The evolution of eyespots in skates and rays.

Animals deploy a wide range of anti-predator adaptations, often combining multiple strategies into a 'defence portfolio'. However, why certain defences evolve together whereas others are mutually exclusive remains unclear. Here we use a phylogenetic approach to examine the evolution of conspicuous visual markings-including eyespots and other conspicuous markings-alongside alternative anti-predator defences in skates and rays (Batoidea), a diverse group of cartilaginous fishes. We compiled data on the presence, type and number of conspicuous markings in 580 species of skates and rays, together with information on robust mechanical and electrical defences (venomous caudal stings and electric organs), adult body size and habitat depth as a proxy for light environment. We show that eyespots evolve stepwise from other conspicuous markings and that evolutionary gains of multiple markings are typically followed by reductions to fewer (often paired) markings. We further find that defence strategies follow alternative evolutionary trajectories: species possessing robust defences rarely evolve eyespots, whereas eyespots are favoured in smaller-bodied species lacking such defences and inhabiting well-lit waters where visual signals are most effective. These results show that accounting for multiple anti-predator defences can resolve why iconic anti-predator defences, such as eyespots, appear in some taxonomic groups but are conspicuously absent in others.