Blue and yellow signal cleaning behavior in coral reef fishes.

Marine cleaning symbioses are classic examples of mutualism: cleaners remove and consume ectoparasites from "client" fish, while clients benefit from a reduction in ectoparasites. However, how clients recognize cleaners and decide not to eat them is unclear. Color and body pattern are thought to be important in signaling cleaning services to coral reef fish; in this study, we tested the long-held belief that cleaner fish display a blue "guild" coloration. Via color analytical techniques and phylogenetic comparisons, we show that cleaner fish are more likely to display a blue coloration, in addition to a yellow coloration, compared to noncleaner fish. Via theoretical vision models, we show that, from the perspective of potential signal receivers, blue is the most spectrally contrasting color against coral reef backgrounds, whereas yellow is most contrasting against blue water backgrounds or against black lateral stripes. Finally, behavioral experiments confirm that blue within the cleaner fish pattern attracts more client reef fish to cleaning stations. Cleaner fish have evolved some of the most conspicuous combinations of colors and patterns in the marine environment, and this is likely to underpin the success of the cleaner-client relationship on the reef.