gentlesharks:

The Lantern shark is one of only three types of shark that can produce its own light. This is the result of a chemical reaction within the sharks skin cells; pigment in the skin oxidizes and creates light. Its spine lights up to ward off predators, and patches on its sides glow to coordinate movement within densely packed schools. But its ability to light up its belly with a blue/green light is the most remarkable adaptation of all. This light allows it to virtually vanish from the sight of predators from below.