What do you call a fish with no eyes? Fsh. What about a fish that can also use its skin as “eyes”? Well, that would be a hogfish.
Hogfish often use their ability to change colours to support their camouflaging abilities. They also have light-sensing skin, or skin vision, that can help them “see” their surroundings.
However, research suggests hogfish are not only using this skin vision to see their surroundings. Lorian Schweikert, a biologist at the University of North Carolina Wilmington, said they “could be using it to view themselves”.
To study this behaviour the team took samples of the hogfish’s skin and analysed them under a microscope. This up-close look showed many cells called chromatophores, which contain granules of colours.
Writing in Nature Communications, the research reports the cellular mechanism in chromatophore pigment activity and how the hogfish uses this ability.
Previous research found that hogfish have a light-sensitive protein called opsin in their skin, which is a different opsin to that found in their eyes.
The granules of colours move around in the cell. When they get close together they become transparent, while when they spread out the colours appear darker. The researchers then located the light-sensitive protein, which resided in cells below the chromatophores.
Schweikert said this meant that light striking the skin of the hogfish had to pass the chromatophores before it got to this light-sensitive layer.
This allows the fish to capture changes in the light and filter through these pigment-filled chromatophores – a bit like a Polaroid.
“The animals can literally take a photo of their own skin from the inside,” said Sönke Johnsen, a biologist at Duke University who was also part of the study. “In a way they can tell the animal what its skin looks like, since it can’t really bend over to look.”
Schweikert said that the skin did not function exactly like an eye: it is rather a sensory feedback mechanism that lets the hogfish monitor its own skin while it changes colours.
Lauren Sumner-Rooney, a researcher at the Museum of Natural History in Berlin, who is not affiliated with the research, said that the ability to colour change and get colours right for the hogfish was a matter of life or death.
She added that many animals could not see their whole body surface with their eyes, and needed another way of knowing whether they had expanded and contracted the right chromatophores.
“This provides a neat and simple mechanism by which fish can tell whether they have successfully changed colour, by using light sensors dispersed across their whole bodies, instead of relying on their eyes,” she said.
“This is the first time we’ve seen a strong body of evidence for exactly how this works in fish – dermal light sensing has been a rather enigmatic ability for a long time.”
