Marine cleaning symbioses are classic examples of mutualism:
cleaners remove and consume ectoparasites from
‘‘client’’ fish, while clients benefit from a reduction in ectoparasites
[1, 2]. 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 [1, 3, 4]; in this study, we tested the longheld
belief that cleaner fish display a blue ‘‘guild’’ coloration
[5–7]. 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.