TYBEE BEACHCOMBER | FEB 2020 13
SPIDER CRABS
We have lots of different species of crabs here in coastal Georgia and
the Low Country. I came to recognize this through many years of teaching
marine invertebrate zoology during my professor days. But it’s been
through lots of exploring while conducting Tybee Beach Ecology Trips over
recent years that I’ve become amazed and impressed with the diversity of
crabs we have around here. There are a few species that I’ll see only once
or twice a year, and it’s usually around the same time of the year that they
make a short, quick appearance around Tybee. Others are likely to show up
at any time of the year.
One of my favorite Tybee crabs is the Spider Crab. They are fairly docile
and slow, and their pinchers are small, so they are safer to handle and are
friendlier in a beach touch tank than are our Blue Crabs. In some aspects,
they look almost like something prehistoric, but I think Spider Crabs are
kind of funny or comical looking. A Spider Crab has a large, round, plump
main body covered with a rough upper surface with lots of bumps. Sticking
out from this big body are ten skinny, jointed legs. So it does have a sort
of giant spider-like appearance. Unlike our Blue Crabs that have back legs
that are flattened and are used for paddling and swimming, the Spider
Crab’s legs end in sharp points (except for the first or front pair of legs
that have those tiny pinchers). Lacking any paddles, Spider Crabs are not
swimmers, but they are good at walking around and holding on to hard
surfaces like rocks, or for digging themselves into the sand.
Along with being slow and harmless, another reason I like Spider Crabs
is because they like to decorate themselves! Often a Spider Crab will grab
pieces of sponges, seaweed, or bushy Hydroids and Bryozoans, and place
these fragments on top of its body. The rough surface of its exoskeleton
is like Velcro, and these objects will become stuck on the fuzzy top of the
crab’s body. In reality, the Spider Crab is attempting to camouflage itself.
But as a result, Spider Crabs are included in a group known as “decorator
crabs.” Being a slow-moving species that likes to sit still, having this
camouflaging activity and ability makes it be less noticeable to predators.
In general, Spider Crabs prefer living on hard or rough bottoms where
they can cling to and walk over the surface. Being slow and having tiny
pinchers, Spider Crabs feed on small animals growing among the turf of
sessile invertebrates that form the fuzzy or hairy-like “fouling community”
often seen covering rocks and dock pilings underwater.
Some readers might remember a previous article about Cannonball
Jellyfish (Tybee Beachcomber, August 2018) that described how sometimes
small Spider Crabs are found living inside Cannonball Jellies. So that’s
another place you might find them on Tybee!
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)
Beach Walks
with Dr. Joe
By Dr. Joe Richardson
/(www.TybeeBeachEcology.com)