EPOW - Ecology Picture of the Week

Atlantic Ghost Crab (Ocypode quadrata), 
Family Ocypodidae, Order Decapoda
St. John, U.S. Virgin Islands

Explanation:   Locally common to shorelines along the Atlantic seaboard and Caribbean, this is an Atlantic ghost crab ... an aptly named nocturnal apparition that emerges in the dark from burrows they dig in gentle sandy beaches.  

At night, ghost crabs feed as active predators, consuming clams, insects, and other crabs, as well as plants and even detritus.  They communicate by rapping their claw on the ground, by rubbing their legs together, and by blowing bubbles from their gill chamber.  

Their eye stalks are sensitive to light intensity but, amazingly, can detect polarized light which commonly occurs from the glare of flat wet sandy beaches. 

As opportunists, ghost crabs probably play key ecological roles as scavengers and predators along the narrow, dynamic edge between sand and sea.  They filter sand through their mouthparts to extract nutrients from algae.  To escape predators itself, ghost crabs emerge mostly at night, are colored to blend into their sandy substrates, and can move surprisingly quickly into their burrow systems.  

To survive, ghost crabs have to literally and figuratively run the nightly gauntlets of raccoons, human alteration of their beach habitats, oil spills, and foot and vehicular traffic.  Healthy ghost crab populations can indicate healthy beach ecosystems with low human impacts.  But where healthy populations begin to wane or disappear, it might signal degradation of the overall health of the coastal ecosystem.  

 
Information:
     Schoene, H., and H. Schoene.  1961.  Eyestalk movements induced by polarized light in the ghost crab, Ocypode quadrata.  Science 134(3480):675-676.
     Turra, A., M.A.O. Gonçalves, and M.R. Denadai.  2005.  Spatial distribution of the ghost crab Ocypode quadrata in low-energy tide-dominated sandy beaches.  Journal of Natural History 39(23):2163-2177.