Excerpts from Jim Conrad's
from the May 29, 2011 Newsletter issued
from written at Mayan Beach
Garden Inn 20 kms north of Mahahual, Quintana Roo, México
Never had I seen crabs atop those rocks, and never since have I seen them. Obviously something was going on that day that I don't understand.
Because this was such a distinctly patterned crab species I figured that volunteer identifier Bea in Ontario would have an easy time learning its name, but she didn't. It turned out that we had a very variable species. Still, eventually we did decide that it must be what people often call the Sally Lightfoot Crab, GRAPSUS GRAPSUS.
Sally Lightfoot, who mostly eats algae on wet rocks in the intertidal zone, occurs along the coasts of subtropical and tropical America, Africa, the Gulf of California, and Chile. The species varies tremendously in color, and juveniles can look very different from adults. Ours, with its cream-colored speckles, is a juvenile. Most photos on the Internet show bright red adults.
On the other hand, descriptions of the species' behavior found on www.TheCephalopodPage.org's page for the species describes exactly what I saw:
At another rocky point a few kilometers away and later in the week I came upon an abandoned Sally Lightfoot shell, or exoskeleton. Crabs have the same problem as insects: They are encased in shells which periodically must be split open, crawled out of and abandoned, so that the crab can grow. A close-up frontal view of this Sally Lightfoot showing the "face" and claws is below: