Fiddler Crabs

In ancient times, sailors on the ocean were in trouble if they couldn’t see the Sun, stars, or any landmark. They had to use a method known as “dead reckoning” to find their way. Today scientists call it “path integration.” It was simply keeping track of how far they traveled and in what directions to figure out where they were. The farther they traveled and the more turns they took, the less accurate the method became. Fiddler crabs use this method to find their way back home.

The fiddler crab gets its name from the fact that it has one vastly oversized claw that looks kind of like it’s carrying a fiddle. These small crabs are defenseless against birds and other predators. They survive by ducking into their burrows when threatened. Their vision is not good, and once they are three or four body-lengths from their burrow, they can’t see it.

Fiddler crabs go as far as 25 body-lengths or more looking for food. When frightened, they make a beeline straight to the burrow. They do this using path integration (dead reckoning). Researchers have determined that the crabs measure how far their eight tiny legs have carried them and they avoid error-inducing turns by keeping their bodies in a fixed orientation as they forage.

Measuring their steps, counting the steps, and keeping error-inducing movements to zero is how they survive as they look for food in a hostile world. Who could have built into their DNA a survival plan that works this well? We don’t think the dead reckoning of the fiddler crab is accidental.
© Roland Earnst

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