Abstract
A recent study showed that triplefins, small cryptobenthic fish, actively reflect downwelling light sideways using their irides. Here, we investigate whether they do this to break the camouflage of cryptic predators by inducing eyeshine, revealing their presence. We attached mini-shades to triplefins to block light redirection and monitored the distance they kept to a cryptic sit-and-wait predator, a scorpionfish with retroreflective eyes. Shaded triplefins stayed significantly closer than two control treatments in replicate laboratory and field experiments. When confronted with a stone as a control, the treatments did not differ in their behaviour. Visual modelling confirmed that the light redirected by a triplefin is sufficient to increase the brightness of a nearby scorpionfish’s pupil to a degree that can be visually detected by that triplefin. We conclude that small fish detect nearby cryptic predators better when allowed to redirect light from their irides. This new form of active sensing, called diurnal active photolocation, has wide implications for fish eye evolution.