Heritability of behavioural tolerance to high CO (2) in a coral reef fish is masked by nonadaptive phenotypic plasticity

Previous studies have demonstrated limited potential for acclimation of adversely affected olfactory behaviours in reef fishes under elevated CO (2), indicating that genetic adaptation will be required to maintain behavioural performance in the future. Adaptation depends on the presence of heritable phenotypic variation in the trait, which may differ between populations and environments. We used parent–offspring regressions to estimate the heritability (h (2)) of variation in behavioural tolerance to high CO (2) (754 μatm) in both field‐collected and laboratory‐reared families of Acanthochromis polyacanthus. Tolerance to elevated CO (2) was measured by determining the behavioural response of individuals to chemical alarm cues. Both populations exhibited high heritability of olfactory behaviour phenotype (father–mid‐offspring h (2) = 0.56 & 0.65, respectively) when offspring were acutely exposed to high CO (2) for 4 days. However, there was no heritability in the behavioural phenotype when juveniles were chronically exposed to high CO (2) for 6 weeks in the laboratory‐reared families. Parental exposure to high CO (2) during the breeding season did not alter this relationship between heritability and length of juvenile exposure to high CO (2). These results demonstrate that variation in behavioural tolerance to high CO (2) is heritable, but adaptive potential may be constrained by a loss of phenotypic variation when juveniles permanently experience a high‐CO (2) environment, as will occur with rising CO (2) levels in the ocean.