Spatio-temporal variations of growth, chemical composition, and gene expression in Mediterranean mussels (Mytilus galloprovincialis): A two-year study in the Venice lagoon under anthropogenic and climate changing scenarios

The Mediterranean mussel Mytilus galloprovincialis represents one of the most important aquaculture species in the Mediterranean Sea. The Venice lagoon is an historically important mussels farming area, contributing 25% of the total mussel production of the Veneto region. This area represents a vulnerable ecosystem, subject to significant spatio-temporal variations due to climate change and anthropogenic interventions as for the recent commissioning of MoSE (Experimental Electromechanical Module), a system of mobile barriers aimed at protecting Venice from high tides. In this study, we monitored for two consecutive production cycles three mussels farming sites located in Chioggia at different distance from the southern Venice lagoon inlet. During each season of the two productive cycles, mussels were collected in the three farming sites and site- and seasonal-specific variations of growth, chemical composition and fatty acid profile were investigated. In addition, we also characterized whole gene expression profiles of mussels grown in two investigated farming sites. Biometric traits and gene expression results highlighted narrow differences between rearing sites, inconsistent between the two production cycles. Conversely, seasonality led to important changes in mussel transcriptional profiles, fatty acid composition and condition index due to mussel reproductive cycle and seasonal conditions of the Venice lagoon (e.g. temperature, food availability). Fatty acid differences were primarily linked to dietary changes, rather than stress-related or physiological differences. Regardless of the rearing site, the lowest condition index was observed in October, probably due to intensified metabolic expenditure and depletion of energy reserves to cope with summer stressful environmental conditions. This hypothesis is corroborated by gene expression profiling and by the reduction in the mussel lipid content observed between July and October. The differences between mussels farmed in different sites were mainly related to the hydrodynamics which influences water physico-chemical parameters and feed availability for mussels. Overall biometric traits (i.e. growth) were more favourable in mussels farmed in the most external site characterized by higher hydrodynamics and more favourable water conditions compared to the other two sites. The most favourable condition of mussels farmed close to Chioggia inlet was also measured by gene expression profiles suggesting stressful conditions in inner site at the end of the second production cycle. Despite comparisons between farming sites at transcriptional level showed different results between production cycles, mussels exhibited comparable growth patterns during corresponding months or seasons, suggesting limited and not yet noticeable effects of MoSE commissioning in mussel production