EPOW - Ecology Picture of the Week

Crabs that Clean the Beach

Sand Balls From Foraging Ghost Crabs (Ocypode gaudichaudii), Family Ocypodidae
Bartolomé Island, Galapagos Islands, Ecuador

Explanation:  This week there is no photo of an organism ... only of its function.  This seemed appropriate, too, given that this week's (hidden) star is a ghost crab. 

Actually, this is a lesson on how hidden organisms sometimes can play key roles in their ecosystems -- little studied, poorly known, often overlooked roles.

This week we are back on Bartolomé Island of the Galapagos Archipelago off Ecuador, South America.  Along this remote back beach we have encountered a sandy stretch that is littered with thousands of tiny sand balls.

These are the work of ghost crabs that emerge at night from their intertidal sand burrows to sift through the beach sand for algae and bits of invertebrates and other animal detritus ... leaving behind these balls of sand that have been sanitized of organic debris, potentially reducing or eliminating development or transmission of diseases and parasites to other organisms.

   
   
This is a key ecological function -- a major role played by an organism in its environment that influences, and usually enhances, the habitat for other species.  For also inhabiting these beaches is a wide variety of invertebrates, birds, reptiles, and mammals.   

Even wilder, as ghost crabs dig their burrows, this very behavior may play a role in their courtship!  The crabs "kick, dump or tamp sand as they excavate from their burrows" ... and that "tamping may be involved in [their] courtship" (Schober and Christy 1993).

Typical burrow of a ghost crab.  
Ghost crabs have eyes on stalks that are raised when
outside the burrow and that are lowered into the carapace
when they "go below" into the sand.
 

Typical beach on the Galapagos
where live the furtive and
functionary ghost crabs.

So the next time you are on a beach and are puzzled by the appearance of thousands of tiny sand balls, now you will realize that a special cleaner function has been at work ... enhancing even your own recreational experience.  Give thanks to the lowly and unheralded ghost crab.

Information:
     Quijon, P., E. Jaramillo, and H. Contreras.  2001.  Distribution and habitat structure of Ocypode gaudichaudii H. Milne Edwards & Lucas, 1843, in sandy beaches of northern Chile.  Crustaceana 74(1):91-103.
     Schober, U.M. and J.H. Christy.  1993.  Sand disposal of the painted ghost crab Ocypode gaudichaudii (Decapoda: Ocypodidae): A possible role in courtship.  Marine Biology 116(1):53-60.
     Trott, T.J.  1988.  Note on the foraging activities of the pained ghost crab Ocypode gaudichaudii H. Milne Edwards & Lucas in Costa Rica (Decapoda, Brachyura).  Crustaceana 55(2):217-219.