Behavioural and morphological defences in a fish with a complex antipredator phenotype

A number of prey species rely on two primary adaptations, behavioural alterations and morphological defences, to reduce their probability of being killed by predators. There should be considerable selection for maintaining adaptive trait combinations between behavioural and morphological defences in natural populations. However, our understanding of how behaviour and morphology interact has been limited to somewhat simple systems. Here we investigate patterns of covariation between behaviour and morphology in juvenile lake sturgeon, Acipenser fulvescens, a fish species with a complex antipredator phenotype. We found strong evidence for trait co-dependence between escape responses and body size, where larger fish were able to show stronger escape responses. Furthermore, we found that cover-seeking behaviour exhibited a complex multitiered relationship, representing a mixture of trait compensation and trait co-specialization that was dependent on specific combinations of mechanically independent traits. The smallest fish sought cover in rocky microhabitats more than fish of any other size (i.e. trait compensation). However, the largest fish also increased the amount of time they spent in rocky microhabitats following exposure to risk, indicating that cover-seeking behaviour may co-specialize with the ability to show strong escape responses. Cryptic fish of intermediate size did not flee from foraging grounds in response to risk (i.e. they showed trait compensation), whereas fish that lacked this pattern fled. Our results demonstrate how behaviour can integrate with complex morphological variation to produce unique antipredator outcomes. As sturgeon morphology has been largely preserved since the lower Jurassic period (∼200 million years ago), it may represent a highly successful morphological trait complex to which behavioural responses are finely tuned. Insight into the extent to which behaviour is linked to morphology is important in order to understand the limitations or reaches of adaptive phenotypic plasticity and the evolution of integrated phenotypes.