Interindividual behavioural variation in response to elevated CO₂ predicts mRNA transcript abundance of genes related to acid-base regulation in medaka (Oryzias latipes)

Rising carbon dioxide (CO₂) in aquatic ecosystems due to climate change is a challenge for aquatic ectotherms. We examined whether interindividual variation in behavioural responses to CO₂ could predict how a teleost fish would respond to elevated CO₂ for multiple phenotypic and molecular traits. To this end, we first quantified behavioural responses of individuals exposed to acute elevated CO₂, and used these to assign individuals as either high or low responders relative to the population mean. Subsequently, we exposed both high and low responders to elevated CO₂ for 6 weeks and quantified the effect on body condition, behaviour, and mRNA transcript responses of gill and liver genes associated with relevant physiological processes. Generally, we found few relationships between the phenotypic groups and body condition and behaviour following the CO₂ exposure period; however, stark differences between the phenotypic groups with respect to gene transcripts from each tissue related to various processes were found, mostly independent of CO₂ exposure. The most pronounced changes were in the gill transcripts related to acid-base regulation, suggesting that the observed behavioural variation used to assign fish to phenotypic groups may have an underlying molecular origin. Should the link between behaviour and gene transcripts be shown to have a fitness advantage and be maintained across generations, interindividual variation in behavioural responses to acute CO₂ exposure may be a viable and non-invasive tool to predict future population responses to elevated aquatic CO₂.