One Crustacean: Many Powers

Mantis shrimp can break the glass of an aquarium using their smasher appendage. Courtesy of Roy Caldwell

Thursday, April 3rd, 2008--

The most complex visual system in the animal kingdom belongs to predatory crustaceans known as mantis shrimps--aggressive invertebrates that live on the muddy ocean floor, according to researchers reporting in the journal Current Biology. Mantis shrimps are an order of organisms that split off from all other crustacea about 400 million years ago. But they are hardly shrimpy: in addition to one-of-a-kind visual powers, they also pack one of the quickest, deadliest weapons in the animal kingdom--a raptorial appendage that can demolish predators with a single strike.

Mantis shrimps can have up to 16 visual cone pigments. By comparison, humans have three cones--red, green and blue. Mantis shrimps have three pigments in the ultra violet spectrum alone, says Roy Caldwell, biologist at the University of California, Berkeley and an author on the paper.

What does the world look like to a mantis shrimp? "I honestly don't know," says Caldwell. What is clear is that their color capabilities alone allow mantis shrimps to see things in the world around them that would be invisible to humans and most other organisms. "What animals want to see is contrast in the world," says Tom Cronin, a biologist at the University of Maryland, Baltimore County and an author on the paper. An object is just a contrast--something that looks different from its surroundings, Cronin says. A mantis shrimp's visual system allows it to see subtle contrasts, which likely comes in handy in a murky, underwater environment, he adds.

But their superior visual powers don't stop with color. Based on the structure of the eye, Justin Marshall, an ecologist at University of Queensland in Australia, and his colleague Horace Barlow at University of Cambridge, suspected that mantis shrimps may be able to see circular polarized light--a type of light that has not been shown to be perceptible to any other organism. "The eye was set up basically the same way that an optical physicist would ... design an analyzer to see circularly polarized light," Caldwell says.

Light travels in waves. Non-polarized light waves move in all directions. If you were to take a cross-section of a beam of non-polarized light, "the arrows would be going in every imaginable direction," explains Marshall, senior author on the paper. A linear polarized light wave--which is what polarized glasses filter out--travels only vertically or horizontally. With circular polarized light, the wave is vertical, but it also spins as it moves. Think of a spiral or a spring, Marshall says.

Through behavioral studies, Marshall and colleagues confirmed that some mantis shrimps can perceive this light. And when they looked at the species Odontodactylus cultrifer and two other mantis shrimp in the Odontodactylus genus through a circular polarized lens they noticed that its tail began to flash--it was not only perceiving circular polarized light, it was reflecting it too.

The parts of the mantis shrimp's body that produce the light signals are sexually dimorphic, meaning that male signals different than those from females. The researchers say the signals are likely used for mating and also perhaps for hashing out dominance among males. "This is the most private channel of communication in the animal kingdom," Caldwell says.

The pressure for mantis shrimps to develop an advanced communication system may be related to its habitat and its weaponry--known as the "smasher" among experts. Cronin says: "Animals that have really good fighting ability, typically they very quickly evolve ways of figuring out whether it’s worth getting into a fight or not." The secret signaling system seems to allow them to communicate in the unprotected environment in which they live, without calling attention to themselves. And when you can kill or be killed in a single blow, clear communication is key.

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--Flora Lichtman