Sometimes they radiate dimly on the undersides of their bodies for camouflage counter-illumination, hiding their shadows on the seafloor by matching the light levels coming from the surface.
As weird as it sounds, bioluminescence turns out to be an incredibly beneficial adaptation.
A new study shows that it has evolved no less than 27 times in biological history, for countless reasons.
University of Kansas evolutionary biologist Leo Smith, who contributed to the study, told Ars that fish use their built-in lights for many different reasons.
They don't depend on finding bacteria to help them glitter.
If a group of fish finds a nice ecological niche that works, and it has a very obvious way of communicating with its immediate group, members of that group of fish will mate only with each other and rapidly diverge from their neighbors.