancova, P<0.01), due to low retention efficiencies of the smaller particle size range, were recorded between the food quality (estimated by the organic content and the total pigment content) measured in the water column and the fraction retained by the oysters gill. Below seston concentrations of 90 mg·l−1 ingestion rate was regulated by pseudofaecal production. Above 90 mg·l−1, a sharp reduction of filtration and rejection rates suggests physical constraints limiting food acquisition. The oyster selectively rejects inorganic from organic particles, enriching the ingested fraction. Amongst the potentially nutritive particles, significantly fewer particles containing phytopigments were rejected relative to organic particles (non-linear regressions, P<0.001). The negative influence, through food dilution, of high seston loads on net absorption efficiency was determined. This efficiency decreases with decreasing organic ingested fraction. Scope for growth calculations confirm the negative influence of seston loads, but show, supported by field growth measurements, that resuspended organic particles play an important role in the oysters nutrition.

Pacific oyster (Crassostrea gigas) feeding responses to a fish-farm effluent

Abstract Bivalves have often been used in integrated fish-farming to enhance the economical value of by-products and/or to improve water quality. However, no physiological studies have dealt with the contribution of the two main sources of organic matter potentially present in a fish-farm effluent: living cells of phytoplankton and detritical matter (fish-faeces and uneaten feed). This study evaluated feeding responses of the Pacific oyster ( Crassostrea gigas ) to a land-based fish-farm effluent comprised mainly of fish-faeces ( Dicentrarchus labrax ) and compared them with those obtained with a diatom ( Skeletonema costatum ). A particular distinction of the main sources of organic matter was made in the experiments, the two diets being evaluated separately and mixed. Feeding responses were evaluated using the biodeposit method with special attention being paid to pre-ingestive processes and absorption efficiency (AE). Experiments were carried out between May and July 1998 in the laboratory at constant water temperature (20°C). Results showed that food quality (nature of particulate organic matter, POM, organic content of the total suspended matter, TSM) had no influence on clearance rates. However, clearance rates were significantly reduced for oysters in advanced stage of gametogenesis. S. costatum was preferentially ingested compared to the fish-faeces when offered in a mixed diet. Absorption efficiency and energy content (56% and 15 J mg −1 , respectively) of the fish-faeces were lower than those estimated for S. costatum (66–70%, 20 J mg −1 ). Nevertheless, these values were high for such a detritic type of food. This study confirms that both concepts of biomechanical filter and economical value improvement of fish-farming waste using oysters are of interest.

Selective feeding of the oystercrassostrea gigas fed on a natural microphytobenthos assemblage

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 (Kützing),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.

Correction for particulate organic matter as estimated by loss on ignition in estuarine ecosystems

Abstract Loss on ignition (LOI) is a simple, inexpensive method widely used to estimate organic matter in the water column and sediment of marine and freshwater ecosystems. Suspended particulate matter in estuarine waters however often contains a large fraction of argillaceous minerals whose loss of structural water can introduce significant bias to the method. The accuracy of LOI, in comparison with elemental analysis, was evaluated in 105 water samples from the Marennes-Oleron Bay (French Atlantic coast). The suspended particulate matter analysed ranged from 7 to 108 mg l −1 , with a mean inorganic fraction of 81.3±s.d. 7.2%. LOI overestimated particulate organic matter (POM) concentrations by up to 300% (mean 98 ± s.d. 28%), and this overestimation was significantly related to particulate inorganic matter (PIM) concentrations. POM overestimation increased linearly with PIM, representing a mean 8.7±s.d. 2.0% of PIM. This value was compared with a theoretical correction factor based on the specific clay composition of PIM at the study site and on thermo-gravimetric analysis for standard clay. The clay fraction, which represents 90% of PIM in the Marennes-Oleron Sound, consists of 40% illite, 30% kaolinite and 30% montmorillonite. The similarity between the theoretical correction factor (9.3% of PIM) and the overestimation based on measurements (8.7%), suggests that a priori correction of POM data is possible. In fact, for three bodies of water along the French Atlantic coast, whose inorganic resuspended matter shows the same mineralogical composition, POM concentrations as estimated by LOI can be corrected by subtracting 9.3% of PIM mass.

Spatiotemporal changes in microphytobenthos assemblages in a macrotidal flat (Bourgneuf Bay,France)1

Spatiotemporal changes in microphytobenthos species composition were investigated in relation to structural variables—biomass; life‐forms; detritus ratio, pheopigment a (pheo a):chl a; and sediment characteristics—at mesoscale in a shellfish macrotidal ecosystem. To characterize algae assemblages, multivariate analyses were performed (multidimensional scaling [MDS] ordination and clustering), supported by correlations between structural variables. Microphytobenthos, dominated by 97% diatoms belonging to 89 taxa, was constituted by two main assemblages: The first one, composed of common species (occurrence >50%), mainly nanobenthic (size <30 μm) and epipsammic (sand‐fixed cells), was associated with mixed sediments. The major species contributing to this assemblage were Navicula perminuta, Achnanthes hauckiana, Fallacia tenera, A. lanceolata var. elleptica, Amphora spp., Plagiogrammopsis vanheurkii, and Plagiogramma tenuissimum (see Table 2 for taxonomic authors). Species diversity was high (N1∼ 11), often linked to high biomass values (>70 mg chl a · m−2) and low detritus ratio. Conversely, the second assemblage comprised occasional species (occurrence from 10% to 50%), mainly microbenthic (>30 μm) and epipelic (moving cells), occurring during summer at muddy sites. This assemblage, characterized by low diversity, high biomass variability, and high detritus ratio, was dominated by Scolioneis tumida or Plagiotropis vitrea and Navicula spartinetensis. Whereas hydrodynamics globally explained the gradual assemblage changes throughout the entire mudflat and the year, oyster beds and ridge and runnel features appeared to be local spatial structuring factors allowing the establishment of specific assemblages. This study suggests a significant role for epipsammon biomass, until now underestimated, in the functioning of this turbid coastal ecosystem.

Rising water temperatures, reproduction and recruitment of an invasive oyster, Crassostrea gigas, on the French Atlantic coast.

The recent appearance and invasion of feral oysters (Crassostrea gigas) along the northern European Atlantic coast, underscores the necessity to investigate the relationship between environmental variables, reproductive physiology, larval development and recruitment. We studied these relationships at both high (HT) and intermediate (IT) - turbidity sites, through historical data on water temperatures, multi-parameter environmental probes, histological analyses, and field collections of planktonic larvae and settled post-larvae in 2005 and 2006. A progressive warming trend was observed, especially since 1995, when oyster proliferation first became severe. Threshold temperatures for oocyte growth, larval development and settlement were achieved in both 2005 and 2006. The HT site showed greater numbers of larvae and post-larvae than the IT site for both years, with the highest numbers of post-larvae observed at both sites during the warmer summer of 2006. These results suggest that increased temperatures in northern European waters allow successful reproduction, larval development, and recruitment of C. gigas. High turbidity conditions further enhance this success.

Finding new diatoms for intensive rearing of the pacific oyster (Crassostrea gigas): energy budget as a selective tool

Abstract Intensive shellfish rearing in the polders of Bourgneuf Bay on the French Atlantic coast (46–47°N, 1–2°W) relies on the diatom Skeletonema costatum (Grev.) Cleve produced in nutrient-rich saline ground water. An episodic reduction in biomass production caused by an unidentified protozoon stressed the need to find new microalgal species as substitutes for S. costatum and diversify the microalgae used by the bivalve industry. Three species met the requirements for year-round availability in Bourgneuf Bay coastal waters and growth potential in saline ground water: Nitzschia acicularis (Kutzing), Nitzschia closterium (Ehrenberg) Wm Smith and Nitzschia gandersheimiensis Krasske= Nitzschia tubicola Grunow. These microalgae were tested as food sources for adult Pacific oysters ( Crassostrea gigas ) by comparing short-term bivalve physiological responses with those obtained with S. costatum as reference. Suspended particulate matter concentrations in experimental diets ranged from 9.3 to 18.6 mg l −1 and particulate organic matter concentrations from 3.3 to 5.7 mg l −1 . Significant differences were observed, with clearance rates ranging from 4.0 l h −1 g −1 for N. acicularis to 7.3 l h −1 g −1 for N. gandersheimiensis . The filtration rate for organic matter was significantly higher for N. gandersheimiensis than the other species, but this algae was also significantly more rejected in pseudofaeces. No differences were found among the four mean faeces production rates. Net energy balance differed significantly among the four microalgae tested, ranging from 282 J h −1 g −1 for S. costatum to 27 J h −1 g −1 for N. closterium . However, no differences were found between S. costatum , N. acicularis and N. gandersheimiensis . These results suggest that N. acicularis and N. gandersheimiensis could be tested on a larger scale involving the production of microalgae in 50-m 3 outdoor tanks in association with Pacific oyster growth experiments.

Efficiency of particle retention and clearance rate in the polychaete Sabellaria alveolata L.

The development of Sabellaria alveolata, a gregarious reef-building polychaete species, is maximal in Mont-Saint-Michel Bay (France), where trophic capacity is now threatened by increasing shellfish farming. As no data are available concerning the ecophysiological response of this species, the purpose of the present study was to obtain clearance rate and retention efficiency values to provide a first order of magnitude for the trophic role of this species. Data were obtained using a flow-through system with novel troughs suitable for 225 cm2 reef blocks containing a mean number of 940 +/- 102 (S.E.) individuals. The experimental diet used consisted of a mixture of two live microalgae, Skeletonema costatum (3800 cell ml-1) and Isochrysis galbana (23,700 cell ml-1), chosen to cover a broad size range (2 to 16 microns equivalent spherical diameter, ESD), as determined by a particle counter. On the basis of a mean clearance rate of 0.7 lh-1 obtained with reef blocks, the mean rate for an individual was estimated at 7.5 x 10(-4) L h-1. Particles larger than 6 microns ESD were cleared with 100% efficiency, but S. alveolata was unable to retain particles smaller than 2 microns ESD. The results are compared with data obtained for other polychaete species, and clearance rate values are extrapolated to an entire reef.

Does bivalve mucus favour the growth of their main food source, microalgae?

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.

REVIVAL CAPACITY OF DIATOMS IN BIVALVE PSEUDOFAECES AND FAECES

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 to a control, confirms that diatoms have significant survival after extracellular digestion by the oyster. Survival was probably facilitated by the large amount of inorganic particles added to experimental diets to reproduce conditions characteristic of intertidal oysters cultivated on mudflats. This variable has probably more significance for the revival process than the size of the microalgae or its ecological type in such environment.