General introduction to light attenuation and observation of natural fluorescence in near-shore marine environments. a. The visual spectrum ranges from 400 to 700 nm at the water surface, but downwelling sunlight loses the red component (600–700 nm) rapidly within 10–15 m (modified from Pinet PR (2000) Invitation to Oceanography. Jones and Bartlett). UV and violet wavelengths are attenuated less rapidly. The attenuation with depth of spectral composition (and light intensity, not shown) varies strongly with the concentration of organic matter in the water column. b. Most red pigmentation is based on reflectance of the red component of ambient light and therefore only appears "red" when close to the surface during daytime or under broad spectral light (e.g. dive torch). Fish with this pigmentation appear dull grey in deeper water. Red fluorescent patterns, however, continue to appear reddish and bright, even in deeper water, where excitation of fluorescent pigments by shorter wavelengths induces redness. Note that red fluorescence is rarely perceived as pure red, but is mostly an enhancer of mixed colours such as pink, lilac or red brown. Even so, it remains clearly visible in deeper water as a contrast enhancer. Closer to the surface, fluorescent patterns are masked by reflective colouration (e.g. yellow and red in Eviota pellucida, Fig. 3). c. Since excitation frequencies (blue-green) are brighter than emission frequencies (red in our example) red fluorescence is best seen when viewed through a filter that blocks the excitation frequencies and only allows the emission frequencies to pass. When looking through a red filter in e.g. 20 m depth, all remaining red light must be "locally produced" through fluorescence or bioluminescence. Given that fluorescence exploits light energy from ambient light, it is more efficient than bioluminescence and therefore likely to be the mechanism of choice for diurnal fish.