Crypsis and communication functions of UV-visible coloration in two coral reef damselfish, Dascyllus aruanus and D. reticulatus

The bold colours of reef fish probably serve various functions ranging from crypsis to advertisement. The presence of UV-sensitive vision in many reef fish demands that we explore these functions beyond the range of human vision. Current knowledge of the transmission of UV and visible radiation through clear ocean water, and knowledge of the likely spectral sensitivity of predatory and prey fish suggest that contrasting UV signals should be selected for social communication. For many juvenile reef fish, close-range detection and being noticed by predators may be much more important than long-range detection.Dascyllus aruanus (three-bar damsel) and D.reticulatus (two-bar damsel) juveniles are boldly coloured white fish with black bars that shelter in mixed-species groups in finely branched corals. Predators on the reef are probably well schooled in the location of their residence coral heads and the major threat is being noticed by a predator when making a defensive error. I took video images of the fish in the field through a series of narrow-band-pass filters from 340 to 600nm, which spans the probable range for visual sensitivity in fish. With the help of colour reference standards and spectral irradiance data, I applied video pixel intensity measures to a model of colour vision to quantify luminance contrast when viewed by both conspecific fish and various predators. The two-bar damsel had a highly UV-reflective colour patch on its flag-like dorsal fin that was lacking in the three-bar. This result, taken together with their longer wavelength coloration, indicates that the two-bar is better adapted for short-range social communication, which would be difficult for predators to detect. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.