Ghost catfish get their rainbow iridescence from transparent muscles

Now you know how ghost catfish get iridescent. These little fish are popular in home aquariums because they are almost completely transparent, emitting rainbow colors from their muscle fibers, unlike many other species that shimmer.

Aside from the head and spines, ghost catfish (Cryptopterus vitreorus) is almost completely transparent. When light hits its body, it emits a colorful iridescent glow, despite the lack of pigment in the transparent parts of its body. His Qibin Zhao and his colleagues at Shanghai Jiao Tong University in China examined samples of fish and their various tissues to elucidate how the iris develops.

“Unlike many fish species that have been confirmed to have structural coloration in reflection, ghost catfish’s structural coloration only appears in transmission, which is unusual,” says Zhao. These other fish tend to get glitter on their scales and skin from photonic structures (shapes that change the color of light when it reflects it).

“In fact, we initially spent a good deal of time looking for the photonic structure of the skin. We started studying muscles just to explain why they are transparent. I realized it was muscle, and it caused diffraction colors,” Zhao said. Researchers have found that the color comes from muscle fibers called sarcomere, which mediate muscle contraction and relaxation.

When light hits a fish, these sarcomeres act like prisms, breaking the light into its constituent colors. These structures can also be found in other types of fish, but they are only found in ghost catfish because they are so small, just a few centimeters long, and so transparent.

In the wild, this iridescence could help communicate with other fish and hide the catfish’s shadow from deeper-swimming predators, says Zhao. Other transparent fish, such as eel larvae and whitefish, may also glow in a similar manner.