A 6-inch peacock mantis shrimp packs a 50 mph punch, and it's for that reason biologist Ilse Daly, at the University of Bristol in Britain, wears thick gardening gloves when handling the little pugilists. She respects the crustaceans' truncheons, but her scientific interests concern their other amazing features — their eyes.
Peacock mantis shrimp eyes perch on the end of stalks. The compound eyes rotate independently in all directions, like ball bearings in greasy sockets. And even when these eyes are spun completely sideways, the animals still know that up is up, according to new research by Daly and her colleagues published in the journal Proceedings of the Royal Society B.
“Their eyes are constantly in motion, up, down, side-to-side,” Daly said. The wandering gaze gives Odontodactylus an air of intelligence, she said, at least compared with the dull forward stares of other crustaceans. (In reality, mantis shrimps may not be smarter than the average crab.)
“Their eyes are built in a really odd way, unlike any other animal eyes,” said University of Maryland Baltimore County biologist Thomas Cronin, who has studied the vision systems of mantis shrimp since 1983 and was not involved with this research.
Mantis shrimps have three “pseudo-pupils” stacked one on top of the other. Each eye has independent depth perception. The crustaceans can see beyond humans on both ends of the light spectrum, peering into ultraviolet and infrared wavelengths. Mammals have three types of photoreceptor cells, whereas mantis shrimps use a dozen. That does not mean mantis shrimps see psychedelic dolphins frolicking through rainbow oceans. In fact mantis shrimp struggle to tell the difference between color shades that human eyes easily discern.
But the mantis shrimp color vision is unique. The shrimp assess their environment in the way a scanner captures a photograph — spot a band of color, move the pseudo-pupil row, repeat. It's a method scientists liken to sweeping a floor with a push broom, stroke by long stroke. “They kind of paint the world into their visual system,” Cronin said.
The new research homed in on the jiggly movement of the mantis eye. “There’s this trade-off in animals,” Daly said: We must move our eyes to see the world. But the more we do so, the blurrier our world becomes. There are several ways to stabilize vision, including supportive muscles around the eyes, to steady the view of the world.
In the new report, Daly and her co-authors examined the way mantis shrimp gaze in a rotational world. The scientists constructed a testing tank with high-speed cameras set above the animals to record their eye movements. In one test, the scientists placed a drum around the aquarium, as if the shrimp were inside a tunnel. They began to spin the drum like a wheel of a car, and watched to see if the shrimp would stabilize their gaze to follow the drum.
The experiment mimicked a popular amusement ride in Britain, Daly said, called a vortex tunnel, in which carnival-goers try to cross a walkway as the wall and ceiling rotate. The rotation is not like a carousel's, rather, the walls loop over a person's head and swing up from below their feet. It's tough to traverse the walkway, and though the floor remains stable, the whirling walls can induce vertigo in humans.
As the world appeared to do aileron rolls around the mantis shrimp, the researchers suspected the shrimp would likewise spin their eyes to maintain a stable view. They did not. In some cases, “their eyes went the opposite way from the drum,” Daly said. This suggests that the orientation of their eyeballs “doesn't seem to have any bearing on their perception of space,” she said.
Scientists still do not understand why the shrimp would require this unique way of seeing the world. “We’re nowhere close” to solving the question of mantis shrimp vision systems, Daly said.
Cronin hypothesized — and this is speculation, mind you — that roving eyes might work best when the animals remain stationary. “It's possible the mantis shrimps just don't have a real sense of how the world is laid out,” he said, as the animals scan for flashes of color and not much else. Most studies like this have observed the shrimp as they stay in place, peeking out of a burrow. Maybe the animals use more stable gazes while scampering across the ocean floor, when returning to the safety of a crevasse becomes a matter of life or death.
The animals still find ways to amaze Daly, she said, despite having worked with them for years. “The more you watch them, the weirder they get.”