Smithsonian.com

Once a fiddler crab (above, a male Uca pugilator) gets big enough, he no longer needs to hide in a periwinkle shell (photo courtesy of flickr user cotinis)

Life can be tough for a fiddler crab. So many other creatures find them tasty: migratory birds, shrimp, fish, raccoons, turtles, even other species of crab. Adults, at least, can dig themselves a burrow and fight off predators. But juveniles don’t—or can’t—seek shelter in the sand. They can hide beneath vegetation, but that’s not always an option. What’s a little crab to do?

They use empty shells from the marsh periwinkle (Littorina irrorata), say biologists from Georgia Southern University, who report their findings in an upcoming issue of the Journal of Experimental Marine Biology and Ecology. The scientists studied juvenile fiddler crabs in six salt marsh sites at Tybee Island, Georgia. They found that up to 79 percent of periwinkle shells were occupied by juvenile fiddler crabs, and that female crabs were more likely to take shelter.

Female fiddlers lack the the larger claw that males use to fight and attract the ladies. With only two small claws, the females are more vulnerable to birds. In addition, the females tend to be smaller than the males. “It was thus not surprising to observe that both size and sex played an important role in shell use among juvenile fiddler crabs,” the biologists wrote.