Schooling of Light Reflecting Fish

Abstract

One of the hallmarks of the collective movement of large schools of pelagic fish are waves of shimmering flashes that propagate across the school, usually following an attack by a predator. Such flashes arise when sunlight is reflected off the specular (mirror-like) skin that characterizes many pelagic fishes, where it is otherwise thought to offer a means for camouflage in open waters. While it has been suggested that these ‘shimmering waves’ are a visual manifestation of the synchronized escape response of the fish, the phenomenon has been regarded only as an artifact of esthetic curiosity. In this study we apply agent-based simulations and deep learning techniques to show that, in fact, shimmering waves contain information on the behavioral dynamics of the school. Our analyses are based on a model that combines basic rules of collective motion and the propagation of light beams in the ocean, as they hit and reflect off the moving fish. We use the resulting reflection patterns to infer the essential dynamics and inter-individual interactions which are necessary to generate shimmering waves. Using an artificial neural network, trained to classify the direction of attack and the shape of the school based only on the flashes, we also provide a proof-of-concept, showing that flash patterns are indeed indicative of the state and dynamics of the school and the individuals composing it. Moreover, we show that light flashes observed by the school members themselves extends the range at which information can be communicated across the school. To the extent that the fish pay heed to this information, for example by entering an apprehensive state of reduced response-time, our analysis suggests that it may speed up the propagation of information across the school.