Love Code: A twist of light only mantis shrimp can see

By Davide Castelvecchi
From Science News, March 22nd, 2008; Vol.173 #12 (p. 179)

For love, some would twist the laws of physics. Short of doing that, mantis shrimp communicate with the other sex by spinning light waves, biologists find. The feat seems to be unique to this animal.

Light is made of electromagnetic waves. These are electric and magnetic fields that wiggle perpendicular to each other and to a light ray’s direction. Many invertebrates have sophisticated eyes that can detect wavelengths of light invisible to humans. Some, including bees, can also distinguish linearly polarized light. That’s when a light ray’s electric field wiggles not in varying directions, but rather in one precise direction that forms a right angle to the ray.

Researchers now show that mantis shrimp—which actually look more like small lobsters—can tell when light is circularly, rather than linearly, polarized. That means that the electric field twists like a corkscrew as the light ray moves. The corkscrew can twist right or left—or, in biological terms, be right- or left-handed.

Roy Caldwell of the University of California, Berkeley, suspected that one species of mantis shrimp, Odontodactylus cultrifer, might be able to distinguish circular polarizations. Animals in this species, especially adult males, are rare. But 2 years ago, thanks to a tip from a crustacean enthusiast, Caldwell obtained a 4 inch-long adult male originally from Indonesia.

The shrimp had a fin with shades of red that looked more or less intense when seen through filters for right- or left-handed circular polarization. This trait was rare enough, but not unique in the animal kingdom. Caldwell’s collaborators at the University of Maryland, Baltimore County (UMBC) and the University of Queensland in Brisbane, Australia also took a closer look at the eyes of O. cultrifer and of two similar species to see whether the animals could distinguish between right- and left-handed polarization.

The researchers found that some of the eyes’ light-sensing cells doubled up as filters, explains Tom Cronin of UMBC. The cells have microscopic structures, like bristles of a toothbrush, that slightly slow light with electric fields parallel to the bristles, but not light with fields that are perpendicular. As a result, the twist of a circularly polarized wave will be flattened into a steady, linearly polarized wiggle, which another layer of sensory cells can then detect. Depending on their arrangement, bristled cells will select right- or left-handed polarization. This parsing enables mantis shrimp to distinguish the two types of light.

Meanwhile, the team trained mantis shrimp to feed from one of a few different tubes based on the circular polarization in the tubes’ reflected light. Results appear online in Current Biology.

Caldwell says the skill, unknown in other animals, most likely helps the shrimp find mates. “It’s the most private communication system imaginable,” he says. “No other animal can see it.”

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