Introduction:
Climate extreme events such as Heat Waves (HWs) are a serious threat for marine ecosystems since they can lead to massive mortality of benthic organisms, to biodiversity loss, ecosystem changes and extensive damage for human activities, e.g. aquaculture. One of the most threatened activities from HWs is bivalve aquaculture, since bivalves are particularly susceptible to HWs and are mostly farmed in coastal ecosystems where the effects of HWs will be more intense. However our knowledge regarding the effects of HWs on bivalves at physiological, immune and molecular level is still limited. In order to fill this gap, we used the Manila clam Ruditapes philippinarum (the most farmed clam species worldwide and an important ecosystem engineer) as a model species to characterise in detail the responses of clams when challenged by an HW.
Materials and Methods:
A population of 240 Manila clams (shell length 23 – 31 mm) was purchased from SATMAR, one of the biggest hatcheries in Europe and acclimated to lab conditions for 15 days (20 ˚C and 32 psu ). After acclimation half population was exposed to HW conditions (30 ˚C for one month) and the remaining half was kept at 20 ˚C as control. After the exposure a series of physiological performances (growth, condition index, hepato-somatic index, clearance rate, burying speed, total haemocyte count) were evaluated in order to reveal the effects of HWs on the physiology of this species.
Results:
No increased mortality in HW-exposed clams was observed during the experiment. Our results indicated a significant decrease in growth, condition index and hepato-somatic index of HW-exposed animals, suggesting that energy reserves were diverted to counteract stressful conditions. Further, the burying speed was significantly increased in animals after the exposure to HW. Interestingly, clearance rate was higher in HW-exposed animals, while no change has been found in total haemocyte count or in the composition of the haemocyte subpopulations (e.g. granulocytes, hyalinocytes).
Conclusions:
Overall, the exposure to HW is able to induce a series of important changes to the physiology of clams, by impairing fundamental processes such as the burying velocity or the energy reserves in the hepatopancreas, thus posing a great concern for the aquaculture sector. Further in depth characterisation of the molecular responses triggered during HW-exposure are currently in place in order to reveal the transcriptomic and metabolomic responses of HW-exposed animals and provide a comprehensive picture of the impact of HWs on the physiology and ecology of the species and on its aquaculture.