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Cases where animals use controlled illumination to improve vision are rare and thus far limited to chemiluminescence, which only functions in darkness. This constraint was recently relaxed by studies on Tripterygion delaisi , a small triplefin that redirects sunlight instead. By reflecting light sideways with its iris, it has been suggested to induce and detect eyeshine in nearby micro-prey. Here, we test whether ‘diurnal active photolocation’ also improves T. delaisi 's ability to detect the cryptobenthic sit-and-wait predator Scorpaena porcus, a scorpionfish with strong daytime retroreflective eyeshine. Three independent experiments revealed that triplefins in which light redirection was artificially suppressed approached scorpionfish significantly closer than two control treatments before moving away to a safer distance. Visual modelling confirmed that ocular light redirection by a triplefin is sufficiently strong to generate a luminance increase in scorpionfish eyeshine that can be perceived by the triplefin over 6–8 cm under average conditions. These distances coincide well with the closest approaches observed. We conclude that light redirection by small, diurnal fish significantly contributes to their ability to visually detect cryptic predators, strongly widening the conditions under which active sensing with light is feasible. We discuss the consequences for fish eye evolution.

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20Active sensing by means of light is rare. In vertebrates, it is known only from chemiluminescent fish 21 with light organs below their pupils, an anatomical arrangement that is ideal to generate eyeshine in 22 the pupils of nearby organisms. Here, we test whether diurnal fish can achieve the same by 23 redirecting sunlight through reflection instead. We recently showed that small (< 5 cm), benthic, 24 marine triplefin fish actively redirect downwelling light using their iris. We hypothesized that this 25 mechanism allows triplefins to improve detection of a cryptic organism by generating eyeshine in its 26pupil. Here, we tested this by attaching small dark hats to triplefins to shade their iris from 27 downwelling light. Two controls consisted of triplefins with a clear or no hat. These treatments test 28 the prediction that light redirection increases the visual detection ability of triplefins. To this end, we 29 placed treated fish in a tank with a display compartment containing either a stone as the control 30 stimulus, or a scorpionfish, i.e. a cryptic, motionless triplefin predator with retroreflective eyes. After 31 overnight acclimatization, we determined the average distance triplefins kept from the display 32 compartment over two days. Both in the laboratory (n = 15 replicates per treatment) and in a similar 33 field experiment at 15 m depth (n = 43 replicates per treatment) fish kept longer distances from the 34 scorpionfish than from the stone. This response varied between hat treatments: shaded triplefins 35 stayed significantly closer to the scorpionfish in the laboratory and in one of two orientations tested 36 in the field. A follow-up field experiment at 10 m depth revealed the immediate response of 37 triplefins to a scorpionfish. At first, many individuals (n = 80) moved towards it, with shaded 38