It's not a mutant fish nor a piscine wizard. The source of the light is actually the defense of the clever, microscopic animal who just wants to get through the day without being gobbled up. Is that too much to ask?
The see-through cardinalfish evolved in lockstep with tiny little crustaceans called ostracods. The millimeter-long creatures sometimes get swallowed by the fish while they're looking for their actual food: plankton. To avoid passing through the digestive system of the fish, the ostracods evolved a brilliant defense: they light up. Since the cardinalfish is translucent, their glowing bodies turn them into easy targets for their own predators. Once the fish realizes that it's turned into a tasty fireworks parade, it spits the ostracod out and both critters can get on with their lives.
It's a fascinating glimpse into the sneaky ways evolution has come up with that allow different animals to avoid becoming someone else's lunch. The cardinalfish probably became translucent to avoid being seen by its predators. But it's a double-edged sword: the ostracods evolved their light show because it renders the cardinalfish's main defense completely useless. And both adaptations are the result of millions of years of slow, unintelligent, random tinkering.
These graphics come from a BBC 2 show called called Super Senses: The Secret Power of Animals.