Your color is quite nice! Do "chameleons" really exist among fish?

Produced by: Science Popularization China

Author: Wu Yu (Institute of Zoology, Chinese Academy of Sciences)

Producer: China Science Expo

Before the article officially starts, let's take a look at a set of pictures:

Figure 1 Long-spined hairy-lipped wrasse

(Image source: Reference [1])

Look carefully, how many kinds of fish are these?

Three? No, one.

The ocean is a huge treasure trove, home to countless creatures, from tiny plankton to giant blue whales, all of which reflect the ocean's all-encompassing nature.

In the western North Atlantic, there is a fish called the long-spined hairy-lipped wrasse . It is the largest and most economically valuable wrasse in the region. Its notable characteristics include hermaphroditism and asexual reproduction. The skin color of this fish can change rapidly under certain circumstances, like a chameleon, and can change between at least three shades: uniform white, uniform reddish brown, and mottled.

The picture we see above shows the state of the long-spined hairy-lipped wrasse in different colors.

Why does the long-spined hairy-lipped wrasse change color?

Because it has special skin cells called "chromatophores" that are able to achieve dynamic color changes .

Let's first look at the structure of its skin. The top layer is epithelial cells, followed by pigment cells, which are divided into black pigment cells, red pigment cells and yellow pigment cells. These three types of pigment cells are arranged horizontally and exist in a thin layer of skin tissue on top of the fish scales.

Figure 2 The skin structure pattern of the long-spined hairy-lipped wrasse

(Image source: Reference [1])

The "beauty" of pale white, dark red and mottled appearance (see Figure 1) is achieved by a certain aggregation and dispersion of the pigments of different pigment cells . In the pigment cells, the color is changed by the intracellular reorganization of pigment granules, crystals or reflective platelets. The incident light either hits the underlying (usually white) tissue or the exposed pigment, giving the skin a light or colored appearance, respectively.

Scientists have made special observations and found that when the long-spined hair-lipped wrasse is swimming in a water tank, it is light white, and when it is still, it immediately changes to a mottled color. When the entire front of the head turns bright reddish brown, the light white color sometimes changes significantly, and the tail will appear more or less blue. In addition, any interference stimulation in the light white state will immediately change it to a mottled appearance.

The magical opsin of skin photoreceptors

So how does the long-spined hairy-lipped wrasse determine changes in ambient light?

First, let's take a look at skin photoreceptors, which are structures in the skin that can sense light stimulation and convert it into neural processes. In vertebrates, skin photoreceptors may contain several types of opsins and transmit light signals in an energy-dependent manner. Both "non-visual" opsins (such as melanopsin) and "visual" opsins (such as RH1 and SWS1) have been shown by researchers to be associated with pigment cell activation in vertebrates , among which the role of SWS1 (short wavelength sensitive opsin-1) opsin is particularly obvious.

In order to find the specific location of SWS1 in the skin cells of the long-spined hairy-lipped wrasse, scientists conducted two experiments to explore:

The scientists first used a special method to make SWS1 have green fluorescence, and then observed it under a microscope (which can be understood as taking a photo of the cell, except that the photo is fluorescent, see Figure 3). They found that SWS1 is located below the pigment of the pigment cells, not in the continuous layer of the skin, but in discrete locations below individual, continuous pigment cells.

Figure 3: White arrows represent red pigment cells, black filled arrows represent melanocytes, and green fluorescence represents opsin SWS1

(Image source: Reference [1])

Since the above method can only see the approximate location of its distribution in the skin, and cannot determine its specific location in the cells, the researchers then conducted a more detailed experiment (which can be understood as the resolution of taking pictures of cells is improved, and the structure inside the cells can be seen more clearly), and found SWS1 protein in the reticular membrane structure of these cells.

Figure 4 Black arrows with white edges point to melanocytes, and black arrows at the reticular membrane structure point to positively immunoreactive SWS1 proteins

(Image source: Reference [1])

So what is the relationship between the aggregation and dispersion of pigment cell pigments and the visual protein SWS1?

As can be seen from Figures 2, 3, and 4 above, the SWS1 opsin exists in a group of morphologically specialized cells beneath pigment cells. So is there a relationship between pigment cells and the opsin SWS1? What kind of relationship is there?

Through research, researchers have found that the light in the environment must pass through the pigment cells before reaching the SWS1 opsin in the skin. In the pigment cells, the pigment will be dispersed and aggregated to a certain extent under the influence of the external environment. The dispersion of pigments can inhibit the irradiation of the type of light required by the SWS1 opsin, while the aggregation of pigments increases the irradiation of the type of light required by the SWS1 opsin. This process closely links the SWS1 opsin to changes in the state of pigment cells, enabling it to monitor internal information about skin color and thus fine-tune color changes.

Figure 5. Dispersed chromatophore pigments inhibit short-wavelength radiation of the SWS1 receptor (left), whereas aggregated pigments allow short-wavelength radiation of the SWS1 receptor (and, therefore, putative opsin activation) (right).

(Image source: Reference [1])

Animals that change color

There are many species of animals in nature that can dynamically change color, including cephalopods, amphibians, reptiles, fish and other warm-blooded animals, such as cuttlefish and chameleons.

Figure 6 Cuttlefish

(Photo source: veer)

Figure 7 Chameleon

(Photo source: veer)

In terms of morphological color changes, physiological color changes can occur over a period of days to months, while changes can occur over a period of minutes or less (in the case of the long-spined wrasse, it takes only 1 second or less). These rate differences are based on differences in regulatory mechanisms, with the most rapid forms of color change being primarily controlled by neurons rather than hormones.

Dynamic color change is a rapid, variable, and context-dependent behavior that shares many common physiological features among different animals.

First, they all use specialized skin cells called "chromatophores" to do this. There are several main types of chromatophores, which change color through intracellular reorganization of pigment granules, crystals, or reflective platelets.

Second, the inherent photosensitivity of their skin and the connection between this sense and their ability to change color, such as the sensitivity to light represented by the opsin SWS1 mentioned above.

The color of some animals changes with the light, temperature and living environment. Sometimes, they change color to attract the attention of the opposite sex or to scare the enemy; some change color because of the manifestation of emotions or mental states such as fear and anger on the body surface. Changing color can better camouflage oneself, confuse the enemy, and facilitate hunting; changing color can hide oneself, avoid natural enemies, and protect oneself.

In short, color-changing is a way for animals to adapt to nature, and this small behavior contains great wisdom. As Lao Tzu said in the Tao Te Ching, "Tao follows nature." For animals that can change color, "Tao" is the law of nature.

References:

[1]Schweikert, LE, Bagge, LE, Naughton, LF, Bolin, JR, Wheeler, BR, Grace, MS, ... & Johnsen, S. (2023). Dynamic light filtering over dermal opsin as a sensory feedback system in fish color change. Nature Communications, 14(1), 4642.

[2]Townsend, CH (1929). Records of changes in color among fishes.

[3]FUJII*, RYOZO (2000). The regulation of motile activity in fish chromatophores. Pigment Cell Research, 13(5), 300-319.

[4]Nilsson Sköld, H., Aspengren, S., & Wallin, M. (2013). Rapid color change in fish and amphibians–function, regulation, and emerging applications. Pigment cell & melanoma research, 26(1), 29-38.

[5]Parker, GH (1930). Chromatophores. Biological Reviews, 5(1), 59-90.