GHOST CRABS
Often one of the things you notice walking through the dry beach sand are
strange holes. They will be various sizes and are often surrounded by little
scratch marks or small piles of sand with a different color or grain size. You
might see these holes anywhere between the most seaward dunes and the
high tide line, but I usually see more of them closer to the recent high tide
line. These holes are the openings to burrows dug by Ghost Crabs.
Folks who walk on the beach in the evening know about Ghost Crabs.
During the daytime, Ghost Crabs generally stay in the burrows they
excavate into the dry sand. But at night, they emerge and patrol the beach,
scavenging things that have washed up for food. They can also locate small
clams, like Coquinas, that live just under the sand, and dig them up to eat.
Ghost Crabs have large, strong claws for protection and feeding. And they
have large eyes that stick up high on stalks. If you have ever watched a
Ghost Crab run from you, you know from experience they have long legs
that allow them to stand up high and run very fast.
For the most part, a Ghost Crab is a terrestrial animal. They don’t have
lungs, but they do have gills enclosed inside their hard exoskeleton with
narrow passageways for air between the gills and outside. This allows
the gills to remain wet even though the crab is not submerged. With this
adaptation, a Ghost Crab needs only to take a quick in-and-out dip at the
water’s edge, and it will be good for hours out of the water. Spending the
hot daytime hours down in its cooler humid burrow also helps keep these
modified gills moist.
Being active at night has helped Ghost Crabs become more of a land
animal than a marine crab. Living on land presents additional problems
34 TYBEE BEACHCOMBER | OCT 2020
besides just drying out and conserving internal water though. Among these
is predation. For a small (especially when younger) crawling land animal,
birds are a constant daytime threat. The nocturnal pattern of behavior helps
minimize this threat of being captured and eaten.
Along with having gills that need to be wet, Ghost Crabs have an additional
connection to the sea. A female carrying eggs has to make occasional trips
into the swash zone to keep her eggs moist. And when it is time to release
the tiny larval crabs, she releases them into the seawater. The larvae then
spend time as part of the plankton drifting in the sea as they develop until
time to settle onto the bottom as baby crabs and eventually make their way
up the beach.
So when you are out for a pleasant evening walk on Tybee’s beach, if you
think you see a small animal scurrying around in the distance, but it looks
more like a small ghost, you are lucky enough to be watching a Ghost Crab
going about its nightly business.
Dr. Joe Richardson (Ph.D. Marine Sciences) is a retired marine science
professor with 40 years of research and teaching experience along GA,
the southeastern coast and Bahamas. Besides research, he conducts Tybee
Beach Ecology Trips (www.TybeeBeachEcology.com) and frequently posts
pictures of their findings on his Tybee Beach Ecology Trips Facebook page.
Beach Walks
with Dr. Joe
By Dr. Joe Richardson
/(www.TybeeBeachEcology.com)