Mantis shrimps became the subject of a new study by researchers of the University of Queensland for their unique eyes. The bulbous eyes of the colorful marine crustaceans boast of 12 photoreceptors and they analyze the outputs from all of their 12 photoreceptors at once into the brain without processing or distinguishing the colors.
The study found that each type of photoreceptor reacts to a specific color, and the mantis shrimps probably just respond to the photoreceptor that is the most active. Their eyes are adapted more for recognizing colors than discriminating between colors.
"This entirely unique form of vision would allow for extremely rapid color recognition without the need to discriminate between wavelengths within a spectrum," said the study.
The study employed a method, which included training the mantis shrimps to respond to certain colors. Food was offered as a reward to a two-way choice test which the mantis shrimps were expected to undergo. They first had to tap the optical fiber that transmitted a particular color. Then they were presented with two fibers of varying wavelength, and they were made to choose the fiber with the wavelength that represents the color of the fiber they first tapped. This method revealed how they perceived and recognized color.
The mantis shrimps are marine crustaceans that are members of the order Stomatopoda. They can grow up to 12 inches in length and can be found in shallow, tropical, and sub-tropical marine habitats. They come in a variety of colors, from brown to neon shades. They have compound eyes on top of moving stalks, and these eyes are made up of tens of thousands of ommatidia units, or long, thin clusters of photoreceptor cells.
To determine an object's color, the mantis shrimp's eye muscles scan a particular area of the object with a sweep of the photoreceptors located across the center of each eye, called the mid-band. This enables them to recognize basic colors by their eyes alone, instead of using the brain to distinguish colors according to how light falls. In effect, the creature is able to perceive distance on its own through a process called trinocular vision, and can perceive circularly polarized light.
"Our experiments suggest that stomatopods use a previously unknown color vision system based on temporal signaling combined with scanning eye movements, enabling a type of color recognition rather than discrimination," the study explained.
As the mantis shrimp does not sends the data to its brain for processing, this is a time-saving way of recognizing color. It presents an advantage to mantis shrimps, since these predators are known to club their prey with very fast blows using appendages that can move up to speeds of 23 meters per second (just like a preying mantis and, hence, the name).
This kind of eyesight gives the mantis shrimp the ability to recognize different types of coral, prey, transparent or semi-transparent organisms, predators, and those with shimmering scales.
