Transcriptomic reaction norms highlight metabolic depression as a divergence in phenotypic plasticity between oyster...

Ocean acidification is occurring at a rate unprecedented for millions of years, forcing sessile organisms, such as oysters, to respond in the short term by relying on their phenotypic plasticity. But phenotypic plasticity has limits, tipping points, beyond which species will have to adapt or disappear. These limits could be related to the adaptation of species to different habitat variabilities. Here, we exposed juvenile pearl oysters, Pinctada margaritifera, to a broad pH range and determined the response at the gross physiological, lipidome and transcriptome levels. Thus, we identified its high tolerance with low pH tipping points at pH 7.3-6.8 below which most physiological parameters are impacted. We then compared the transcriptomic reaction norms of the tropical subtidal P. margaritifera with those of an intertidal temperate oyster, Crassostrea gigas, reusing data from a previous study. Despite showing similar tipping points to C. gigas, P. margaritifera exhibits strong mortality and depletion of energy reserves below these tipping points, which is not the case for C. gigas. This divergence relies mainly on the induction of metabolic depression, an adaptation to intertidal habitats in C. gigas but not P. margaritifera. Our method makes it possible to detect divergence in phenotypic plasticity, probably linked to the species' specific life-history strategies related to different habitats, which will determine the survival of species in the face of ongoing global changes. Such an approach is particularly relevant for studying the physiology of species in a world where physiological tipping points will increasingly be exceeded.