In the waters off the Florida Keys, biologist Lori Schweikert stumbled upon an astonishing revelation about a well-known fish species. On a fishing excursion, Schweikert observed the peculiar behavior of a hogfish that she had caught.
The fish, known for its uncanny ability to shift its skin color rapidly, adapted to both the shade and pattern of the boat deck, even after its demise.
Though renowned for its chameleon-like abilities, shifting between white, mottled, and reddish-brown to merge seamlessly with the corals, sands, and rocks of its surroundings, this particular observation piqued Schweikert’s curiosity.
Schweikert wondered if hogfish can detect light using only their skin, independently of their eyes and brain. “That opened up this whole field for me.”
To investigate, Schweikert took up the research on “skin vision” during her postdoctoral tenure at Duke University and Florida International University.
In collaboration with Duke biologist Sönke Johnsen in 2018, she discovered that the hogfish possess a unique gene responsible for a light-sensitive protein, opsin, which gets activated within their skin. Interestingly, this opsin gene was distinct from the ones present in their eyes.
This discovery is not entirely isolated. Animals ranging from octopuses to geckos have been found to possess light-sensing opsins in their skin. But the purpose behind this phenomenon remains elusive.
Reflecting on the hogfish finding, Schweikert recalled her conversation with Johnsen: “When we found it in hogfish, I looked at Sönke and said: Why have a light detector in the skin?”
While one prevailing theory suggests light-sensitive skin aids animals in better understanding their environment, Schweikert proposes a fascinating alternative. She believes that they might be utilizing this mechanism “to view themselves.”
In a study that is now published in the journal Nature Communications, Schweikert, Johnsen, and their colleagues closely examined hogfish skin.
The team discovered that under microscopic scrutiny, the hogfish skin resembled a pointillist artwork, each dot a specialized pigment-filled cell called a chromatophore. The chromatic shifts are governed by the movement of these granules within the cells.
Utilizing a technique known as immunolabeling, the team pinpointed the location of opsin proteins, finding them not in the chromatophores, but in an adjacent layer below.
Furthermore, electron microscope imagery revealed an unknown cell type beneath the chromatophores that was loaded with opsin protein.
Schweikert explained the process: “Light striking the skin must pass through the pigment-filled chromatophores first before it reaches the light-sensitive layer.”
Given that the opsins in hogfish skin are most responsive to blue light – coincidentally the same light wavelength absorbed best by the chromatophores – it’s plausible that these light-sensitive opsins function like an internal photographic film, capturing shifts in light that permeates the pigment cells above.
“The animals can literally take a photo of their own skin from the inside,” said Johnsen. However, Schweikert noted, “Just to be clear, we’re not arguing that hogfish skin functions like an eye.”
Schweikert emphasizes that while eyes form images, there’s no evidence suggesting similar capabilities in their skin. Instead, it’s a mechanism allowing the hogfish to monitor its own skin, adjusting its colors based on what it perceives with its eyes. “They appear to be watching their own color change,” said Schweikert.
According to the researchers, their 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,” said 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?” Schweikert said. “For us it may not matter,” 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.