Omphalotus olearius
Through metabolic and physiological functions, all living things must produce and ultimately dispose of waste. This fungus exudes its waste through its gills, and some of these wastes are luciferases. Luciferases are enzymes / secondary metabolites commonly used for bioluminescence (or emission of light by a living organism) such as in fireflies (Photinus pyralis). There are five commonly accepted reasons for bioluminescence: camouflage, attraction, repulsion, communication, and illumination (mostly in underwater creatures). As for why bioluminescence occurs in Omphalotus, I haven’t been able to track down a reference. However, for more on the particulars of this species, see Omphalotus olearius on Tom Volk’s website.