Mysterious Sight of the Mantis Shrimp Closer to Being Solved?
The mantis shrimp (a.k.a. stomatopod) are colorful marine crustaceans, renowned for their ability to pack a punch. Found in tropical and sub-tropical marine habitats, the marine shrimp’s powerful claws can deliver over 90 kilograms of force – an ability that has earned the creature the nickname “the thumb splitter.” In fact, the creature’s arm can accelerate through water at up to 10,000 times the force of gravity.
The mantis shrimp does not perceive color in the same way as humans. Crustaceans have numerous types of light-detecting photoreceptors – many more types than are found in human beings. However, according to a study published in the journal Science, the creature’s ability to discriminate between different colors remains relatively limited.
Mantis Shrimps Have 12 Photoreceptors
The researchers have concluded that the mantis shrimp’s color vision is based upon a rather rudimentary method that works at the level of individual photoreceptors. Possessing 12 different types of photoreceptor cells – a specialized neuron used for converting light into electrical signals – the mantis shrimp can see hues that humans cannot. According to Justin Marshall, a marine neuroscientist at the University of Queensland, Brisbane, and co-author of the latest research paper, human beings have far fewer types of photoreceptors, partially explaining the difference in visual perception.
The extraordinary crustacean’s eyes sit on stalks that can move separately of one another, each having trinocular vision and depth perception. The mantis shrimp’s 400-million-year-old visual system can gauge both depth and distance by focusing on particular objects with three separate regions.
Although researchers have long known the mantis shrimp to possess 12 photoreceptors, they were unable to fathom the precise reason for this. Mathematical models have shown that no more than five receptors are necessary to effectively perceive the spectrum of light. Human beings – like most other animals – possess three color photoreceptors, each of which responds to red, green and blue light. Electrical signals are then sent along the optic nerve to the brain, which interprets the ratio of activity from each receptor type to interpret overall color.
Meanwhile, the mantis shrimp’s 12 photoreceptors comprise of four types that cover the ultraviolet region of the spectrum and eight receptors that cover the spectrum that humans can see. In questioning the basis for, what appears to be, an excess of photoreceptors, one would assume that mantis shrimps were better equipped to discriminate between different colors than most other animals. However, Marshall indicates this is simply not the case; in fact, the mantis shrimps are much worse than humans at differentiating between colors.
Behavioral Wavelength Discrimination Experiments
Hanne Thoen, of the University of Queensland, performed studies on Haptosquilla trispinosa – a small stomatopod that occupies rocky, intertidal reef and consumes a diet of crustaceans and gastropods. Thoen presented the creatures with two optic fibres, each of which presented a different color. If the mantis shrimp attacked the “correct” color, it was awarded food. The researchers then changed the color of the “incorrect” target fibre, up to the point where the subject was unable to distinguish between the two colors. Although humans can identify difference in colors that are five nanometers apart, in terms of wavelength, the researchers found that mantis shrimps can only differentiate between two colors that are around 15 to 25 nanometers apart.
The authors claim that the mantis shrimps appear to identify colors with less sensitivity than humans; although this makes comparison of similar colors less accurate, the simple process does not require the same level of brain-power. Roy Caldwell, a behavioral ecologist at the University of California, Berkeley, suggest this could give the mantis shrimp a “speed advantage” that permits the creature to rapidly determine the different colors of potential prey.
Reflecting on their future plans, Marshall discusses his desire to determine how the mantis shrimp is capable of integrating information using their unique visual system. The shrimp have distinctive, species-specific marks along their claws that vary in shade; it has been suggested that these marks enable mantis shrimps to “talk to each other,” and tell one another apart. The researchers, however, admit there is more research that needs to be conducted before the mantis shrimp’s unusual vision is comprehensively understood.
By James Fenner