Fiddler on the Beach

A sand fiddler crab emerges from his burrow. Photographed at North Peninsula State Park, Florida.

In the last post (and quite some time ago) I wrote about how closely connected our world is to off-world things. The Sun, Moon, and tides are all non-living components of the natural world that interact with one another.  But they also interact with many living components as well.  In fact, there are many species that live their lives completely governed by the rise and fall of the tides.  One of my favorite is the sand fiddler crab (Uca pugilator).  I mean, who doesn't love a boy with a giant claw? Fiddler crabs live in burrows created in the mud or sand along coastal areas. And although I don't normally see them on the beach, I do see them up and down Jefferson Creek where I kayak in Delaware.  But only if I'm very quiet, and very lucky to catch a glimpse of them.  These crabs will scatter and retreat to their burrows when startled.  They also do this during high tides, only to emerge again during low tide when they spend their time feeding and trying to attract mates.  The crabs live by the tidal patterns; their behavior is triggered by the Earth’s rotation itself.  These triggers can also come in the form of changes in temperature, light, or even color.   

As fiddlers are affected by non-living components of environment, they too have an effect on it.  They feed on detritus and parts of dead organisms that would be left behind as the tide retreats. Almost frantically, they use their one small claw to shovel bit after bit of mud into their bristly mouthparts where water pumped in from their gills separate food particles from sediment.  Their digging oxygenates the sand and soil in the process making it richer and more productive.  The idea that they only have the time during low tide to feed as much as they can might account for why they always appear to be in a rush to eat.  They know when to get the job done, and don't waste a second.  In fact, this biological clock, centered around the tidal cycles, is so engrained in some fiddler crabs that even when taken away from their natural environment they continue to follow it.  When placed in an artificial environment with a controlled temperature and amount of light, the crabs still follow the movement of the tides. Fascinating little critters, am I right?

Reference

Castro, P., & Huber, M. E. (2010). Marine biology (8th ed.). New York: McGraw-Hill Higher Education.