Ontogeny of escape response and body shape in threespine stickleback ( <scp> <i>Gasterosteus aculeatus</i> </scp> L.)

Escape responses in fishes provide insight into accelerative motions and behavioural response times of these animals, linking numerous fitness-related traits. We sought to connect escape response performance to genotype and phenotype across ontogenetic stages within a single population of threespine stickleback (Gasterosteus aculeatus L.) to determine if a gene of major effect (ectodysplasin; Eda) on adaptive lateral armour plate phenotype and fitness conveys a performance advantage in brackish and saltwater environments. We predicted that the possession of one or more low-plated alleles would result in better escape performance in brackish environments across ontogeny, and that Eda genotype would result in variation in shape phenotype that would be predictive of escape performance before the completion of lateral plate development. We assessed three kinematic variables derived from C-start escape responses in wild-caught, F1 juvenile and F1 adults, and found that Eda genotypes did not directly impact maximum velocity or curvature coefficient, but linked phenotypes contributed to an increase in total distance travelled in F1 adults. We further observed that Eda genotype impacted growth, producing variation in size, which, in turn, influenced escape performance variables across age groups, with a particular focus on total distance travelled. The influence of Eda genotype on shape was confounded by sexual dimorphism, indicating that further sex-specific studies on shape and performance are needed. The presence of at least one low-plated allele conferred a growth advantage, particularly in a freshwater rearing environment, consistent with known pleiotropic effects of Eda, including skeletal development, that may be modulating anti-predator responses before plate phenotype is fully expressed. Altogether, more ontogenetic studies are needed to ensure that the influence of development on adaptive traits is more clearly understood.