A few years ago, during a fishing trip in the Florida Keys, biologist Lori Schweikert witnessed an unusual phenomenon: she caught a dogfish (Lachnolaimus maximus), a specimen of a species that can weigh ten kilos and measure about one meter, whose pointed snout is very characteristic. When he was going to move him from the deck of the ship to the refrigerator, he realized something strange: his skin had taken on the same color and design as the deck of the ship. Common in the western Atlantic Ocean from North Carolina to Brazil, the dogfish is known for its color-changing skin. The species can transform from white to speckled to reddish brown in a matter of milliseconds to blend in with corals, sand or rocks. Still, Schweikert was surprised because this fish had continued its camouflage even though it was no longer alive. Which made her wonder: Can pigfish detect light using just their skin, independently of their eyes and brain? A pointed-snouted reef fish called a hogfish can change from white to spotted brown to reddish depending on its environment Dean Kimberly and Lori Schweikert “That opened up this whole field for me,” Schweikert says. In the following years, the biologist began researching the physiology of "skin vision" as a postdoctoral fellow at Duke University and Florida International University. Previous work In 2018, Schweikert and Duke biologist Sönke Johnsen published a study in the 'Journal of Comparative Physiology A' showing that pigfish have a gene for a light-sensitive protein called opsin that is activated in your skin, and that this gene is different from the opsin genes found in your eyes. Other animals that change color, from octopuses to geckos, have also been found to produce light-sensitive opsins in their skin. But it's unclear exactly how they use them to help change color. "When we found it in the dogfish, I looked at Sönke and said, 'Why have a light detector in the skin?'" explains Schweikert, now a professor at the University of North Carolina at Wilmington. One hypothesis is that light-sensitive skin helps animals assimilate their environment. But the new findings suggest another possibility: "They could be using it to see themselves," Schweikert says. Taking a closer look In a new study just published in the journal Nature Communications, Schweikert, Johnsen and their colleagues teamed up to take a closer look at dogfish skin. The researchers took pieces of skin from different parts of the fish's body and took photographs of them under a microscope. Up close, their skin looks like a pointillist painting: each dot of color is a specialized cell called a chromatophore that 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. When the granules spread throughout the cell, the color appears darker. When they clump together into a tiny spot that is difficult to see, 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 dogfish, opsins are not produced in chromatophore cells that change color. Instead, opsins reside in other cells directly below them. Images taken with a transmission electron microscope revealed a previously unknown cell type, just below the chromatophores, filled with opsin protein. This means that light hitting the skin must first pass through the pigment-filled chromatophores before reaching the light-sensitive layer, explains Schweikert. Viewed through a microscope, the skin of a pigfish looks like a pointillist painting. Each color dot is a specialized cell that contains pigment granules that can be red, yellow or black. Pigment granules may spread or clump closely together within the cell, making the color appear darker or more transparent Lori Schweikert, University of North Carolina Wilmington Researchers estimate that opsin molecules in dogfish skin are more sensitive in the blue light. This is the wavelength of light that is best absorbed by the pigment granules in fish chromatophores. Like a Polaroid The findings suggest that the fish's light-sensitive opsins act like an internal Polaroid film, capturing changes in light that is able to filter through the pigment-filled cells above as the pigment granules They group together or fan out. "Animals can literally take a photo of their own skin from the inside," Johnsen notes. "In a way, they can tell the animal what its skin looks like, since it can't actually bend over to look." "To be clear, we are not arguing that dogfish skin functions as an eye," adds Schweikert. The eyes do more than simply detect light: they form images. "We don't have any evidence to suggest that this is what happens in their skin," the biologist points out. Rather, it is a sensory feedback mechanism that allows the dogfish to monitor its own skin as it changes color and adjust it to fit what it sees with its eyes. "They seem to be seeing their own color change," he says. The researchers say the work is important because it could pave the way to new sensory feedback techniques for devices like robotic limbs and self-driving cars that must adjust their performance without relying solely on vision or camera images. "Sensory feedback is one of the tricks that technology is still trying to solve," says Johnsen. "This study is a good dissection of a new sensory feedback system." MORE INFORMATION news No China observes, for the first time, structures hidden 300 meters below the surface on the far side of the Moon news Yes India aspires to become the fourth country to reach the Moon «If you didn't have a mirror and didn't If you could bend your neck, how would you know if you were dressed appropriately? -says Schweikert-. For us it may not matter. But for creatures that use their color-changing abilities to hide from predators, warn rivals or court mates, "it could be life or death."
