| Summary: | The increase of CO2 in the oceans, termed ocean acidification, is projected to have detrimental effects on marine organisms. The magnitude of the impact on the marine ecosystems will depend on species capacity to adapt. Recent studies show that the behaviour of reef fishes is impaired at projected CO2 levels; however, individual variation exists that might promote adaptation. Offspring of CO2-tolerant and CO2-sensitive parents were reared at nearfuture CO2 (754 uatm) or present-day control levels (414 uatm) and exposed to higher levels of CO2 at different life stages. We study the transcriptomes and proteomes in the brain of Acanthochromis polyacanthus (spiny damselfish), to evaluate short-term, longterm (one generation) and transgenerational molecular responses to more acidified oceans. Withingeneration CO2 exposure lead to an increased expression of genes involved in GABAergic neurotransmission as well as the potassium-chloride cotransporter 2 (kcc2). The reversal of the transmembrane gradient for HCO3- and Cl- with elevated levels of extracellular bicarbonate is the likely cause for the excitement of the neuronal transmission which in turn causes the behavioural impairment in fish at near-future high CO2 levels. We find a clear signature of the parental sensitivity to CO2 in the molecular phenotype of the offspring, mainly driven by circadian rhythm genes. Furthermore, expression patterns with prior parental CO2 acclimation largely differ to short-term or long-term exposures emphasizing the importance of transgenerational acclimation in exposure experiments.
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