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What do fiddler crabs see?

April 15, 2013

What do fiddler crabs see?

Martin How lifts a male fiddler crab from a mudflat at the Pacific entrance to the Panama Canal and points at its periscope eyes

Martin How lifts a male fiddler crab from a mudflat at the Pacific entrance to the Panama Canal and points at its periscope eyes. “Watch this,” he says, titling its carapace. The crustacean's little beads stay parallel to the horizon. That's not the only interesting thing fiddler eyes do.

By human vision standards, a fiddler can't see well. A fiddler loses sight of his burrow once he's a hand's length from it. With feathery predators a quick swoop away and claw-snapping rivals ready to steal his mate or home, a fiddler has a lot to watch out for. How does he know what's going on?

The trick is polarized light - sunbeams reflected off wet mudflats, scattered by particles in the air or refracted through murky water. Humans cannot differentiate polarized light from non-polarized light but many animals, including fiddlers, can.

Martin, a visiting scientist from the Queensland Brain Institute, Australia, conducts one polarization experiment in a broom closet at STRI's Naos laboratories, using a lightweight ball suspended in the air by a blow dryer.

He will track the movements of tethered crabs on the ball as they respond to polarized images, which are invisible to humans, on modified computer screens.

“We assume we can see everything,” says Martin. “It turns out that there is a whole load of extra information out there that we don't see.”

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