Some animals give off bioluminescence. Some contain bacteria, which they stimulate chemically to glow, and some use filters to modify this light. Others produce their own alchemy of compounds, which when allowed to mix give off light. . . . Some bioluminescence has a sexual function. Fireflies certainly atract mates with their light displays, and several species of deep-sea squid and fish would appear to use bioluminescence in their courtship displays.

—Peter Parks, from the essay Colour in Nature, from Colour: Art & Science edited by Trevor Lamb and Janine Bourriau, 1995.