These transparent fish turn rainbow in white light. Now we know why

The ghost catfish transforms from glassy to glamorous when white light passes through its mostly transparent body. Now scientists know why.

The iridescence of the fish comes from the bending of light as it travels through microscopic striped structures in the animal’s musclesresearchers report March 13 in the Proceedings of the National Academy of Sciences.

Many iridescent styled fish have small crystals on your skin or scales that reflect light (Serial Number: 4/6/21). But the ghost catfish (Kryptopterus vitreolus) and other transparent aquatic species, such as eel larvae and icefish, lack such structures to explain their brightness.

The transparent body of the ghost catfish caught the eye of physicist Qibin Zhao when he was in an aquarium shop. The approximately 5 centimeters long freshwater fish is a popular ornamental species. “I was standing in front of the tank and looking at the fish,” says Zhao, from Shanghai Jiao Tong University. “And then I saw the iridescence.”

To investigate the fish’s colorful properties, Zhao and his colleagues first examined the fish under different lighting conditions. The researchers determined that its iridescence arose from light passing through the fish rather than reflecting off it. By using a white light laser to illuminate the animal’s muscles and skin separately, the team found that the muscles generated the multicolored glow.

The researchers then characterized the properties of the muscles by looking at how X-rays scatter as they travel through tissue and observing them under an electron microscope. The team identified sarcomeres, regularly spaced banded structures, each about 2 micrometers long, that stretch along the length of muscle fibers, as the source of the iridescence.

The repetitive bands of the sarcomeres, made up of proteins that overlap in varying amounts, bend white light in a way that separates and enhances its different wavelengths. The collective diffraction of light produces a variety of colors. When the fish contracts and relaxes its muscles to swim, the sarcomeres change slightly in length, causing a changing rainbow effect.

The purpose of the ghost catfish’s iridescence is unclear, says Heok Hee Ng, an independent ichthyologist in Singapore who was not involved in the new study. Ghost catfish live in murky waters and rarely rely on sight, he says. But the iridescence could help them visually coordinate movements when riding in schools, or it could help them blend in with the shimmering water to hide from land-based predators, such as some birds, he adds.

Regardless of the role, Ng is excited to see scientists explore the unusual features of the ghost catfish.

“Fish actually have quite a few of these interesting structures that serve them in many ways,” he says. “And many of these structures are very poorly studied.”