An accidental discovery of a flatworm’s color-changing ability could help develop treatments for porphyria, a disease that causes pain and light sensitivity in humans. Schmidtea mediterranea, a flatworm native to southern Europe and Tunisia, turns from brown to white when exposed to sunlight for 24 or more hours, as scientists discovered when they were studying regeneration in the species. The vampirelike bleaching happens as the flatworm’s skin cells begin to overproduce porphyrin, a pigment that gives them their natural color. Porphyrins in high levels are toxic, killing off the cells that produce them and turning the flatworms pale white. The flatworms gain their pigment back after a few days, and were otherwise unaffected, according to a study published this week in the journal eLife. Humans with porphyria overproduce porphyrins, which poison their skin cells, making them pale and extremely sensitive to light. The disease can also cause extreme pain and neurological issues, and there is no cure or effective treatment. As a result, scientists see S. mediterranea as a useful test organism for drugs and therapies that may one day yield a treatment.