The Shrimp Whose Eyes Work Like Reversible Light Switches
Mantis shrimp are famous for their eyes. Humans get by with three visual pigments for colour vision. Mantis shrimp have around thirty. For decades, scientists have wondered why one animal needs so many — and what each pigment actually does. The question was hard to answer because all the pigments work together inside the eye, and the proteins involved are notoriously difficult to produce and study in the laboratory.
Now an international team has measured the pigments one by one. Researchers from the University of Liège (ULiège) in Belgium and Humboldt University in Berlin, with collaborators in Germany, the United Kingdom and the United States, analysed seventeen visual pigments, called opsins, from the mantis shrimp's median band — the central strip of the eye dedicated to colour vision. Their findings are published in Science Advances.
Seventeen pigments, counted and tested
Vision relies on proteins called opsins. Combined with retinal, a small molecule derived from vitamin A, opsins form visual pigments that absorb light and convert the signal into an electrical message the brain can interpret. The team combined an in vitro expression platform developed by Marjorie Liénard at ULiège with the electrophysiology expertise of Peter Hegemann's laboratory in Berlin, so each pigment could be produced and tested separately.
Of the seventeen opsins measured, twelve 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.
Four of the pigments have a distinctive property: they are bistable, meaning they can reversibly switch between two stable states when they absorb light, without needing 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 round: they are activated by blue light and deactivated by orange light," explains Hegemann, professor of biophysics at Humboldt University in Berlin.
That is different from human vision. In humans, when a pigment absorbs a photon of light, retinal changes shape and eventually detaches from the opsin; it must then be regenerated before it can work again. That helps explain why spots persist for a while after looking at an intense light source. Bistable pigments, by contrast, can be reactivated directly by a second photon of light that restores the retinal, so the visual system can remain continuously operational.
Seeing into the deep red
The team also found that some of these pigments detect red light that very few animals can perceive. "This is the first time such pigments have been described in an animal whilst using the same chromophore as humans," Liénard explains. "Combined with the coloured filters present in the mantis shrimp's eye, they enable sensitivity to be achieved at 687 nanometres," adds Alina Pushkarev, a marine biologist in Hegemann's laboratory. A nanometre is a billionth of a metre; 687 nanometres is deep red, at the far edge of what human eyes can see.
The researchers suggest the shrimp may use different pigment combinations depending on the light around it. In bright light, when the ultraviolet, blue and green pigments are saturated, the red opsins keep working. At dawn and dusk, the animal can rely on its dozen green opsins and its bistable blue opsins. Liénard compares it to wearing sunglasses during the day and using a torch at dusk.
There is an important caution. How the mantis shrimp actually uses this information — to locate prey, avoid predators or defend its territory — remains unconfirmed and will need to be demonstrated through behavioural experiments, which are difficult to carry out with these robust predators. Measuring a pigment in the laboratory is not the same as proving how the animal sees.
Beyond biology, the proteins interest researchers working in optogenetics, a technique that controls cell activity using light. Unlike the channelrhodopsins used in much optogenetics work, these shrimp opsins belong to the family of G-protein-coupled receptors, which trigger signalling cascades inside cells. G-protein signalling is involved in many diseases and is the target of a significant proportion of approved medicines. Opsins that can be switched on by red light and off by blue light could one day serve as precise, reversible molecular switches — red light penetrates deeper into tissues and is less harmful to cells than blue or ultraviolet light. Those applications have not yet been developed.
If your child finds animal senses fascinating but gets tangled in the vocabulary — opsins, retinal, wavelengths, receptors — a biology tutor can untangle it one clear diagram at a time, with the processes behind the words. Book your first lesson and compare verified biology tutors, online, in person or in a group class.
Sources:
- University of Liège, GIGA Institute, "Red-sensitive bistable pigments identified in the eye of the mantis shrimp": https://www.giga.uliege.be/cms/c_13745357/en/red-sensitive-bistable-pigments-identified-in-the-eye-of-the-mantis-shrimp?et_rid=17037716&et_cid=6100240