Enhanced visual detection by active redirection of downwelling light in marine fish

Diurnal active photolocation (DAP) is the process by which diurnal fish re-direct downwelling light into the horizontal visual plane using their irides to generate reflections in cryptic target organisms, facilitating their detection. Although nocturnal active photolocation is known from chemiluminescent fishes, diurnal active photolocation is a controversial hypothesis. This debate follows from the seemingly unfavourable conditions during the day such as bright backgrounds, and a lack of relevant data. Our research focuses on quantifying the components of this process, and testing predictions experimentally.

This allows us to narrow down the morphological, perceptual, and ecological niche in which DAP may function. Important properties that will affect the contribution DAP can make to target detection are (1) the presence of highly reflective eyes in target organisms, (2) the ability to redirect light from structures around the pupil in the observer, (3) short distances between sender and target, (4) small-scale light gradients, and (5) the receiver's visual system. Our work focuses on a single species, the triplefin T. delaisi, a small (3-5 cm), crypto-benthic micro-predator [1].

We present data from visual modelling and manipulative experiments that support the idea that DAP can supplement visual detection of cryptic predators in an ecologically relevant part of the parameter space. Given the ubiquity of small benthic species with specialised forms of iris radiance and reflective eyes in many target prey and predator species, this is likely to be a widespread phenomenon.