Mapping the natural visual world of the zebrafish (Danio rerio): from sensory input to behavioural output

Vision is one of the most crucial senses for animals to catch prey, find mates and stay alive. The tetrachromatic zebrafish (Danio rerio) is a widely used model animal in visual neuroscience with four cone photoreceptors sensitive to UV, blue, green and red light. However, a detailed understanding of how their visual system is adapted to the natural environment, and what is important for the fish to see in their shallow freshwater habitats of the Indian subcontinent, has been missing. Therefore, it also has not been possible to carefully assess the importance of different parts of the light spectrum for their natural behaviours. In this thesis I introduce a new method for natural imaging, characterise the spectral composition of zebrafish’s natural visual world and demonstrate the role of UV light in their prey capture behaviours. To characterise the light conditions in natural environments, I developed and built two hyperspectral scanners to take spectrally detailed light measurements in shallow ponds and slowly moving streams in North-East India. As expected, the spectral profile becomes increasingly monochromatic and red shifted when moving from surface to the bottom. However, the short wavelength dominated surface and long wavelength dominated bottom are separated with colour-rich horizon. These spectral statistics match rather perfectly with the cone densities and colour processing abilities of the bipolar cells in the larval zebrafish retina. Previous work has demonstrated how prey capture behaviours on larval zebrafish can be triggered by small, bright spots. The short wavelength dominated upper part of the visual field projects light from UV bright prey items perfectly to the ventro-temporal part of the retina (“strike zone”) with high density of UV cones. Finally, with my behaviour experiments I demonstrate how prey capture behaviours are strongly driven by UV bright paramecia detected with the strike zone.