Patterns of fluorescence and colony tissue, colour were studied (field observations and epifluorescence microscopy) in six species of the coral genus Madracis over depth from 10 to 60 m at a reef slope in Curaçao. Two functions showed up: (1) Decrease in number of colourmorphs (n = 25) with depth suggests a photo-protective function where short wavelengths (e.g. UV) are transformed to long wavelengths, (2) Green fluorescence, observed in four species over their entire depth range, transforms radiation to wavelengths useful for photosynthesis. The observed patterns in fluorescence between species did not correspond to the current taxonomic classification. Our results do not support the usefulness of fluorescence as a taxonomic tool in corals.