Now we know how ghost catfish get their rainbow glow. These small fish, popular for home aquaria due to their almost complete transparency, derive their iridescence from the fibers of their muscles, unlike many other species that sparkle.
Aside from their heads and spines, ghost catfish (Kryptopterus vitreolus) are almost completely transparent. When light shines through their bodies, they take on a colorful iridescent sheen despite having no pigment in the transparent parts of their bodies. Qibin Zhao at Shanghai Jiao Tong University in China and his colleagues examined the fish and samples of their various tissues to understand how this iridescent scene occurs.
“Different from most fish species that have been identified as having structural colors in reflection, the structural color of ghost catfish only appears in transmission, which is unusual,” Zhao says. These other fish tend to get their shimmer from photonic structures – shapes that change the color of light when they reflect it – in their scales and skin, which is how we assumed the Ghost Catfish had its glow, too.
“We actually spent quite a bit of time at first looking for photonic structures in the skin, and we started studying the muscle just to find out why it’s so transparent, until after For several months, we realized that it was the muscles that caused the colors to diffract,” says Zhao. The researchers discovered that the colors come from muscle fibers called sarcomeres that are involved in muscle contraction and relaxation.
When light shines through the fish, these sarcomeres act like prisms, breaking the light down into its constituent colors. Although these structures can also appear in other types of fish, we can only see them in the ghost catfish because they are so small – only a few centimeters long – and so transparent.
In the wild, this iridescence can be useful for communicating with other fish or hiding catfish shadows from deeper-swimming predators, Zhao says. Other transparent fish, such as eel larvae and icefish, can get their shine in a similar way.