We began by studying the zebrafish natural habitat. Zebrafish live in shallow freshwaters, typically in low-current side-pockets on the side of streams across the Indian subcontinent. We visited several field sites and took video footage of the habitat. In addition, we used a custom device capable of collecting accurate spectral information, from ultraviolet to infrared. This is key to understanding how zebrafish may see colour. From this work we find that different parts of the visual field contain different important information for these animals. The world below the fish tends to be red-dominated and is rich in spatial detail. The horizon is very colourful but lacks spatial detail. Next, the world above the animal is first a ground-reflection on the underside of the water surface, and beyond this, in a region called Snell’s window, the fish see the world above the water surface. The latter part of visual space is largely devoid of structure and colour. Together, these findings laid out a series of predictions how the visual system of zebrafish should be organised for efficient function. For example, it predicted that the lower part of the eye, which surveys the upper part of visual space, should not invest in circuits that support in colour vision, as there is little colour in this part of visual space. Conversely, the upper part of the eye, which surveys the ground beneath the zebrafish, should invest in colour-sensitive circuits to make best use of colour information on the ground.
Turning to functional recordings in the eye by way of laser-scanning microscopy combined with genetically encoded reporters of neuronal activity, this is exactly what we found. Looking at different sets of nerve cells in the zebrafish eye, in different parts of the retina, we found that the lower eye of larval zebrafish is all but colour-blind, while the upper part of the eye supports rich colour vision. In this work, we also stumbled across another key feature of these animal’s eyes. The part of the retina that looks just above the horizon in front of the fish is extremely UV-light sensitive. Further work revealed that this is probably a specific adaptation of small underwater animals to support prey-capture. While this first part of our project is published (Zimmermann, Nevala, Yoshimatsu et al. 2018), we now follow up on these points using further functional recordings in different populations of neurons both in the retina, and in the brain.
