Introduction
The Venice Lagoon represents a vulnerable ecosystem prone to significant variations of biotic and abiotic factors based on the spatiotemporal conditions as well as extensive anthropogenic interventions (Deheyn and Shaffer., 2007). These stressors can have consequences on the biology and ecology of sessile and highly sedentary organisms, such as bivalve mollusks, that cover, in turn, important ecological and economical roles. Shellfish, like the Manila clam (Ruditapes philippinarum), are extensively farmed in several areas of the Venice Lagoon, mostly due to their high tolerance and adaptability to different conditions and to strong variations in environmental parameters (Boscolo Brusà et al., 2013). Recent studies have demonstrated that clam health depends also on the microbiota, or associated microbial community, which plays key roles in nutrient assimilation, homeostasis, immunological regulation (Aceves et al., 2018) and the protection against pathogens and environmental stressors (Milan et al., 2018). However, seasonal variations, adverse climatic conditions, chemical-physical features of different farming sites, and anthropogenic activities can influence the fitness of farmed clams as well as their associated microbial communities, with potential negative consequences on the clam farming aquaculture industry. In addition, the inauguration of the new Experimental Electromechanical Module (MoSE), system of flood barriers, designed to safeguard Venice and the lagoon from high tides, may lead to further modifications of the delicate Venice lagoon ecosystem. In this study, a long-term monitoring campaign was carried out on clams from four farming sites in the south of the Venice Lagoon characterized by different environmental conditions. The aim of this study was to investigate the correlations between seasonal trends of microbiota and gene expression profiles in the digestive gland and gills of clams, and the biometric characteristics, condition index, and site-specific environmental chemical-physical parameters. Overall, this study reports key information for predicting and identifying the potential impacts of the MoSE system on Manila clam farming activities.
Material and Methods
Twenty thousand individuals of Manila clam spat (Ruditapes philippinarum), supplied by the Satmar Company (France) in August 2018, were partitioned in 4 groups and placed at gradual distances from the Chioggia inlet, from the outmost to the innermost site. Clam sampling was performed at five intervals throughout a one-year period, representative of the seasonal variations (May 2019, July 2019, October 2019, February 2020 and May 2020). After each sampling, biometric parameters and condition index of clams were recorded. Digestive glands and gills were collected for transcriptomic analyses (RNA-sequencing) and microbiota characterization (16S rRNA Amplicon Sequencing). Raw data elaboration from molecular analyses is still ongoing. To link potential alterations in clam health with changes in environmental conditions, the sediments of each sampling area were collected to measure levels of dibenzo-p-dioxins/furan (PCDD/F), polychlorinated biphenylenes (PCB), hexachlorobenzene (HCB) and heavy metals at each sampling time. Moreover, two multiparametric probes located at different distances from the inlet continuously measured water temperature, turbidity, pH, chlorophyll-a, salinity, dissolved oxygen, and saturation throughout the year.
Results
Results regarding biometric parameters showed no relevant differences between clams harvested in different sites at any single sampling time during the monitoring year. The highest growth rate was detected between May and July 2019, in all sites. The lowest condition index was recorded in October 2019 in all study areas, while the highest value was detected in the outermost site in May 2019 and February and May 2020. Detectable levels of the investigated pollutants were not different between investigated areas. Regarding environmental parameters, probes in both sites recorded a similar seasonal trend of each environmental parameter. Generally, the average temperatures, dissolved oxygen, and turbidity are always higher in the southern part (innermost probe), with the exception of the winter period in which a higher average temperature was observed near the inlet. Finally, the ongoing analyses on gene expression profiles and microbiota characterization performed in the two investigated tissues of clams will allow to evaluate seasonal alterations in clams farmed in different areas of the lagoon; their potential link to biometric analyses and environmental parameters will be also presented and discussed in this presentation.
Conclusion
By merging results obtained through the multidisciplinary approach applied in this study, important information regarding the influence of several environmental factors on the health status of farmed clams may be underlined. Furthermore, results of 16S rRNA sequencing and RNA-seq will make it possible to describe the host-microbiota interactions in the Venice lagoon farming areas, which may be associated with different environmental conditions. Overall, results obtained by this study will represent the “baseline” information which to evaluate the potential consequences of the MoSE system on Manila clam farming areas in the future from.
References
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Boscolo Brusà R., Cacciatore F., Ponis E., Molin M., Delaney E.. 2013. Clam culture in the Venice lagoon: stock assessment of Manila clam (Venerupis philippinarum) populations at a nursery site and management proposals to increase clam farming sustainability. Aquat. Living Resour. 26. https://doi.org/10.1051/alr/2013042.
Deheyn D. D., Shaffer L. R., 2007. Saving Venice: Engineering and ecology in the Venice lagoon, Technology in Society, 29,
Milan M., Carraro L., Fariselli P., Martino M.E., Cavalieri D., Vitali F., Boffo L., Patarnello T., Bargelloni L., Cardazzo B.. 2018. Microbiota and environmental stress: how pollution affects microbial communities in Manila clams. Aquatic Toxicology, 194, https://doi.org/10.1016/j.aquatox.2017.11.019.