Effects of incident light and background conditions on potential conspicuousness of Hawaiian coral reef fish

Many coral reef fish are assumed to be conspicuous in their natural environ ment due to their bold and distinct colour patterns. However there have bee n felt attempts to quantify the conspicuousness of coral reef fish under na tural photic conditions. In this study; the effects of incident light and b ackground conditions on the potential conspicuousness of four species of Ha waiian reef fish (Chaetodon auriga, Labroides phthirophagus, Thalassoma dup errey, Zebrasoma flavescens) were examined. The optical properties of water , coral and fish colour patterns were measured with an underwater spectrora diometer. Potential conspicuousness was quantified by calculating luminance (brightness) contrast and spectral (colour) contrast within fish colour pa tterns, and between patterns and natural backgrounds, i.e, water and coral. Under different illuminating and background conditions, the contrast from these fishes was highly variable. Different incident light conditions resul ted in significant differences in inherent spectral contrast for L. phthiro phagus. All fish had significantly different luminance contrast when the op tical properties of the water backgrounds varied. Against coral backgrounds , T. duperrey and Z. flavescens had significantly different luminance contr ast and L. phthirophagus and T. duperrey had significantly different spectr al contrast. These results have important implications for theories on the adaptive significance of reef fish coloration since the conspicuousness of reef fish appears to, be highly dependant on local photic conditions and th erefore, some colour patterns may not be conspicuous in the wild as is ofte n presumed.