Sources of red fluorescence in fishes. a. Guanine crystals from the eye ring of Eviota pellucida (fluorescence microscopy). b. Same from Ctenogobiops tangaroai (fluorescence overlaid with phase contrast). c. As in b, diamond form (fluorescence microscopy). d. Guanine crystals falling apart in characteristic platelets (from eye ring of E. pellucida, scanning electron micrograph). e. Single red fluorescent guanine crystal among normal crystals in eye ring of the non-fluorescent goby Trimma cana (fluorescence microscopy, see also Fig. 3). f. Scale of Pseudocheilinus evanidus, in which the red fluorescent pigment is associated with bony scales and fin rays (fluorescence microscopy). Scale bar = 50 μm unless indicated otherwise.