Several years ago, biologist Lori Schweikert found herself face-to-face with a fascinating phenomenon during a fishing trip in the Florida Keys. She encountered a hogfish, a reef fish known for its remarkable color-changing abilities. However, what caught her attention wasn’t just the fish’s ability to blend seamlessly with its environment but the fact that the hogfish’s skin continued to change its color and pattern to match the boat’s deck even after it was dead.

This encounter sparked Schweikert’s curiosity and led her down a path of research into the concept of “skin vision”: the detection of light using only skin cells, independent of eyes or the brain.

“That opened up this whole field for me,” Schweikert said.

The hogfish  (Lachnolaimus maximus), a common inhabitant of the western Atlantic Ocean, can morph its skin color from white to mottled to reddish-brown in milliseconds. This way, the fish perfectly blends in with its surroundings, which include corals, sand, and rocks.

Schweikert’s curiosity led to years of research and collaboration between her and Duke biologist Sönke Johnsen, culminating in a 2018 study published in Nature Communications. The report unveiled some remarkable findings about the hogfish’s skin vision. For instance, the researchers discovered that hogfish carry a gene for a light-sensitive protein called opsin, which is present not only in their eyes but also in their skin. This unique gene differed from the opsin genes responsible for their vision, raising the question of why they have light-sensing capabilities in their skin.

“When we found it in hogfish, I looked at Sönke and said: Why have a light detector in the skin?” said Schweikert, now an assistant professor at the University of North Carolina-Wilmington.

One potential hypothesis was that these light-sensitive skin cells helped animals sense their environment. However, the recent findings propose an intriguing alternative: these skin-based “eyes” might allow the hogfish to view itself.

“The animals can literally take a photo of their own skin from the inside,” Johnsen said. “In a way, they can tell the animal what its skin looks like, since it can’t really bend over to look.”

In the newest study, published in Nature Communications, the researchers found that the opsin proteins were not located in the color-changing chromatophore cells but rather in other cells beneath them. This arrangement suggests that the skin functions as a sensory feedback mechanism, allowing the hogfish to monitor its own changing skin colors and adjust them to match its visual perception.

The intricate mechanism of color change in hogfish involves specialized cells known as chromatophores, each containing pigment granules that can be red, yellow, or black. The movement of these granules, which either cluster together or spread out, causes the changes. The opsin molecules in the skin are most sensitive to blue light, the same wavelength that the pigment granules absorb best.

The system can be compared to internal Polaroid film, where the opsin-rich layer captures the light-filtering changes through the pigment-filled cells above. This novel sensory feedback mechanism is what enables the hogfish to essentially “see” its color changes.

“Just to be clear, we’re not arguing that hogfish skin functions like an eye,” Schweikert added. Eyes do more than merely detect light — they form images.

“We don’t have any evidence to suggest that’s what’s happening in their skin,” Schweikert said.

The hogfish’s ability to self-monitor and adjust their coloration significantly impacts their survival. This adaptation is crucial for animals that rely on color change for various purposes, from evading predators to attracting mates.

“If you didn’t have a mirror, and you couldn’t bend your neck, how would you know if you’re dressed appropriately?” Schweikert said. “For us, it may not matter,” she added. But for creatures that use their color-changing abilities to hide from predators, warn rivals or woo mates, “it could be life or death.”