2023-08-22 17:08:08
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 up to ten kilos and measure close to one meter, whose pointed snout is very characteristic. When she was going to move him from the deck of the ship to the refrigerator, she noticed something strange: her skin had taken on the same color and pattern 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 mottled to reddish-brown in a matter of milliseconds to blend in with corals, sand, or rocks. Still, Schweikert was surprised that this fish had continued its camouflage even though it was no longer alive. Which made her wonder: can hogfish detect light using just their skin, independent of their eyes and brain?
A sharp-nosed 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,” says Schweikert. In subsequent years, the biologist began researching the physiology of “skin vision” as a postdoctoral fellow at Duke University and Florida International University.
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In 2018, Schweikert and Duke biologist Sönke Johnsen published a study in the journal ‘Journal of Comparative Physiology A‘ which shows that hogfish have 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 geckos, have also been found to produce light-sensitive opsins in their skin. But it’s not clear exactly how they use them to help change the color.
“When we found it in the dogfish, I looked at Sönke and said, why have a light detector on 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 surroundings. But the new findings suggest another possibility: “They could be using it to see themselves,” Schweikert says.
looking closer
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 pictures of them under a microscope. Up close, his 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 are spread around the cell, the color appears darker. When they clump together into a tiny point that’s hard to see, the cell becomes more transparent.
Next, the researchers used a technique called immunolabeling to localize the opsin proteins within the skin. They found that in dogfish, opsins are not produced in color-changing chromatophore cells. 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, Schweikert explains.
Seen through a microscope, the skin of a hogfish 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 can spread out or clump together within the cell, making the color appear darker or more transparent Lori Schweikert, University of North Carolina Wilmington
The researchers estimate that the opsin molecules in dogfish skin are more sensitive to blue light. This is the wavelength of light that is best absorbed by pigment granules in fish chromatophores.
like a polaroid
The findings suggest that the fish’s light-sensitive opsins act like internal Polaroid film, capturing changes in light that is able to filter through the pigment-filled cells above as the pigment granules clump together or clump together. they fan out. “Animals can literally take a picture of their own skin from the inside,” Johnsen notes. “In a way, they can tell the animal what its fur looks like, since it can’t actually lean down to look.”
“To be clear, we’re 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 on their skin,” the biologist points out. Rather, it’s a sensory feedback mechanism that allows the dogfish to monitor its own skin as it changes color and adjust it to match what it sees with its eyes. “They seem to be seeing their own color change,” she says.
The researchers note that the work is important because it could pave the way toward new sensory feedback techniques for devices like robotic limbs and self-driving cars that must adjust their performance without relying solely on sight or camera images. “Sensory feedback is one of the tricks that technology is still trying to figure out,” says Johnsen. “This study is a nice dissection of a new sensory feedback system.”
“If you didn’t have a mirror and you couldn’t bend your neck, how would you know if you’re 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.”
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