How a fish sees with its skin

How a fish sees with its skin

The boar wrasse (Lachnolaimus maximus) can change its coloration, here a juvenile in the normal spotted color variant. © Lorian Schweikert

In a matter of seconds, the boar wrasse can change color to camouflage or communicate. Now a study shows that it can perceive its own color with the help of sensory cells in the skin. For this purpose, the fish has a layer of light-sensitive cells directly under the color cells of its skin. From the researchers' point of view, monitoring the color of one's own skin could be vital for the fish, for example when it comes to hiding from enemies.

The boar wrasse (Lachnolaimus maximus) is found in the western Atlantic from North Carolina to Brazil. It prefers to roam around lagoons and coral reefs and is known for being able to change its color to match its surroundings at lightning speed - from an inconspicuous white to speckled to reddish-brown. Previous studies have also shown that the fish produces the light-sensitive protein opsin in its skin. This also occurs in a different form in the eyes. However, the function of this photopigment in the skin was previously unclear.

Color change in the boar wrasse
Color variations of the Boar Wrasse and closeup of the chromatophores in its skin. © Lorian Schweikert et al./ Nature Communications

Light-sensitive cells under the pigments

A team led by Lorian Schweikert from Florida International University in North Miami has now examined the skin of the boar wrasse more closely. The researchers first visualized the cells responsible for the color changes in the fish under the microscope. These so-called chromatophores contain small pigment granules in red, yellow and black. If the pigment granules are closely packed together, the cell is almost transparent. On the other hand, if the pigments are distributed, the cell takes on a darker color.

In the next step, the team used a technique called immune labeling to stain the opsin proteins in the examined skin pieces of the boar wrasse. They found that the opsin was not in the chromatophores as expected. "The immune labeling showed that it was located directly under the chromatophores," the researchers report. "The cells with the opsin do not form a continuous layer in the skin, but are located individually just below the chromatophores."

Inside shot of your own skin

This positioning of the photosensitive cells means that light hitting the skin must first pass through the pigment-filled chromatophores before reaching the photosensitive layer. Because the pigments absorb the exact wavelengths of light that lead to opsin activation, opsin is only activated in fair skin. If the pigments are distributed in the chromatophores and the skin of the fish is dark, the light no longer reaches the opsin.

According to the researchers, this mechanism suggests that the light-sensitive cells function as a kind of monitoring device. "The animals can literally take a picture of their own skin from the inside," explains co-author Sönke Johnsen of Duke University in Durham. "In this way, the sensory cells can give the fish feedback on what its skin looks like, since it cannot turn around to look." This information can be vital for the fish. Finally, skin color allows him to hide from enemies, warn rivals, or woo mates.

No extra eyes

However, according to the research team, the findings don't mean the fish's skin functions like extra eyes. Because eyes can not only perceive light, but also link the information to images. "We have no evidence that this happens in the skin," says Schweikert. Instead, it is more of a sensory feedback mechanism. The research team suspects that similar mechanisms could also play a role in other color-changing animals such as squid or geckos. The technology could possibly also learn something from the fish: "Sensory feedback is one of the tricks that technology is still trying to fathom, for example for robots or self-driving cars," says Johnsen.

Source: Lorian Schweikert (Florida International University, North Miami) et al., Nature Communications, doi: 10.1038/s41467-023-40166-4

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