Young mantis shrimp camouflage themselves by using reflectors that give off opalescent blue-green light to make their eyes invisible to anyone who’s looking, according to a new study.
Adult mantis shrimp are known for having one of the swiftest strikes in the animal kingdom. Yet their young are tiny and vulnerable, depending on transparent bodies to escape the attentions of predators. Researchers have speculated for years on how these animals camouflage their eyes—which by necessity can’t be transparent. But a study published online today in the Journal of Experimental Biology is the first to test this.
Image-forming eyes need colored pigments for proper vision, says Kathryn Feller, a doctoral student at the University of Maryland, Baltimore County who studies animal vision. The receptors that catch light in the eye need to be isolated from one another using opaque materials, like pigment; otherwise, the animal ends up seeing one large blur rather than a sharp image, she explains. (See “The Mantis Shrimp Sees Like a Satellite.”)
A Study in Blue-Green
Mantis shrimp larvae—which often look like “floating, clear pterodactyls”—are known for having eyes that shine an opalescent blue or green under the right light, says Feller. That’s actually what drew her to these animals in the first place. Researchers suspected that the larvae were using that eyeshine to hide their eyes, Feller says, but no one had tested it.
By studying captive and free-living mantis shrimp larvae near Lizard Island (map), Australia, Feller was able to measure the contrast between the reflected light coming from the animals’ eyes and the background light underwater.
Feller figured that the eyeshine would somewhat reduce the contrast between the light coming from the eye and the background. “No camouflage works all the time,” she says. But her data showed that there was almost no difference between the reflected light and the background illumination. “That was very surprising,” Feller says.
This was the case for all three species she looked at, no matter the depth, time of day, angle from which she viewed the animals, or angle at which the sun hit the water.
Feller and colleagues speculate that this eyeshine is finely tuned to the particular light habitat of an animal. When they calculated the eyeshine of a species of mantis shrimp larvae from the western Atlantic Ocean, the researchers found that the reflected light from that animal probably wouldn’t be a match to the light environment around Lizard Island.
“It was a very thorough, convincing paper,” says Christos Ioannou, a behavioral ecologist at the University of Bristol in the U.K. who wasn’t involved in the study.
Daniel Osorio, a visual scientist at the University of Sussex in the U.K. who also wasn’t involved in the study, agrees that the researchers did a good job of showing how this eyeshine hides a mantis shrimp’s eyes. But he would also have liked an explanation for how the mantis shrimp are creating the reflections.
Feller says that she’s currently looking into how the eyeshine works, “and we’re finding some weird stuff.” The reflective layer that produces the shine is made up of very precisely structured crystal spheres and lies right over the top of the pigments that isolate the receptors within the shrimp’s compound eyes.
“[They’re] arranged in such a way that they scatter light of a particular wavelength,” Feller explains.
These kinds of mirrored or reflective crystals aren’t unheard of in the animal world, Feller says. Beetles and weevils have them.
But their arrangement in the mantis shrimp eye is pretty bizarre, she says. “We’re a little stumped as to what some of them are doing.”
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