The Ghost Catfish Can Turn Rainbow Through Its Muscles

Many fish can achieve an iridescent glow through the tiny crystals in their skin or through their scales that can reflect light. The Kryptoterus vitreolus, however, achieves its iridescent glow using its muscles. A team of scientists led by physicist Qibin Zhao discovered the fish's characteristics in their recent investigation.
Zhao's interest in the fish began one day in an aquarium store when he noticed it in front of him. Then the freshwater fish became iridescent. Intrigued, Zhao decided to investigate the fish in the lab, examining it under different lighting conditions. Using a white laser to illuminate the animal's muscles and skin separately, the team found out that its muscles — specifically, its sarcomeres — were the ones responsible for its rainbow glow.
As light passes through the muscles, the sarcomeres' repeating bands bend it, separating and enhancing the light's wavelengths, which creates the rainbow look.
While the iridescent glow is pleasing to the eyes, scientists are unsure about its purpose, but they have a few theories. For one, the iridescence could help the fish visually coordinate movements when they travel in groups (as they live in murky water). It might also help them blend in shimmering water to avoid predators like birds.