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Scientists Find Fish That 'sees' With Its Skin 'even After It Dies'
Reef-dwelling hogfish monitor changes to their own skin colour based on the environment in which they swim, sensing their surroundings using special light-sensing cells on their skin "even after they die", a new study reveals.
The research, published in the journal Nature this week, improves our understanding of the behaviour and evolution of these fish, and how certain animals are able to monitor their own skin colour changes and quickly adapt.
"They appear to be watching their own colour change," study co-author Lori Schweikert said.
"In a way they can tell the animal what its skin looks like, since it can't really bend over to look," explained Sonke Johnsen, another author of the study.
Many fish, including squid, amphibians, reptiles and fish, have the natural ability to rapidly change colour, with the trait evolving multiple times in several different animal species.
Creatures find this trait useful for adapting to environmental temperature changes, attracting mates and providing camouflage, said researchers, including those from University of North Carolina, Wilmington in the US.
Cells on their bodies called chromatophores, which contain pigments, crystals or tiny reflective plates, enable these animals to rapidly change their colours within minutes or less.
Hogfish, for example, change their colour to camouflage and escape predators, or for social signalling.
The reef fish is common in the western Atlantic Ocean from North Carolina to Brazil and is known for its colour-changing skin.
It is known to morph from white to mottled to reddish-brown in a matter of milliseconds to blend in with corals, sand or rocks.
They do this by moving pigment around within their body's chromatophore cells to expose or cover the white tissue underneath.
However, it remains unclear how hogfish regulate and perceive these colour changes.
What particularly surprised scientists in the study is that the fish continued its camouflage even though it was no longer alive.
In the new research, they used microscopy to examine hogfish skin in detail by measuring the impact of light on different parts of the fish.
Researchers found that light receptors, called SWS1, that are underneath the chromatophore may be involved in the process.
They said these cells are sensitive to the light shining through colours expressed by chromatophores, specifically the wavelength of light that is present in their coral reef habitat.
These receptors, according to scientists, provide feedback to the fish on where and how colour changes are occurring in different parts of their skin.
"By examining the morphology, physiology, and optics of dermal photoreception in hogfish (Lachnolaimus maximus), we describe a cellular mechanism in which chromatophore pigment activity (ie, dispersion and aggregation) alters the transmitted light striking SWS1 receptors in the skin," scientists wrote in the study.
This feature enables the reef-dwelling fish is to monitor the chromatophores and sense information about their own colour change performance.
"The animals can literally take a photo of their own skin from the inside. In a way they can tell the animal what it's skin looks like, since it can't really bend over to look," Dr Johnsen explained.
"Just to be clear, we're not arguing that hogfish skin functions like an eye," Dr Schweikert said, adding that eyes are capable of more than just detecting light.
This Fish Doesn't Just See With Its Eyes -- It Also Sees With Its Skin.
Published August 22, 2023Now researchers think they know why.
DURHAM, N.C. -- A few years ago while on a fishing trip in the Florida Keys, biologist Lori Schweikert came face to face with an unusual quick-change act. She reeled in a pointy-snouted reef fish called a hogfish and threw it onboard. But later when she went to put it in a cooler she noticed something odd: its skin had taken on the same color and pattern as the deck of the boat.
Former Duke postdoc Lori Schweikert holds a hogfish she caught while on a fishing trip in the Florida Keys. A common fish in the western Atlantic Ocean from North Carolina to Brazil, the hogfish is known for its color-changing skin. The species can morph from white to mottled to reddish-brown in a matter of milliseconds to blend in with corals, sand or rocks.
Still, Schweikert was surprised because this hogfish had continued its camouflage even though it was no longer alive. Which got her wondering: can hogfish detect light using only their skin, independently of their eyes and brain?
"That opened up this whole field for me," Schweikert said.
A pointy-snouted reef fish called the hogfish can change from white to spotted brown to reddish depending on its surroundings. Photos courtesy of Dean Kimberly and Lori Schweikert. In the years that followed, Schweikert started researching the physiology of "skin vision" as a postdoctoral fellow at Duke University and Florida International University.
In 2018, Schweikert and Duke biologist Sönke Johnsen published a study showing that hogfish 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 geckos have been found to make light-sensing opsins in their skin, too. But exactly how they use them to help change color is unclear.
"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 hypothesis is that light-sensing skin helps animals take in their surroundings. But new findings suggest another possibility -- "that they could be using it to view themselves," Schweikert said.
In a study published Aug. 22 in the journal Nature Communications, Schweikert, Johnsen and colleagues teamed up to take a closer look at hogfish 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, a hogfish's skin looks like a pointillist painting. Each dot of color is a specialized cell called a chromatophore containing granules of pigment that can be red, yellow or black.
When viewed under a microscope, the pearly white (left) or reddish (right) skin of a hogfish looked like a pointillist painting. It's the movement of these pigment granules that changes the skin color. When the granules spread out across the cell, the color appears darker. When they cluster together into a tiny spot that's hard 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 the hogfish, opsins aren't produced in the color-changing chromatophore cells. Instead, the opsins reside in other cells directly beneath them.
Images taken with a transmission electron microscope revealed a previously unknown cell type, just below the chromatophores, packed with opsin protein.
This means that light striking the skin must pass through the pigment-filled chromatophores first before it reaches the light-sensitive layer, Schweikert said.
The researchers estimate that the opsin molecules in hogfish skin are most sensitive to blue light. This happens to be the wavelength of light that the pigment granules in the fish's chromatophores absorb best.
The findings suggest that fish's light-sensitive opsins act somewhat like internal Polaroid film, capturing changes in the light that is able to filter through the pigment-filled cells above as the pigment granules bunch up or fan out.
"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 it's skin looks like, since it can't really bend over to look."
"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.
Rather, it's a sensory feedback mechanism that lets the hogfish monitor its own skin as it changes color, and fine-tune it to fit what it sees with its eyes.
"They appear to be watching their own color change," Schweikert said.
The researchers say the work is important because it could pave the way to new sensory feedback techniques for devices such as robotic limbs and self-driving cars that must fine-tune their performance without relying solely on eyesight or camera feeds.
"Sensory feedback is one of the tricks that technology is still trying to figure out," Johnsen said. "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?" 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."
The study was co-authored by researchers from the 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 (1556059).
Beyond Eyes: This Fish Can See With Its Skin
Researchers discovered that hogfish possess a unique light-sensitive protein in their skin, enabling them to change colors. The study suggests that these fish can monitor their own skin color changes, possibly allowing them to adapt to their environment more effectively.
Now researchers think they know why.Several years back, during a fishing expedition in the Florida Keys, biologist Lori Schweikert came face to face with an unusual quick-change act. She caught a pointy-snouted reef fish known as a hogfish and placed it on her boat's deck. However, when she later intended to transfer it to a cooler, she observed a peculiar phenomenon: its skin had taken on the same color and pattern as the deck of the boat.
A common fish in the western Atlantic Ocean from North Carolina to Brazil, the hogfish is known for its color-changing skin. The species can morph from white to mottled to reddish-brown in a matter of milliseconds to blend in with corals, sand, or rocks.
Still, Schweikert was surprised because this hogfish had continued its camouflage even though it was no longer alive. This got her wondering: Can hogfish detect light using only their skin, independently of their eyes and brain? "That opened up this whole field for me," Schweikert said. In the years that followed, Schweikert started researching the physiology of "skin vision" as a postdoctoral fellow at Duke University and Florida International University. In 2018, Schweikert and Duke biologist Sönke Johnsen published a study showing that hogfish 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.
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