Fiddler Crabs: an unusual but important member of the salt marsh community

Fiddler crabs are small but striking members of the salt water marsh community. They are also a perfect example of the roles that different species play within a particular ecosystem and how these roles form a complex web of interdependency.

There are actually some 96 different species that fall within the fiddler crab genus. Reaching a maximum of only two inches, the fiddler crab gets its name from its disproportionately larger claw and how the crab uses it during feeding in what appears to be a violin-playing like motion. The claw is actually used for signaling potential mates as part of a "hey, look at me" courtship behavior in addition to battling possible rivals during the courting process. If the crab loses its large claw to a predator or a rival suitor, it will grow back but only as a much smaller claw. However, as if to make up for the loss, the crab's other smaller claw will increase in size.

Fiddler crabs feed by filtering sand and ingesting any algae, fungus, microbes, or detritus it finds. In so doing, it plays an important role in cleaning and aerating the salt marsh sediment. Unfortunately, being such a small crab makes it a delicious morsel for other predators, so the fiddler crab population is kept in check by shore birds, fishes, and larger crabs.

While adult fiddler crabs make a home by burrowing in the sand, the juvenile crab seeks shelter by using an empty snail shell, much like the hermit crab. But whereas the hermit crab carries its protective housing with it, juvenile fiddler crabs simply use the empty shells as temporary shelter. A recent study by researchers from the Georgia Southern University in Statesboro (United States) found that with every 100 snail shells, almost 80 would typically have baby fiddler crabs inside.

"Small mammals and reptiles feed on fiddler crabs as they move across the salt," said Prof. Sophie George, the study's leader. "An abundance of Littoraria irrorata [snail] shells [provide] a welcome refuge for juvenile fiddler crabs."

So, the tiny fiddler crab plays important functions both as a filter feeder of marsh sediment and as a prey item for other animals within the community. And its success depends on the presence of snails which provide sustenance to some predators and a temporary home to the juvenile fiddler crab.

Such is the complexity of nature's ecological web.

Read more about the fiddler crab in the BBC Earth News.