Two probiotics were tested as alternative treatment to limit the prevalence of the vertebral column compression syndrome (VCCS) in rainbow trout, compared with a preventive treatment with florfenicol. Either the antibiotic, or a lactic acid bacterium, Pediococcus acidilactici, or a yeast, Saccharomyces cerevisiae var. boulardii, was introduced into experimental diets, which were compared with the control diet without supplementation. The antibiotic caused some mortality during the first week after treatment, likely because of the toxicity of the compound. However, this antibiotic treatment limited the occurrence of VCCS to 3%, vs. 13% of fish affected in the control group. Pediococcus acidilactici provided the same level of protection as the antibiotic, but on condition that the treatment was applied during the 5 months of the experiment, without any adverse effect on survival. A treatment with P. acidilactici only during the first 20 days of feeding was not sufficient to limit VCCS, nor was the treatment with the yeast sufficient. These results were discussed while comparing the microbiota associated with the intestine. The experiment reinforced the hypothesis that pathogenic bacteria are involved in VCCS. The long‐term dietary supplementation with P. acidilactici seemed promising as a preventive treatment against the syndrome, but large‐scale investigation in fish farms will be necessary to confirm its prophylactic relevance.

-A reliable anaesthesia and sampling protocol for Pacific oysters will enable experiments to be conducted without sacrificing animals and will facilitate successive sampling of individuals for gametogenesis studies. As no such techniques were available for Crassostrea gigas, the present study aimed to define suitable anaesthetic conditions for use with this species. Three groups of ten oysters (mean weight ± SD, 32.1±9.0 g) were anaesthetised in 5 L containers. Among different chemicals: benzocaine, eugenol and three different types of magnesium chloride (a laboratory oneFlucka -and two designed for agriculture -DEUSA International and Dead Sea Works ) and concentrations tested, one laboratory (concentration: 72 g L −1 ) and one agricultural (Dead Sea Works 50 g L −1 ) type of magnesium chloride were the most effective, respectively inducing anaesthesia in 73 ± 3% and 80 ± 3% after three hours. Lower oyster weight and a two day period of starving prior to treatment significantly increased the number of anaesthetised animals. Using this protocol, losses of 0 to 10% of oysters were observed one week after anaesthesia. Increasing anaesthesia duration from 3 to 16 h resulted in a significant increase in the number of anaesthetised oysters (from 50 ± 10 to 97 ± 7%) but no increase in mortality (7 ± 11%). On the other hand, reducing water temperature from 19.5• C to 15.3• C, resulted in a significant decrease in anaesthesia efficacy. A reliable anaesthesia protocol was developed: 100% of Pacific oysters are anaesthetised using 50 g L −1 MgCl 2 Dead Sea Work for a 16 h duration. This long duration facilitates tissue sampling and does not increase the working time needed. This protocol was validated by monthly anaesthesia and gonad sampling during a three month period with the loss of only a single oyster. It can thus be used for routine applications.

The emergence and worldwide spread of SARS-CoV-2 raises new concerns and challenges regarding possible environmental contamination by this virus through spillover of human sewage, where it has been detected. The coastal environment, under increasing anthropogenic pressure, is subjected to contamination by a large number of human viruses from sewage, most of them being non-enveloped viruses like norovirus. When reaching coastal waters, they can be bio-accumulated by filter-feeding shellfish species such as oysters. Methods to detect this viral contamination were set up for the detection of non-enveloped enteric viruses, and may need optimization to accommodate enveloped viruses like coronaviruses (CoV). Here, we aimed at assessing methods for the detection of CoV, including SARS-CoV-2, in the coastal environment and testing the possibility that SARS-CoV-2 can contaminate oysters, to monitor the contamination of French shores by SARS-CoV-2 using both seawater and shellfish. Using the porcine epidemic diarrhea virus (PEDV), a CoV, as surrogate for SARS-CoV-2, and Tulane virus, as surrogate for non-enveloped viruses such as norovirus, we assessed and selected methods to detect CoV in seawater and shellfish. Seawater-based methods showed variable and low yields for PEDV. In shellfish, the current norm for norovirus detection was applicable to CoV detection. Both PEDV and heat-inactivated SARS-CoV-2 could contaminate oysters in laboratory settings, with a lower efficiency than a calicivirus used as control. Finally, we applied our methods to seawater and shellfish samples collected from April to August 2020 in France, where we could detect the presence of human norovirus, a marker of human fecal contamination, but not SARS-CoV-2. Together, our results validate methods for the detection of CoV in the coastal environment, including the use of shellfish as sentinels of the microbial quality of their environment, and suggest that SARS-CoV-2 did not contaminate the French shores during the summer season.

International audienceFour microalgae species (Rhodomonas salina, Thalassiosira weissflogii, Thalassiosira pseudonana and Pavlova lutheri) were evaluated to estimate their potential as food for Ostrea edulis (L.) reproductive conditioning. Best ingestion and absorption were observed with R. salina (3.44 and 1.59 mg g− 1 h− 1, respectively), followed by T. pseudonana (2.75 and 0.98 mg g− 1 h− 1) and P. lutheri (2.40 and 0.91 mg g− 1 h− 1). Oysters fed T. weissflogii exhibited the lowest ingestion and absorption values (1.40 and 0.68 mg g− 1 h− 1). Proximate composition (proteins and carbohydrates) and lipid content (fatty acids and sterols) analysed in four main tissues (gonad, digestive gland, muscle and gills) also differed significantly with diet. Protein ranged from 355 mg g− 1 in the gonad of oysters fed P. lutheri to 837 mg g− 1 in gills of oysters fed T. weissflogii; whereas carbohydrates ranged from 17.5 mg g− 1 in gills of oysters fed P. lutheri to 271 mg g− 1 in gonads of oysters fed R. salina. An overall poor enrichment in total PUFAs across all diets masked some of their potential impact on nutrition. In gonad, however, the major polyunsaturated fatty acids (polar lipid fraction) were EPA (≈ 19% for oysters fed T. weissflogii and 14% for those fed P. lutheri) and DHA (17% for oysters fed P. lutheri and 15% for those fed R. salina). Sterol contents showed a clear transfer from food to oyster tissues except with P. lutheri, from which neither methylpavlovol nor ethylpavlovol (characteristic of Pavlophyceae) were detected in oyster tissues. Histological analysis showed that gametogenesis was active in oysters fed R. salina and T. weissflogii, whereas only low gonadic development occurred in unfed oysters or those fed P. lutheri. R. salina is accordingly highly recommended for O. edulis broodstock conditioning whereas P. lutheri should be excluded