Humans see the world with their eyes, but fish can "see the world" with their skin?

Produced by: Science Popularization China

Author: Tian Dawei (popular science author)

Producer: China Science Expo

When it comes to seeing things, our first reaction is to see with our eyes.

But recently, scientists discovered that there is a kind of fish that can not only see things with its eyes, but also "observe" the surrounding environment with its skin. Even after death, the receptors in the skin are still functioning.

Pigfish See the World with Their Skin

This fish is called Lachnolaimus maximus. It has a long nose that looks a bit like a pig's nose, and uses it to search for crustaceans hidden in the sand. This searching action is also very similar to the way a pig uses its nose to push and pull, so they are also called hogfish.

Pigfish

(Image source: Wiki)

The most special thing about pigfish is not its nose, but its ability to change color .

Pigfish can quickly adjust the color of their skin to blend in with the surrounding coral reef environment, or change to bright colors to drive away competitors or attract the opposite sex.

The familiar chameleons often take several minutes to change color, but pigfish change color very quickly, even changing the color of their skin in less than a second, which is comparable to animals such as octopuses and squids.

Octopus that can change color instantly

(Image source: Wiki)

What's even more interesting is that these fish can still change color even after death . The person who discovered this phenomenon was biologist Lori Schweikert. A few years ago, she caught a hogfish while fishing. The hogfish died soon after leaving the water. Schweikert threw the dead hogfish on the deck. After a while, when she was about to put the hogfish in the refrigerator, she found that the hogfish's skin had turned the same color as the deck.

This discovery surprised her. Previously, scientists believed that pigfish used their eyes to observe the surrounding environment, and then sent instructions to the skin through the brain to change color. But after the pigfish died, the visual system of the eyes and brain stopped working, so how could they still change color? Could it be that pigfish can change color without relying on the brain and eyes?

Schweikert fishing for the long-haired lipped wrasse

(Image source: Nature Communications)

Although it is known that mollusks such as octopuses and squids can detect their surroundings through photoreceptors on their skin, this has not been seen in fish. So Schweikert and his colleagues began to study the color change of pigfish.

In 2018, Schweikert and his colleagues published an article in which scientists pointed out that there is a light-sensitive protein in the skin of pigfish - opsin SWS1 . This opsin is present in the cones of many animals (including humans).

Five years later, in August 2023, Schweikert and his colleagues published another paper in Nature Communications, explaining how the photosensitive proteins in pigfish skin help them adjust their skin color.

To understand this, we must first talk about the color change principle of pigfish.

The principle of biological color change

The principle of pigfish's color change is very similar to that of chameleons, octopuses, and squids, and is achieved by regulating the pigment cells in the body .

chameleon

(Image source: Wiki)

In the latest study, Schweikert and his colleagues detected red, yellow and black pigment cells in the skin of pigfish. By adjusting the relative size of these pigment cells, the skin can present different types of colors , similar to how we use paint to mix colors.

In addition, Schweikert and his colleagues also observed rainbow pigment cells and white pigment cells under an optical microscope. These two pigment cells are different from red, yellow and black pigment cells.

Iridescent pigment cells do not actually exist in many colors, but they adjust the structure of guanine crystals to make the skin appear in brilliant colors.

Similarly, leukocytes do not actually have white pigments, but also reflect light by adjusting the structure of guanine crystals .

These pigment cells, distributed in the skin, are the basis for color change in organisms including pigfish .

Pigfish skin and pigment cells in the skin

(Image source: Reference [2])

In this process, the visual protein SWS1 plays the role of "seeing" from the inside of the skin to the outside. It fine-tunes the pigment cells based on the subtle differences it "sees".

In the pigfish's skin, the SWS1 opsin is located beneath the pigment cells, so the size of the pigment cells affects the light that reaches the opsin .

By using the differences in the light it receives , SWS1 can fine-tune pigment cells to make the skin appear closer to the color of the surrounding environment.

Conclusion

People have previously known about the presence of visual proteins in animal skin, but have never studied how these visual proteins help animals change color.

In fact, the mechanism of "seeing things" by visual proteins in biological skin is different from the "eye-brain" visual mechanism. It can judge the difference between skin and environmental colors more simply and quickly.

By understanding the principle of biological color change, people may be able to create visual sensors based on this principle in the near future, allowing artificial intelligence, including self-driving cars, to "see" the world more easily and quickly.

References:

[1]Schweikert, LE, Fitak, RR & Johnsen, S. De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus). J Comp Physiol A 204, 475–485 (2018).

[2]Schweikert, LE, Bagge, LE, Naughton, LF et al. Dynamic light filtering over dermal opsin as a sensory feedback system in fish color change. Nat Commun 14, 4642 (2023).