US biologist Lori Schweikert was looking at a hogfish caught in the Florida Keys when she noticed that, even though it was dead, its skin had the colour and patterning of the boat-deck on which it lay.
Hogfish (Lachnolaimus maximus) are a species of wrasse familiar to western Atlantic and Caribbean reef divers and, while their ability to camouflage themselves by colour-shifting was known about, continuing to do this even after death led Schweikert to wonder whether the fishes’ skin might detect light independently of their eyes and brain.
She explored the idea of “skin vision” as a post-doctoral fellow at Duke University and Florida International University and, with fellow-biologist Sönke Johnsen, published a 2018 study showing that hogfish carry a gene for the light-sensitive protein opsin activated in their skin – an opsin gene distinct from those found in their eyes.
Now the scientists’ hypothesis that light-sensing skin helps hogfish and other animals such as octopuses take in their surroundings has been overtaken by new findings suggesting a more surprising possibility – that the fish use it to view themselves, to ensure that their camouflage is appropriate for their surroundings.
As Schweikert puts it: “If you didn’t have a mirror, and you couldn’t bend your neck, how would you know if you’re dressed appropriately?”
Unknown type of cell
Schweikert, now an assistant professor at the University of North Carolina Wilmington, and Johnsen put together a team that included scientists at Florida Institute of Technology, Florida International University and the Air Force Research Laboratory to examine skin from different parts of a hogfish's body.
Each chromatophore (skin-cell) contains granules of red, yellow or black pigment that can fan out across the cell to darken its colour, or bunch together so that the cell becomes more transparent.
Hogfish opsins were not produced in the skin-cells, the team discovered. Instead they found a previously unknown type of cell packed with opsin protein lying just below the chromatophores.
Light striking the skin had to pass through the pigment-filled chromatophores before reaching this light-sensitive layer. Opsin molecules in hogfish skin were estimated to be most sensitive to blue light, which the pigment granules in the chromatophores were best designed to absorb.
The light-sensitive opsins appear to capture light changes and filter through the pigment-filled cells above as the pigment granules either bunch up or fan out.
Watching their own colour change
“The animals can literally take a photo of their own skin from the inside,” says Johnsen. “In a way they can tell the animal what its skin looks like, since it can’t really bend over to look.”
Eyes detect light but also form images. “We don’t have any evidence to suggest that's what’s happening in their skin,” says Schweikert, but believes that hogfish “do appear to be watching their own colour change” to allow them to fine-tune their colour to fit what they see with their eyes.
For creatures that change colour to evade predators, hunt, warn off rivals or win mates, such an ability “could be life or death”.
The team believe their work could pave the way to new sensory feedback techniques for devices such as robotic limbs and self-driving cars that need to fine-tune their performance without relying solely on eyesight or camera feeds. Their study has just been published in Nature Communications.
Also on Divernet: Octopuses dream – but what about?