Ghost catfish (K. vitreolus) showing iridescent colors with a backlight.

Some creatures in the animal kingdom display impressive colors when exposed to light, such as some snakes, insects and peacocks. They are known as iridescent animals.

While snakes have iridescent scales and peacocks have iridescent feathers, a recent study published in PNAS shows that in the case of the transparent ghost catfish, the iridescent glow it emits is not from the fish’s scales.

Read more: Inside the secret world of iridescent animals

The rainbow fish

Ghost Catfish (Kryptopterus vitreolus) is a transparent fish native to the slow-flowing rivers and streams of Thailand, and they are also popular in freshwater aquariums. His tiny heart and spine can be seen beating through his translucent scales.

(Courtesy of: Nan Shi, Xiujun Fan and Genbao Wu) Transparent ghost catfish (K. vitreolus).

When light from the sun or an artificial source passes through these fish, they can emit a vibrant rainbow light. However, unlike other iridescent animals, the study found that the ghost catfish gets its iridescent glow from its internal organs, not its scales.

According to a press releaseresearchers used laser light, electron microscopy and X-ray scattering to characterize the optical properties of fish skin and muscle samples.

Through these methods, the researchers found that ghost catfish muscles have “sarcomeres with periodic band structures inside tightly packed myofibril sheets.” This means that the light passes through the muscle fibers (myosin) and reasons diffraction — or the slight bending of light.

As a ghost catfish swims, the sarcomeres in the muscles expand and contract, changing their lengths and creating a “rapidly flashing dynamic diffraction pattern.” According to the press release, the study authors note that although other aquatic species have similar muscle fibers in similar structures, the ghost catfish’s transparent skin allows more than 90 percent of light to pass through its muscles.

Thanks to this discovery, researchers now have a better understanding of how other aquatic species become iridescent and how other structural colors have evolved in nature.

Read more: Some animals use iridescent colors to communicate

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