Introduction
According to the predictive models, in the near future, the heat waves (HWs) will increase their duration up to 30 days, with negative consequences for the shellfish farming sector. Nowadays the farmers are facing the loss of mussels which come off the socks when the water temperature increases. In order to figure out the physiological mechanism behind these events and what to expect in the future, this study simulated an exposition of mussels to a prolonged HW. Morphometric indices, byssus quantity and quality through analysis of the fiber morphology were assessed.
Materials and Methods
260 specimens of farmed M. galloprovincialis reared into 12 aquaria were divided in two groups: the control one at temperature of 20±0,5°C and the heat wave exposed group at 28°±0,5°C. Mussels destined to byssus mechanical analysis, will be placed on square of polypropylene (PP) film in order to simulate the attach to PP socks used in farming.
At 3, 5, 10, 30 days exposure, mussels were sampled in both groups to determine the biometric indices of animal weight, wet and dry soft body and shell weight, the condition index (CI), meat yield (MY) and hepatosomatic index (HIS), clearance rate (CR), survival at air exposure. At final sampling time (30 days), byssus quantity and quality of the animals attached to PP, was evaluated by analysis under SEM observation.
Results and Discussion
About biometric indices, significant differences were found for hepatosomatic index, clearance rate and survival at air exposure where the lowest values were reported by the exposed group at the end of trial. For the HSI, the exposed group at 30 days of exposure showed a significant reduction in the digestive gland energy reserve, compared to other time points. Although, the heat wave reduced the energy reserve of the stressed group but only in long-term and did not alter the pulp yield. Moreover, the exposed group, after 5 days, started to filter at a rate significantly lower than the beginning and the control group, a sign of metabolic activity decrease. Concerning the survival at air exposure, the exposed group, at the end of thermic stress period, revealed a significant reduction in the capacity to survive compared to the other samplings and group. It proves that the prolonged heat wave has made these animals more susceptible to additional stress input (Fig. 1). Counting the byssus filaments, at 30 days of experiment, produced by the animals attached on the PP plate, it was found that exposed specimens made significantly fewer filaments, less than half of the other group. Probably, stressed mussels did not have enough energy to spend for the byssus formation (Fig 1D).
Conclusion
Prolonged feat waves cause a reduction in the quality of the mussel as a food product, not in terms of loss in pulp yield, but animal energy reserve and the shelflife of a product that has to be sold alive: in fact, this thermic stress reduces the survival duration outside the sea. Moreover, with the reduction in byssus filaments, stressed animals are able to detach more easily from the socks under the force of the currents, with the risk for the farmers to lose a great quantity of product.
Acknowledgements
The research leading to these results has been conceived under the International PhD program “Innovative Technologies and Sustainable Use of Mediterranean Sea Fishery and Biological Resources” (www.FishMed-PhD.org).
References
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