When it comes to versatile vision, the mantis shrimp reigns supreme. Its specialized eyes can pick up several types of light, including infrared and ultraviolet, and its color vision tops ours. Now scientists report that this reef-dwelling crustacean has done itself one better: It can see a type of polarized light that no other animal is known to be able to detect. The function of this new form of vision is still a mystery, but researchers speculate that the crustacean may use it in mating displays or as a secret form of communication.
The key to the mantis shrimp's (Odontodactylus scyllarus) extraordinary vision is in the structure of its eyes, which consist of six rows of numerous smaller eyes called ommatidia. Justin Marshall first suspected that the shrimp could see a new type of light based on the way light-sensing cells in some ommatidia are arranged. They sit at just the angle to convert circularly polarized light (CPL)--a type of light wave that travels in a spiral--to a form that other cells underneath can detect, says Marshall, a sensory neurobiologist at the University of Queensland in Brisbane, Australia. Certain beetles have shells that change color when various types of CPL are shone on them, but no animal is known to have the ability to see this type of light. (To humans using special goggles, CPL appears as a bright light.)
Marshall and colleagues tested the mantis shrimp to see if they could distinguish between various forms of the light. Three of four animals learned to correctly identify a tube emitting left-handed CPL (corkscrewing to the left) in exchange for a bite of food, whereas two of three did the same for right-handed CPL, the researchers report online today in Current Biology. Although these sample sizes are small, co-author Roy Caldwell, an invertebrate biologist at the University of California, Berkeley, says they are sufficient to demonstrate that the shrimp can detect CPL. The question now is what they use it for.
Marshall says CPL vision may play a role in mating displays. Males--but not females--of at least three mantis shrimp species have body parts that reflect CPL. Another possibility is a mode of secret communication: Reflections of CPL off mantis shrimp bodies would likely be invisible to predators and competitors.
Whatever its role, CPL vision must be important to the animals, says Eric Warrant, an invertebrate biologist at Lund University in Sweden. "Evolution wouldn't have gone to all this trouble to develop this if it wasn't playing a role in some vital aspect of their lives," he says.