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'Scary' Movie Proves Crabs Can See Different Colors

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Photo/Andrew Higley
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UC Creative Services
UC biologist John Layne developed a simple color vision test for crabs and other animals that uses a liquid crystal display and adjustable light-emitting diodes. The screen projects a fast-approaching object of 1 color set against a background of another.

How do you know what colors a fiddler crab can see if they can't answer you? You show them a "movie" that would scare them and cause a reaction if they did see all the colors. A University of Cincinnati professor did just that and the crabs were running scared.

Associate Professor of Biology John Layne decided to study fiddler crabs because they are one of very few creatures that apparently can see lots of colors. They have three color receptors, much like humans who can see millions of shades.

What Layne set out to answer is that even though they have cells that make proteins to catch light in three different colors, can they tell the difference between them?

To find out, he built a miniature movie theater for the saltwater crustaceans. With a stripped-down liquid crystal display from inside a computer monitor, fiddler crabs watched a kind of motion picture.

"And the movie is very, very scary to a crab. It's very scary, and what it consists of is a little dot in the middle of the screen and the background is one color and the dot is another color. The background is blue and the dot is green," he says.

"They run like a bat out of hell. Their reaction is not subtle. They will sprint really fast and bang into the wall," when they see the ball coming at them, Layne says.
Credit Photo/Andrew Higley / UC Creative Services

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The dot gradually increases in size and gets faster. It looks like it's coming right at them. Think: the boulder scene in Raiders of the Lost Ark.

https://www.youtube.com/watch?v=c6XHLe94SJA

Layne says even his students flinched when watching the video with the ball that the crabs saw.

Researchers are still in the early stages of discovering who can see what in the animal world. While we can see about one million colors, some spiders are believed to see 100 times that and mantis shrimp even more. As scientists learn from the animal world, some of the knowledge can improve life for humans.

"There's all kinds of different ways that technology is taking essentially cues from experiments that have been run for millions of years and seem to have hit on a solution, " Layne says.

His research is published in the journal Methods in Ecology and Evolution.