Genetic effect of domestication selective pressures on Pacific oyster at larval stage

Among bivalve species, the Pacific oyster, Crassostrea gigas, is the most economically important bivalve production over the world. Today, C. gigas is subject to an important production effort that leads to an intensive artificial selection. Larval stage is relatively unknown, specifically in a domestication context. Genetic consequence of artificial selection is still at a preliminary study. We aimed to tackle the consequence of inconscient domestication on the variance reproductive success focusing on larval stage, keystone of the life cycle. We studied two kinds of specific selective processes that common hatchery rearing practices exert : the effect of discarding the smallest larvae on genetic diversity and the artificial environment rearing effect via the temperature providing a contrast resembling wild versus hatchery conditions (20 and 26°C). In order to monitor the effect of the selection of fast growing larvae by sieving, growth variability and genetic diversity in a larval population descended from a factorial breeding was studied. We used a mixed-family approach to reduce potentially confounding environmental biais. The retrospective assignment of individuals to family groups has been performed using a three microsatellite markers set. Two different rearing were carried out in parallel. For three (replicates) 50-l tanks, the smallest larvae were progressively discarded by selective sieving, whereas for the three others no selective sieving was performed. The intensity of selective sieving was adjusted so as to discard 50% of the larvae over the whole rearing period in a progressive manner. As soon as the larvae reached the pediveliger stage, ready to settle larvae were sampled for genetic analysis. Regarding the artificial environment rearing effect via the temperature, we used a similar mixed-family approach. The progeny from a factorial breeding design was divided as follows: three (replicates) 50-l tanks were dedicaced to a rearing at 26°C versus 20°C for three others 50-l tanks. The whole size variability was preserved for this experiment. Individual growth measurements for larvae genetically identified have been performed at days 22 and 30 after fertilization for both conditions. In a same way, we collected individual measurements for genotyped juvenile oysters (80 days after fertilization). At a phenotypic scale, relative survival and settlement success for larvae with sieving were higher. Sieving appears as a time-saving process associated with a better relative survival ratio. But in the same time, our results confirm that a significant genetic variability exist for early developmental traits in the Pacific oyster. This is congruent with the results already obtained that investigated genetic variability and genetic correlations in early life-history traits of Crassostrea gigas. Discarding around 50% of the smallest larvae can lead to significant selection at the larval stage.

Rearing effect of biofloc on antioxidant and antimicrobial transcriptional response in Litopenaeus stylirostris shrimp facing an experimental sub-lethal hydrogen peroxide stress

This study compares the antioxidant and antimicrobial transcriptional expression of blue shrimps reared according to two different systems, BioFloc Technology (BFT) and Clear sea Water (CW) and their differential responses when facing an experimental sublethal hydrogen peroxide stress. After 30 days of rearing, juvenile shrimps were exposed to H2O2 stress at a concentration of 30 ppm during 6 hours. The oxidative stress caused by H2O2 was examined in the digestive glands of the shrimp, in which antioxidant enzyme (AOE) and antimicrobial peptide (AMP) gene expression were analysed by quantitative real-time PCR. Results showed that rearing conditions did not affect the expression of genes encoding AOEs or AMPs. However, H2O2 stress induced a differential response in expression between shrimps from the two rearing treatments (BFT and CW). Comparative analysis of the expression profiles indicates that catalase transcripts were significantly upregulated by H2O2 stress for BFT shrimps while no change was observed for CW shrimps. In contrast, H2O2 caused down-regulation of superoxide dismutase and glutathione transferase transcripts and of the three AMP transcripts studied (penaeidin 2 and 3, and crustin) for CW shrimps, while no effect was observed on BFT shrimp transcript levels. These results suggested that BFT shrimps maintained antioxidant and AMP responses after stress and therefore can effectively protect their cells against oxidative stress, while CW shrimp immune competence seems to decrease after stress.

Relative contribution of natural productivity and compound feed to tissue growth in blue shrimp (Litopenaeus stylirostris) reared in biofloc: Assessment by C and N stable isotope ratios and effect on key digestive enzymes

The aim of this study was to assess the relative contribution of natural productivity and compound food to the growth of the juvenile blue shrimp Litopenaeus stylirostris reared in a biofloc system. Two experiments were carried out based on the same protocol with three treatments: clear water with experimental diet (CW), biofloc with experimental diet (BF) and biofloc unfed (BU). Shrimp survival was significantly higher in biofloc rearing than in CW rearing. The contribution of the biofloc to shrimp diet was estimated through measurement of carbon and nitrogen stable isotope ratios in shrimp and food sources. Different isotopic compositions between feeds were obtained by feeding natural productivity with a mixture rich in fish meal and the shrimps with a pellet containing a high level of soy protein concentrate. Using a two source one-isotope mixing model, we found that the natural productivity of the biofloc system contributed to shrimp growth at a level of 39.8% and 36.9%, for C and N, respectively. The natural food consumed by the shrimps reared in the biofloc system resulted in higher gene expression (mRNA transcript abundance) and activities of two digestive enzymes in their digestive gland: α-amylase and trypsin. The growth of shrimp biomass reared in biofloc was, on average, 4.4 times that of those grown in clear water. Our results confirmed the best survival and promoted growth of shrimps using biofloc technology and highlighted the key role of the biofloc in the nutrition of rearing shrimps. Statement of relevance In this study, we have applied an original protocol to determine the respective contribution of natural productivity and artificial feeds on the alimentation of the juvenile blue shrimp L. stylirostris reared in biofloc system by using C and N natural stable isotope analysis. Moreover, we have compared, in shrimp digestive gland, the α-amylase and trypsin enzyme activities at biochemical and molecular levels for two different shrimp rearing systems, biofloc and clear water. In our knowledge, the use of molecular tool to study the influence of biofloc consumption on digest process of shrimp was never carried out. We think that our research is new and important to increase knowledge on biofloc topic.

Biofloc contribution to antioxidant defence status, lipid nutrition and reproductive performance of broodstock of the shrimp Litopenaeus stylirostris: Consequences for the quality of eggs and larvae

The aim of this study was to determine biofloc contributions to the antioxidant status and lipid nutrition of broodstock of Litopenaeus stylirostris in relationship with their reproductive performance and the health of larvae produced. Shrimp broodstock reared with Biofloc technology (BFT) compared to Clear water (CW) exhibited a higher health status with (i) a better final survival rate during the reproduction period (52.6% in CW against 79.8% in BFT); (ii) higher glutathione level (GSH) and total antioxidant status (TAS), reduced oxidized/reduced glutathione ratio and a higher spawning rate and frequency as well as higher gonado-somatic index and number of spawned eggs. Finally, larvae from broodstock from BFT exhibited higher survival rates at the Zoe 2 (+ 37%) and Post Larvae 1 (+ 51%) stages when compared with those from females from CW treatment. The improved reproductive performance of the broodstock and higher larvae survival rate resulting from BFT treatment may be linked to the dietary supplement obtained by the shrimp from natural productivity during BFT rearing. Indeed, our study confirms that biofloc particulates represent a potential source of dietary glutathione and a significant source of lipids, particularly essential phospholipids and n-3 highly unsaturated fatty acids (HUFA) for shrimps. Thus, broodstock from BFT treatment accumulated phospholipids, n-3 HUFA and arachidonic acid, which are necessary for vitellogenesis, embryogenesis and pre-feeding larval development. The predominant essential fatty acids, arachidonic acid (ARA), eicopentaeonic acid (EPA) and docosahexaenoic acid (DHA), had levels in the eggs that were, respectively, 2.5, 2.8 and 3 fold higher for BFT compared to the CW treatment. Statement of Relevance Today, the influence of biofloc technology on shrimp broodstock is not enough described and no information was available on the larvae quality. Moreover, two key pieces of new information emerge from the present study. Firstly, biofloc is a source of further dietary lipids that can act as energetic substrates, but also as a source of phospholipids and essential fatty acids necessary to sustain reproduction, embryonic and larval development. Second, improving the reproduction of the broodstock also leads to an improvement in the quality of the larvae. We think that our research is new and important to increase knowledge on biofloc topic. We believe the paper will contribute to the development of more efficient and therefore more sustainable systems.

Bacterial community characterization of water and intestine of the shrimp Litopenaeus stylirostris in a biofloc system

Background Biofloc technology (BFT), a rearing method with little or no water exchange, is gaining popularity in aquaculture. In the water column, such systems develop conglomerates of microbes, algae and protozoa, together with detritus and dead organic particles. The intensive microbial community presents in these systems can be used as a pond water quality treatment system, and the microbial protein can serve as a feed additive. The current problem with BFT is the difficulty of controlling its bacterial community composition for both optimal water quality and optimal shrimp health. The main objective of the present study was to investigate microbial diversity of samples obtained from different culture environments (Biofloc technology and clear seawater) as well as from the intestines of shrimp reared in both environments through high-throughput sequencing technology. Results Analyses of the bacterial community identified in water from BFT and “clear seawater” (CW) systems (control) containing the shrimp Litopenaeus stylirostris revealed large differences in the frequency distribution of operational taxonomic units (OTUs). Four out of the five most dominant bacterial communities were different in both culture methods. Bacteria found in great abundance in BFT have two principal characteristics: the need for an organic substrate or nitrogen sources to grow and the capacity to attach to surfaces and co-aggregate. A correlation was found between bacteria groups and physicochemical and biological parameters measured in rearing tanks. Moreover, rearing-water bacterial communities influenced the microbiota of shrimp. Indeed, the biofloc environment modified the shrimp intestine microbiota, as the low level (27 %) of similarity between intestinal bacterial communities from the two treatments. Conclusion This study provides the first information describing the complex biofloc microbial community, which can help to understand the environment-microbiota-host relationship in this rearing system.