Divergent Giant Clam Strategies to Cope with Thermal Stress Revealed by Fatty Acid Dynamics

Giant clams are mixotrophic bivalves that obtain food from filtration and photosynthetic Symbiodiniaceae algae, making them vulnerable to climate change, as elevated sea temperatures trigger bleaching. While the species-specific reliance of giant clams on autotrophy has been reported, their metabolic responses to thermal stress remain poorly understood. Differences in symbiont density and photoefficiency, as well as total lipid and fatty acid (FA) composition of Tridacna derasa and Tridacna maxima and respective symbionts, highlighted varying abilities to cope with stress during a 31 day thermal stress regime, followed by an 8 day recovery period. T. derasa lost 56% of its symbionts, while T. maxima lost 89% but nearly recovered. The FA composition was altered in both species, emphasizing the importance of symbiont photosynthate transfers (e.g., C18:2n-6) and suggesting oxidative damage. In addition, FAs exhibited species-specific differences in intensity and rate (e.g., C20:5n-3). Our study is the first to demonstrate the importance of giant clam symbionts in providing essential FAs to their hosts. Our findings highlight significant differences in how species respond to thermal stress, with T. derasa exhibiting a resistance strategy and T. maxima exhibiting a resilience strategy. This is critical knowledge to understand the biological responses of these species and to inform future conservation strategies.