Metabolic Shifts and Muscle Remodeling as Pro-Survival and Energy Compensation Strategies in Photosymbiotic Giant Clams after Bleaching

Tropical photosymbiotic giant clams are increasingly threatened by climate change, leading to widespread bleaching. Decline in density of symbionts caused mortality events in symbiotic organisms; however, giant clams appear to exhibit prior survival capacity against these detrimental effects. It remains unclear whether giant clams can mitigate the adverse impacts of bleaching. Herein, we found that bleaching events after chronic heat stress induce remarkable changes and remodeling in symbiotic tissue of the giant clam Tridacna crocea. The density and structure of unique muscle fibers with high collagen content, observed in siphonal mantle of these animals, were negatively altered after heat stress. These changes were associated with a metabolic shift from carbohydrates and fatty acids to amino acids as the breakdown of collagen-rich muscle fibers can partially compensate for energy loss during bleaching. Such a shift was proposed to be regulated by AMP-activated protein kinase (AMPK) signaling and FoxO-atrogin pathways. Overall, our study highlights a pro-survival mechanism in giant clams through plastic regulation, which likely contributes to their relatively high environmental resilience during bleaching.