Weekly “What is it?”: Ghost crabs
I didn’t grow up on the coast, but we came down to the Gulf Shores/Perdido area every year when I was a kid. We spent a week getting sunburned and eating seafood and generally soaking up every minute next to the Gulf. At night, though, I always loved running along the moonlit beach, watching ghost crabs darting about. With their signature sideways crab scuttle, they seemed to float across the white sand. In fact, they can travel up to 10 mph! You could sometimes mesmerize them for just a minute with your flashlight, but as soon as the spell was broken, they would disappear into one of their countless holes on the beach.
The sand-colored ghost crabs (Ocypode quadrata) are active in the cooler parts of the day—morning, sunset, and evenings—and typically spend midday in their burrows. Like many burrowing animals, theirs often have multiple entrances/exits. The “J”-shaped burrows can be quite deep and run below the surface of the beach, but above the water level in the sand. While a college student in summer term at Dauphin Island Sea Lab, I made a plaster cast of a ghost crab burrow with my classmates. After pouring the plaster into the hole, waiting for it to harden, and digging it out (no crabs harmed!), we were astonished to see the burrow was at least two feet deep, with an undulating tunnel stretching a few feet from opening to opening.
Daily life on an exposed beachfront is not necessarily a vacation. With full sun, salt exposure, and dry, sandy conditions (with no access to beach umbrellas or fruity drinks!), wildlife must adapt ways of surviving. Besides building their burrows to hide from predators and heat, ghost crabs have developed stiff hairs (setae) around their gills which allow them to suck water from the sand like a straw. If you ever see a ghost crab running into the surf and back out, it is likely swapping out a pocket of seawater kept in its gill chamber, which keeps its gills moist and oxygenated. Ghost crabs also use their antennae to keep sand out of their eyes and communicate with fellow crabs using body and claw positioning.
Ghost crabs have a surprisingly varied diet. They feed on clams, insects, mole crabs, sea turtle eggs and hatchlings, but also on dead jellyfish and other detritus. Like their fiddler crab relatives, they can use their pincer claws (chelae) to grab sand, deposit it in their mouths, and efficiently separate food off the grains. For more fascinating information on beachfront critters, I highly recommend the “Beachcomber’s Guide to Gulf Coast Marine Life” by Susan B. Rothschild.