Red-sensitive pigments in mantis shrimp eyes act as reversible light switches

With about 30 visual pigments, compared to just three in humans, mantis shrimp possess a visual system that remains a mystery. Researchers from ULiège and Humboldt University in Berlin have, for the first time, analyzed these pigments one by one and discovered that some detect red light that very few animals can perceive. This finding, published in Science Advances, has potential applications in optogenetics.

Vision is one of the most sophisticated sensory systems; it relies on proteins (opsins) that capture light and convert the signal into an electrical message that the brain can interpret. When combined with retinal, a small, universal molecule derived from vitamin A, opsins form the visual pigments (rhodopsins) that absorb light in the photoreceptors of the retina. In humans, three cone pigments are sufficient for color perception.

In many invertebrates, such as insects and crustaceans, the eye has multiple facets, and different regions may specialize in ultraviolet, blue or green light. Mantis shrimp take this a step further. Their eye is divided into two hemispheres separated by a central band, known as the median band, dedicated to color vision, and this band expresses an exceptional number of opsins.

But why does this crustacean possess such powerful vision? This question has puzzled ecologists and biologists for decades. It remains difficult to resolve because all these proteins coexist in the eye, and invertebrate opsins are notoriously difficult to produce and study in the lab.

An international team of scientists has combined an in vitro expression platform developed by Marjorie Liénard, a researcher at GIGA—ULiège, with the electrophysiological expertise of Peter Hegemann's laboratory at Humboldt University in Berlin, with the support of biologists, biochemists and theorists from other universities in Germany, the United Kingdom and the United States.

A scientific team led by Camille Brouillon, Alina Pushkarev and Liénard has measured the light sensitivity and signal-transmission mechanisms of the 17 visual opsins found in the mantis shrimp's median band. It turned out that 12 of them were "standard" green opsins, similar to our own green pigment. Three are infrared opsins, including two red-blue bistable opsins that straddle the boundary between the visible and infrared spectrum. The remaining two are blue-orange bistable opsins.

"This is the first time such pigments have been described in an animal while using the same chromophore as humans," explains Liénard, a molecular biologist at ULiège. "Combined with the colored filters present in the mantis shrimp's eye, they enable sensitivity to extend to 687 nanometers," explains Pushkarev, a marine biologist at Hegemann's laboratory.

Four of these pigments have a distinctive property: They are bistable, meaning they can reversibly switch between two stable states when they absorb light, without requiring an external cellular mechanism to regenerate.

"Two of them are activated by red light and deactivated by blue light. The other two work the other way around: they are activated by blue light and deactivated by orange light," explains Hegemann, professor of biophysics at Humboldt University in Berlin and co-recipient of the 2026 Nobel Prize in Medicine.

In humans, when a pigment absorbs a photon of light, retinal changes conformation and eventually detaches from the opsin; it must then be regenerated to function again. This process helps explain why, after looking at an intense light source, spots persist for a while in our field of vision.

"Bistable pigments, on the other hand, can be reactivated directly by a second photon of light that restores the retinal, suggesting that the visual system remains continuously operational," explains Brouillon, a biophysicist at Humboldt University Berlin. The team also identified the molecular mechanism behind the shift in sensitivity between red and blue.

These results suggest that the mantis shrimp uses different pigment combinations depending on ambient light levels. "You could compare this to wearing sunglasses during the day and using a flashlight at dusk," explains Liénard. "In bright light, when the ultraviolet, blue and green pigments are saturated, the red opsins continue to function. At dawn and dusk, the animal can rely on its dozen green opsins and its bistable blue opsins."

However, how the mantis shrimp uses this information to locate prey, avoid predators or defend its territory remains unconfirmed and will need to be demonstrated through behavioral experiments, which are difficult to carry out with these robust predators.

Applications in optogenetics

Beyond biology, these proteins are of interest in optogenetics, a technique that controls cell activity using light. The opsins of the mantis shrimp, unlike channelrhodopsins of the single-celled alga Chlamydomonas reinhardtii, belong to the family of G-protein-coupled receptors (GPCRs), which trigger signaling cascades within cells. G-protein signaling is implicated in numerous diseases, including cardiovascular, neurological and inflammatory conditions, and is the target of a significant proportion of approved medicines.

Opsins of this type, which can be activated by red light and deactivated by blue light, could serve as molecular switches to modulate these signaling pathways precisely and reversibly. Red light penetrates deeper into tissues and is less harmful to cells than blue or ultraviolet light.

These applications have not yet been developed. The team is now working to make these opsins' signaling more selective, search for similar proteins in closely related species, and understand what gives some of them their bistable nature.

For Liénard, this study illustrates the value of studying organisms with unique ecological characteristics: "Nature still harbors many innovations, which can only be uncovered through an approach combining molecular biology, biochemistry, physiology and sensory ecology."