The hogfish, a remarkable denizen of the Atlantic Ocean spanning from North Carolina to Brazil, is renowned for its skin’s remarkable ability to undergo color changes, enabling it to blend with its coral reef habitat seamlessly.
However, a recent study published in the journal Nature Communications has unveiled an intriguing facet of these fish’s sensory capabilities: in addition to camouflage, hogfish may employ their skin as a means to perceive their underwater environment and, perhaps, even observe themselves.
Biologist Lori Schweikert from the University of North Carolina Wilmington led the research, motivated by her firsthand observations of hogfish behavior in the Florida Keys. She noted that these fish could continue to mimic their surroundings by changing colors even after death, prompting her to explore the possibility that hogfish might utilize their skin as a light-detecting organ, independent of their eyes and brain.
In a prior study, Schweikert and Duke University biologist Sönke Johnsen had uncovered that hogfish possess a gene responsible for a light-sensitive protein called opsin, which becomes active in their skin. Importantly, this opsin gene differs from those found in their eyes.
Similar light-sensing opsins have been identified in the skin of other color-changing animals like squid and geckos. Still, scientists have yet to decipher precisely how this phenomenon assists in color adaptation.
While one hypothesis suggests that light-sensing skin helps animals perceive their surroundings, another intriguing possibility is that it enables these creatures to view themselves.
In their latest investigation, Schweikert and Johnsen scrutinized samples of hogfish skin from various body regions under a microscope. They made a noteworthy discovery: each colorful spot on the skin corresponds to a specialized cell called a chromatophore.
These cells contain pigment granules that can be black, yellow, or red. The skin’s color transformation occurs as these pigment granules move within the chromatophores. Dispersion leads to darker hues, while clustering results in transparency.
To pinpoint the presence of light-sensitive opsin proteins within the skin, the researchers used an immunolabeling technique. They found that, in hogfish, opsins are not produced within the color-changing chromatophore cells themselves.
Instead, they are synthesized in adjacent cells located directly beneath the chromatophores. Detailed images obtained through transmission electron microscopy unveiled a previously unknown cell type beneath the chromatophores, brimming with opsin protein.
Schweikert explained that for light to reach the light-sensitive layer beneath, it must first traverse the pigment-laden chromatophores. The researchers estimated that hogfish opsins are most sensitive to blue light, the same wavelength absorbed most effectively by the pigment granules in hogfish skin.
In essence, these light-sensitive opsins serve as an internal photographic system for the hogfish. They capture variations in light and interpret them as pigment granules expand or contract, allowing the fish to effectively take a photograph of their own skin from the inside.
This remarkable ability to perceive their own color transformation proves valuable, as hogfish lack the means to inspect themselves physically.
While it may be tempting to draw parallels between hogfish skin and a giant eye, the researchers emphasize that such a comparison is not apt. They are not proposing that hogfish skin functions akin to an eye; rather, they suggest that hogfish exhibit the capacity to monitor their own color changes, offering an intriguing glimpse into the concealed talents of these aquatic chameleons.
