Like a chameleon, a hogfish can quickly change the color of its skin. These reef-dwelling creatures can toggle between appearing white, reddish-brown and spotted at the drop of a hat, both for camouflage and for social signaling.
But on a fishing trip in the Florida Keys several years ago, Lorian Schweikert observed something strange about this hue-shifting ability: She reeled in a hogfish, and even after death, it changed color to blend in with the deck of the boat.
This surprising event suggested hogfish might not need their eyes, or even their brains, to camouflage, according to a statement from Duke University. Now, in a paper published Tuesday in the journal Nature Communications, Schweikert and her colleagues suggest a mechanism for how hogfish switch to the correct color—and it has to do with their unique skin.
A light-sensitive protein in a hogfish’s skin might be monitoring changes in its color, the researchers theorize. The fish “could be using it to view themselves,” Schweikert, first author of the study and a biologist at the University of North Carolina Wilmington, says in the statement.
The research is “a really exciting and considerable step toward answering the long-standing question, ‘How can these animals tell what color they are?” Lauren Sumner-Rooney, an evolutionary biologist at the Museum of Natural History in Germany who did not contribute to the study, tells Scientific American’s Rachel Nuwer.
No mirror and can't bend your neck? Our study shows that light receptors underlie color change cells in a 1:1 ratio, perhaps to monitor the status of dynamic skin color change. #sciTwitter #colSci @laura_bagge @lyd_love_laugh @BrackenGrissom @sonkelab https://t.co/yofzziHgNv pic.twitter.com/XeoDoLa7kE— Lorian Schweikert (@LoriSchweikert) August 22, 2023
Chameleons, octopuses, squids and cuttlefish can all change color to match their surroundings using special skin cells called chromatophores. Hogfish, too, use chromatophores to alter their hue in less than a second, by shifting the way that pigments are organized in the cells. But Schwiekert wondered how the fish knew they were changing into the correct color, writes Scientific American.
In a 2018 study, Schweikert and colleagues found the fish sensed light with their skin using a protein called opsin. This time around, the researchers tried to pinpoint how this light-sensing ability worked.
The team used electron microscopy to image how these cells were arranged and revealed that beneath each chromatophore was a previously unknown type of cell that was full of opsins, according to the New York Times’ Elizabeth Anne Brown. In this structure, the behavior of the pigments in the chromatophores governs how much light reaches the opsin receptors below.
“I absolutely screamed,” Schweikert tells the New York Times. “Nothing had ever been seen like this.”
“The animals can literally take a photo of their own skin from the inside,” Sönke Johnsen, a co-author of the study and biologist at Duke University, says in the statement. “In a way, they can tell the animal what its skin looks like, since it can’t really bend over to look.”
But the light receptors in a hogfish’s skin wouldn’t be forming images like an eye does—just detecting light, as Schweikert notes in the statement.
By tracking changes in skin color, the light-sensitive opsins could allow hogfish to fine-tune their color changing to match their surroundings. However, the researchers have yet to find a mechanism through which the cells with opsin communicate with the chromatophores to adjust their color, they note in the study.
The work makes progress in explaining the connection between chromatophores and opsins in hogfish, Todd Oakley, an evolutionary biologist at the University of California, Santa Barbara, who wasn’t involved in the research, tells the New York Times. It’s the first time scientists have found strong evidence to describe how light-sensing skin works in fish, as Sumner-Rooney tells the Guardian’s Gabriella Sotelo.
The researchers are now working to determine more precisely what the newly discovered light-sensitive cells are, per Scientific American. Next, they hope to find out whether other creatures use a similar strategy to change color.