No Other Animal In The World Sees Color Like The Mantis Shrimp - And Researchers Finally Know Why
The eyes of the mantis shrimp have more types of photoreceptors, or color-detecting cells, than any animal on the planet. But the bottom-dwelling sea creatures are surprisingly bad at discriminating one color from another, a discovery that has not made sense to scientists until now.
In a new study, published Jan. 23 in the journal Science, Hanne Thoen of the University of Queensland and colleagues, say that the mantis shrimp perceives colors in a way that has not been documented in the animal kingdom before - the creatures immediately recognize basic colors just by scanning an object with their eyes, rather than using the brain to distinguish different colors of light.
The time-saving trick helps it to survive in one of the most colorful environments on Earth - tropical coral reefs.
What do animals see?
Most animals have two to four types of photoceptors - cells that detect different wavelengths in electromagnetic spectrum - in their eyes. Humans, for instance, have three different types of photoreceptors, each one tuned to pick up green, red, or blue wavelengths of light. When light hits our eyes, the receptors turn these colors into electrical signals that are sent to the brain. Our brains determine the color that we see by blending the signals that each receptor senses - like how a screen made of millions of different-colored pixels makes an image.
Certain species, like birds, reptiles, and butterflies, have even more receptors than humans, although scientists believe that three to four are just fine for discriminating all the colors of the visible spectrum.
But, for some reason, the mantis shrimp has 12 distinct photoreceptor types. Each photoreceptor is sensitive to a different wavelength of light, ranging from deep ultraviolet colors to red. In addition, the crustaceans are also able to perceive linear and circular polarized light, which refers to the angle and direction of traveling light waves. Previous studies have suggested that mantis shrimps use the ability to detect circular polarized light, which reflects off their bodies, as a secret channel of communication, especially during mating.
The mantis shrimp super-eye There are more than 500 known species of mantis shrimp, which range in size from less than an inch to over a foot long.
To determine the color of an object, the mantis shrimp's eye muscles must scan the band across a particular area.
In the study, mantis shrimps were trained to associate different color wavelengths with a food reward. The animals had to first tap an optical fiber transmitting one color. The food was then removed and the mantis shrimps were presented with two fibers of varying wavelengths. The mantis shrimps aced the eye test if they chose the fiber representing the color wavelength they were originally trained to grab.
The mantis shrimp is a ferocious creature, known for spearing or clubbing their prey with their powerful claws (like those of a praying mantis, hence the name).
"The extraordinary sensory machinery of the eye midband may be useful for species, and even individual, recognition, as well as more generally in detecting prey and other other objectes," Michael F. Land and Daniel Osorio, from the University of Sussex wrote in a commentary on the study, published in the same issue of Science.
Of course, without being able to get into the head of the mantis shrimp, we may never know how they actually experience those colors in the brain.