Physiological and biochemical responses of clams to recurrent marine heatwaves

In the past decade, the frequency, intensity and duration of marine heatwaves (MHWs) in the South China Sea have been increasing strikingly, resulting in serious impacts on intertidal bivalves and their ecosystems. The Manila clam, Ruditapes philippinarum, is one of the most ecologically and economically important bivalve species in the South China Sea, yet very little is known about its fate under intensifying MHWs events. Here, we examined how R. philippinarum responded to two consecutive scenarios of MHWs, with each composed of 4 °C and 8 °C rises of seawater temperatures, respectively. Up to 87% of Manila clams survived recurrent MHWs events, and significant increases in standard metabolic rate occurred predominantly under extreme conditions (+8 °C), indicating that the clams could trigger compensatory mechanisms to mitigate MHWs-induced thermal stress. Following acute and repeated exposures to MHWs, Manila clams showed similar responses in enzymes underpinning energy metabolism (NKA, CMA, and T-ATP), antioxidant defence (SOD, CAT, and MDA), and biomineralization (AKP and ACP), most of which exhibited significantly increasing and then decreasing trends with the intensification of MHWs. Of eight genes associated with physiological tolerance and fitness, ATAD3A, PFK, SOD, and C3 were significantly down-regulated in response to recurrent MHWs events, demonstrating the certain resistance to MHWs. These findings provide a better understanding that marine bivalves hold the potential to acclimate simulated MHWs events from the physiological and molecular processes.