Researchers from the University of Queensland in Australia have discovered that the mantis shrimp which has 12 photoreceptors can actually see fewer color hues than humans can, even though humans have only three photoreceptors.
Most scientists previously believed that the mantis shrimp could see more color hues than humans because they have 12 different types of photoreceptors. Human eyes can distinguish between millions of different colors, so scientists believed that shrimp could distinguish the difference between even more colors because the shrimp has four times as many light-detecting cells in their body.
In previous research, scientists have proven that animals which have four to seven photoreceptors are able to see practically every color possible. These finding made the scientists believe the theory that mantis shrimp would be able to see even better.
At the University of Queensland, researchers have instead found that the mantis shrimp’s vision is actually worse than humans. To be able to test the vision of the mantis shrimp, first the team of researchers had to train the shrimp to recognize one of 10 specific color wavelengths, ranging from 400 to 650 nanometers. The way they trained the shrimp was rewarding them with a frozen prawn or mussel when they picked the right color from two color choices, such as blue and red.
After the shrimp were trained, they could distinguish between the distinct colors, but when they were offered colors which were closer together on the color spectrum, like red and yellow or red and orange, the shrimp had trouble distinguishing the different colors and got confused. The head researcher of the project, Hanne Theon, said, “We tested their ability to discriminate between colors that differ a lot, such as red and blue, and then changed colors that were closer and closer together along the spectrum – red-green, red-yellow, red-orange – and noted when they started making mistakes.”
As the testing continued, the shrimp could discriminate between the wavelengths they were trained at and another color which fell 50 to 100 nanometers up or down the color spectrum. Once the testing had reached the point where the wavelengths were reduced to 12-25 nanometers, the shrimp were no longer able to tell them apart.
The authors of the study say that if the shrimps’ eyes worked in the same fashion as a human eye, it would have allowed the shrimp to distinguish between two colors with wavelengths as close as 1-5 nanometers. But the shrimp’s photoreceptors work in a different way. They seem to pick up a specific color, identifying it with a vision system that is less sensitive than the human eye which relies heavily on brain power to discriminate between the different colors.
According to Ray Caldwell, a behavioral ecologist at the University of California, Berkeley, this way of seeing without relying heavily on brain powered comparisons is what give the predatory shrimp a speed advantage in distinguishing between different colored prey.
Professor Justin Marshall, a neurobiologist and Theon’s PhD supervisor said, “There is no other animal out there that has anything remotely like this.” He added, “They’re definitely not seeing the world of color in as much detail as other animals.”
Marshall also said that this was the first time they had determined how an animal processed color in a different manner. “It solves a long-standing problem of why mantis shrimp have 12 color receptors because they process colors and the contrast they provide in a totally different way to anything we have previously seen in an animal.”
The researchers aren’t sure how the 12-receptor system works, but they believe that the mantis shrimp processes color very quickly by developing patterns of receptor excitation that correspond to certain colors. So the mantis shrimp might not process colors within the brain as they see them in the eye. This technique could help the shrimp quickly pick out colors within their environment, especially in the bright multi-colored reef environment.
According to Michael Bok, a biologist at Lund University in Sweden, who studies vision, all of the information gained in this study is an important step towards understanding the unbelievably complexity of the mantis shrimps’ vision. He said, “The next step, really, is to figure out what these visual signals tell the brain and how the brain uses these signals.”
Professor Marshall believes the next step is to figure out how the mantis shrimp incorporates all of the information from their bizarre color-sensing system into reactions. He also believes the shrimp have specific spots on the claws that come in different shades and that those indicate their species. Shrimp may use these spots to tell each other apart. Marshall said, “The color messages they’re sending each other are something we don’t understand, but they definitely use color to talk to each other.”
Researcher Hanne Theon believes that the information gathered in this study could help in innovations and advancements in visual technology. She also thinks it will aid us in processing more color efficiently.