Blue is also an effective color for long-distance transmission in marine waters [17], and it is interesting that the category Blue appears not to be exhibited by cleaner organisms. Blue was more highly contrasting than UV/Blue or Blue/Red for C. strigosus and S. helleri against all three backgrounds (p < 0.05; Figure 3). The category of blue that is exhibited by cleaner fish may be limited by the physical structure of chromatophores, cells that contain pigments giving the fish its coloration