Non-parallel impacts of predators on the evolution of colouration plasticity in Trinidadian killifish

Animal colouration is a key trait affecting survival and reproduction, yet the evolutionary drivers of colouration plasticity remain poorly understood. Here, we examine how predation shapes both baseline colouration and physiological colouration plasticity in the Trinidadian killifish (Anablepsoides hartii). This species inhabits streams that vary in fish predator presence, creating a replicated natural experiment across three rivers. We hypothesised that fish from high-predation sites would exhibit lighter baseline colouration due to associations with open canopy and increased light, and that predation would select for stronger plasticity in background-induced colour change. We assessed colouration before and after exposure to black or white backgrounds and analysed variation by predation regime, river, and sex. Our results show that baseline colouration differed significantly by predation regime, river identity, and sex, with high-predation fish generally lighter. A. hartii also displayed strong plasticity, darkening on black backgrounds and lightening on white. However, the effect of predation on plasticity was inconsistent across rivers, suggesting that local ecological factors mediate these responses. Our study provides rare empirical evidence for both baseline and plastic variation in colouration driven by ecological context, offering new insight into how phenotypic plasticity evolves in response to environmental pressures like predation.