Now researchers think they know why.
Several years ago, during a hunting trip in the Florida Keys, biologist Lori Schweckert came face to face with unusually rapid change. She catches a coral fish with a pointed snout known as a pigfish and puts it on the deck of her boat. However, when I later resolved to take him to the cooler, I noticed a strange phenomenon: his skin had assumed the same color and pattern as the deck of the boat.
The pigfish is a fish common in the western Atlantic Ocean from North Carolina to Brazil, and is known for its discolored skin. the classify It can go from white to speckled to reddish brown in a matter of milliseconds to blend in with coral, sand or rocks.
However, Schweckert was surprised because this pigfish continued its camouflage even though it was no longer alive. This made her wonder: Can the pigfish detect light using only its skin, independently of its eyes and brain?
“It opened up a whole field for me,” Schweckert said.
In the years that followed, Schweckert began researching the physiology of “skin vision” as a postdoctoral fellow at Duke University and Florida International University. In 2018, Schweckert and Duke biologist Sonky Johnson published an article Stady Pigfish appear to carry a gene for a light-sensitive protein called opsin that is activated in their skin and that this gene differs from the opsin genes found in their eyes.
Other color-changing animals, from octopuses to geckos, have been found to make light-sensitive opsins in their skin as well. But it’s not clear exactly how they use it to help change color.
“When we found it in the pigfish, I looked at Sonky and said, ‘Why is there a photodetector in the skin?'” said Schweckert, now an assistant professor at the University of North Carolina Wilmington.
One hypothesis is that the light-sensing skin helps the animals to assimilate their surroundings. But the new findings point to another possibility, “that they might use it to see themselves,” Schweckert said. In a study recently published in the journal Nature CommunicationsSchweckert, Johnson, and colleagues teamed up to take a closer look at pigfish skin.
The researchers took pieces of skin from different parts of the fish’s body and took pictures of it under a microscope.
Up close, the pigskin looks like a pointillist painting. Each dot of color is a specialized cell called a chromatophore that contains granules of pigment 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 across the cell, the color appears darker. When they clump together in a small, hard-to-see spot, the cell becomes more transparent.
Next, the researchers used a technique called immunolabeling to locate the opsin proteins within the skin. They found that in porcine, opsins are not produced in the discolored chromatophores. Instead, the opsins reside in other cells just below them.
The transmission electron microscope images revealed a previously unknown cell type, just below the chromatophores, that was full of the opsin protein. This means that light hitting the skin must pass through the pigment-filled chromatophores first before it reaches the photosensitive layer, Schweckert said.
The researchers estimate that the opsin molecules in pigfish skin are the most sensitive to blue light. It just so happens that this is the wavelength of light that the pigment granules in fish chromatophores absorb best. The results indicate that photosensitive opsins in fish act somewhat like an inner Polaroid film, capturing changes in light that are able to filter through the pigment-filled cells above where pigment granules aggregate or diffuse.
“The animals can take a picture of the inside of their skin,” Johnson said. “In a way, they can tell the animal what its skin looks like, because they can’t bend over to look at it.”
“Just to be clear, we’re not saying that pork skin works like an eye,” Schweckert added. Eyes do more than detect light, they create images. “We don’t have any evidence to suggest that this is what happens in their skin,” Schweckert said.
Rather, it is a sensory feedback mechanism that allows the pigfish to monitor its skin as it changes colour, and adjust it to match what it sees with its eyes.
“It looks like they’re watching their color change,” Schweckert said.
The researchers say this work is important because it could pave the way for new sensory feedback technologies for devices such as robotic limbs and self-driving cars that must adjust their own performance without relying solely on sight or a camera feed.
“Sensory feedback is one of the tricks that technology is still trying to figure out,” Johnson said. “This study is a fine dissection of a new sensory feedback system.”
“If you don’t have a mirror, and you can’t bend your neck, how do you know if you’re properly dressed?” Schweikert said. “For us, it may not matter,” she added. But for creatures that use their color-changing abilities to hide from predators, warn off rivals, or attract mates, “it could be life or death.”
Reference: “Dynamic light filtering over cutaneous opsins as a sensory feedback system in fish color change” by Laurian E. Schweckert, Laura E. Page, Lydia F Naughton, Jacob R. Bolin, Benjamin R. Wheeler, Michael S. Grace Heather D. Bracken-Grissom and Sonky Johnson, Aug. 22, 2023, Available here. Nature Communications.
The study was co-authored by researchers from Florida Institute of Technology, Florida International University, and the Air Force Research Laboratory. Financial support came from Duke University, Florida International University, the Marine Biological Laboratory, and the National Science Foundation.