A few years ago, while fishing in the Florida Keys, biologist Lori Schweikert was confronted with an unusual phenomenon of rapid change. She caught a reef fish called a pig and threw it into the sea, she. But when he wanted to put it in the fridge later, he noticed something strange: his skin had taken on the same color and pattern as the boat’s deck.

Found in the Western Atlantic Ocean from North Carolina to Brazil, the porpoise is known for its color-changing skin. This species can change from white to mottled and reddish brown in a few milliseconds, mixing with coral, sand or rocks.

But Schweikert was surprised because this porpoise continued to disguise itself even though it was no longer alive. This made him wonder: Can a porpoise detect light using only its skin, independent of its eyes and brain?

“For me, it opened up all that space of possibilities,” Schweikert said.

In later years, Schweikert began researching the physiology of “skin vision” as a postdoctoral researcher at Duke University and Florida International University. In 2018, Schweikert and Duke biologist Zenke Johnsen published research showing that porpoises carry a gene for a light-sensitive protein called opsin that is activated in their skin, and that this gene is different from the opsin genes found in their eyes.

Other color-changing animals, from octopuses to lizards, have also been found to produce photosensitive opsins in their skin. But exactly how they use it to change colors is unclear.

“When we discovered this in porpoise, I looked at Zienki and said, ‘Why is there a light detector in the skin?’ said Schweikert, now an associate professor at the University of North Carolina at Wilmington.

One hypothesis is that photosensitive skin helps animals perceive their environment. But the new results suggest another possibility — they “could use it to see for themselves,” says Schweikert.

In a study published in the journal Nature Communications, Schweikert, Johnsen and colleagues teamed up to take a closer look at pig skin. The researchers took skin scraps from different parts of the fish’s body and photographed them under a microscope.

It looks like a polka dot painting next to the pigskin. Each colored spot is a special cell called a chromatophore, which contains pigment granules that can be red, yellow or black. It is the movement of these pigment granules that changes the color of the skin. As the granules spread inside the cell, the color darkens. When they collect in a spot too small to be seen with the naked eye, the cell becomes more transparent.

The researchers then used an immunolabeling technique to detect opsin proteins in the skin. They found that in porcine fish, opsins are not produced in the color-changing chromatophore cells. Instead, opsins reside in other cells directly below them.

Images taken using a transmission electron microscope revealed a previously unknown cell type located just below the chromatophores and packed with the opsin protein.

This means that light hitting the skin must pass through pigment-filled chromatophores before reaching the photosensitive layer, Schweikert says.

According to the researchers, opsin molecules in pig skin are most sensitive to blue light. This is the wavelength of light that the pigment granules in the fish’s chromatophores absorb best. The results show that the fish’s photosensitive opsins act as an internal Polaroid film, capturing changes in light that can filter through the pigment-filled cells above as clusters or branches of pigment granules.

“Animals can literally take pictures of their own skin from the inside out,” Johnsen said. “In a sense, they can tell the animal what its skin looks like because they can’t bend over to look.”

“To be clear, we’re not claiming that pigskin functions like eyes,” Schweikert added. Eyes do more than perceive light – they create images. “We have no evidence that this is exactly what happened to her skin,” Schweikert says.

Rather, it is a sensory feedback mechanism that allows the porpoise to monitor the color of its skin and fine-tune what it sees with its own eyes.

“They seem to be watching the color changes,” says Schweikert.

The researchers say the work is important because it could pave the way for new methods of sensory feedback for devices such as robotic limbs and self-driving cars that need to fine-tune their performance without relying solely on vision or a video camera.

“Sensory feedback is a technique that technology is still trying to understand,” Johnsen said. “This work is a good analysis of a novel sensory feedback system.”

“If you don’t have a mirror and you can’t bend your neck, how will you know you’re dressed appropriately?” – says Schweikert. “It may not matter to us,” he added. But for creatures that use their color-changing abilities to hide from predators, warn their opponents, or find mates, it can be a matter of life and death.