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The first aim of this study was to determine the stocking density limits for Pacific oyster Crassostrea gigas larvae reared in flow-through system (FTS) and recirculating aquaculture systems (RAS). The second aim was to examine biofilm formation on the larval tank wall and its interaction with larvae growth. Three larvae concentrations were tested: 50, 150, and 300 mL−1. Chemical parameters and larvae performance were measured. The biofilm was observed by scanning electron microscopy, and its bacterial composition was investigated by pyrosequencing analysis of part of the 16S rRNA gene. The highest growth (13 µm day–1), survival (87%) and metamorphosis (50%) rates were observed in FTS at 50 larvae mL–1, while lower and similar performances occurred at 150 larvae mL–1 in both systems. At 300 larvae mL−1, performances dropped with occurrence of mortality. Biofilm thickness increased with larval density. The pioneer bacteria were coccobacilli followed by filamentous bacteria. The latter constituted abundant braids at the end of rearing at high larval concentrations. The first colonizers were mainly Rhodobacteraceae (α-Proteobacteria). The filamentous bacteria were Saprospirae (Bacteroidetes) and Anaerolineae (Chloroflexi). The biofilm was also made up of other minor groups, including Actinobacteria, Planctomycetes, δ-, γ-Proteobacteria, and Flavobacteriales. The biofilm's composition was more similar to that found in a sewage reactor than in open-sea collectors, which might negatively influence larval rearing due to potential metabolites. This first study on biofilms provides insights into the interaction between rearing density and larvae performance.

00000 ăWOS:000383370300057; International audience; This study has two main objectives: (1) to implement a recycling aquaculture system (RS) for the larvae of the oyster Crassostrea gigas, and (2) to characterise the bacterial communities established in different compartments of this system. An RS with 25% fresh seawater addition per hour and another with no addition (0%) were compared with a flow-through system (FT). Larval survival was equivalent in RS and FT, but growth rate was 17% slower in RS than in FT. The physical chemical parameters remained stable, except for pH that decreased to 7.75 and salinity that increased to 37.5% in the RS 0%. In both systems, the cultivable bacteria were present in similar numbers in seawater (around 10(5) ml(-1)) and in larvae (10(3) larva(-1)) on day 15. Bacterial assemblages, characterised by 454 pyrosequencing of the V1-V3 region of 16S rRNA, were highly similar (50-65%) for compartments, regardless of rearing system and sampling time, but the compartments were clearly different from one another. At the beginning of rearing, larval microbiota was mostly composed of Proteobacteria (similar to 90%), with 47% Rhodobacteraceae (gamma-Proteobacteria). beta-Proteobacteria, including Pseudoalteromonas, Alteromonas and a few vibrios, declined in the rearing period (25% on day 7 to 9% on day 15). At the end of rearing, colonisation by two members of the Burkholderiales (beta-Proteobacteria), 45% on average on day 15, had decreased overall diversity. Seawater microbiota was more stable with in all batches as one unclassified bacterium present in all batches (27 +/- 7%), 42 OTUs of alpha-Proteobacteria (19 +/- 7%) and 26 of gamma-Proteobacteria (14%). Change was due notably to a species of Cryomorphaceae (Flavobacteria) that reached 15 +/- 7% on day 15. Predatory bacteria, Bdellovivrio spp. and Bacteriovorax spp. were present (3-12%) and could participate in the regulation of bacterial populations. Bacterial assemblages in RS bioreactors remained stable and were mainly composed of Rhodobacteraceae, Rhizobiales and Planctomycetes. Only a few nitrite oxidisers were detected and no ammonia oxidisers, although nitrification was efficient in RS without addition of new water. The larval microbiota was made up of bacteria growing in seawater, but some such as the Burkholderiales could have come from the broodstock. Several bacteria predominant in seawater were first harboured in the algal culture. Finally, despite sanitary measures in the hatchery (UV treatment and frequent cleaning), the diversity of microbiota remained high, and it did not contain pathogenic bacteria. The presence of this microbiota might even be indispensable to ensure normal ontogenesis, as suggested by the holobiont concept. Statement of relevance RAS and control of microbiota should improve bivalve larval culture.