Cyclical training enhances the melanophore responses of zebrafish to background colours.

Zebrafish respond to visual stimuli to adapt their body colour to the background. If, rather than being a simple on/off reaction to visual stimulation, the colour change involves cognitive and memory-related processes, training fish with cyclical changes of the background would be expected to increase its ability to change colour. To test this, we developed a standardized procedure for quantifying the responses of melanophores to background changes in living adult specimens of leopard, a zebrafish mutant with spotted stripes. After training with 2-day cyclical alternation of white and black backgrounds for over 20 days, the proportion of the melanosome-filled area of dorsal melanophores, which was 20% on the black background before the training, increased up to 97%. In these trained fish, a rapid melanosome aggregation occurred within 10 s of the background change from black to white. The results indicate that the zebrafish melanophore responses can be modulated by learning, in which areal and speed control of melanosome movement are important for dispersion and aggregation, respectively.