Visual detection of a cryptic predator by its prey fish

Animal interactions based on visual signals have been one of the oldest and most interesting research topics for early naturalists and biologists, that led to a better understanding of animal behaviour and its evolutionary implications. This study underlines the importance of considering the perspective of the species of interests when investigating visual communication in prey-predator interactions. Indeed, inaccurate conclusions are often made when we interpret animal behaviour basing ourselves on human vision, which outperforms that of most animals. Built on this premise, this dissertation focuses on the visual interaction between a small marine fish, the yellow black-faced triplefin, and one of its common cryptic predators, the black scorpionfish. The research approach first aims at better understanding the visual perspective of a triplefin when facing its predator (chapter 1 and 2), and then behaviourally and theoretically tests a new form of active sensing in the context of this prey-predator interaction (chapter 3). The first chapter describes the contrast sensitivity function of triplefins, later combined with other known visual features to estimate the amount of information that this species can visually perceive from natural scenes where predators might be concealed. The second chapter investigates how triplefins perceive the eye of a scorpionfish, and focuses on the unusual daytime eyeshine featured by this predator. This study describes, quantifies the phenomenon and tests its potential role for pupil camouflage in the context of visual inspection by triplefins under different light scenarios. The last chapter finally introduces and tests “diurnal active photolocation”, a new mechanism of active sensing that redirects ambient light (rather than emitting sound waves or electric fields) to detect reflective targets. By combining behavioural experiments with theoretical visual modelling, this final study provides first evidence for the functionality of diurnal active photolocation in triplefins by means of light redirected from their iris. This process supplements regular vision by increasing the chances of detection of a cryptic predator by exploiting its daytime eyeshine, and may have strong implications for the evolution of fish eyes