SAND DOLLAR CRABS
You never know about Sand Dollars on Tybee. Sometimes you find one
and sometimes you don’t see any. A Sand Dollar is actually a type of sea
urchin that has become flattened and its spines are reduced to very short
tiny spines that cover the lower surface, creating a felt-like fuzzy bottom
surface. Soon after it dies, these tiny spines fall off, so the bottom will be
smooth just like the top side. A dead, white, sun bleached, smooth Sand
Dollar is one of every beachcomber’s favorite treasures to find.
Live Sand Dollars slowly burrow through the sand, usually about an inch
below the sand. Most live just offshore of the low tide zone, but sometimes
they move shoreward into the intertidal zone. Here, it might create a slight,
subtle bump or hump in the wet sand when the tide is out.
If you are fortunate enough to discover a live Sand Dollar, its worth taking
a close look at it before returning it back below the wet sand near the water
line. With your fingers, carefully scrape away the sand from above the Sand
Dollar so that you can gently lift it out of the sand. They are so fragile and
break so easily, you want to handle it carefully. Gently rinse away the sand
and take a good look at the tiny spines, and you can see them moving in
slow waves. It’s by using those tiny spines that the Sand Dollar is able to
slowly plow through the sand.
When we find a live Sand Dollar during our Tybee Beach Ecology Trips,
I’ll usually look closely at the underside to see if it is home to a pair of tiny
Sand Dollar Crabs. These tiny crabs often occur in pairs crawling around
in the tiny spines on the bottom of the Sand Dollar. Sometimes there may
be only one, but there could be three of four. Look carefully, and be sure
to examine closely inside the slots of the Sand Dollar. The tiny crabs like to
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move into these spaces when they feel exposed. If I remember right, this is
the smallest crab in the Western Atlantic Ocean.
Although it has been suggested that the tiny crab might sometimes nip at
the short tube feet of its Sand Dollar host, it is more likely feeding on mucus
and tiny organisms that live between and on the surface of sand grains.
As such, the Sand Dollar Crab is considered a commensal rather than a
parasite. Commensalism is when two species live in a close relationship,
and one of the species benefits, and the other species is neither helped nor
harmed. In this case, the crab benefits by having a place to live, but the
Sand Dollar is neither helped nor harmed by the crab’s presence.
So, when you find a live Sand Dollar on Tybee, handle it gently; but take
a close look at its underside and check for the smallest crab on our side of
the Atlantic Ocean. I get excited every time I see one! And then be sure to
carefully return the live Sand Dollar back under the wet sand.
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)