Beach Walks
with Dr. Joe
Holey Shells By Dr. Joe Richardson
Shell collecting on Tybee can be unpredictable. Sometimes a stretch of
the beach will have a bunch, and a few days later, some other area will seem
to have more shells. Tybee has a good variety of species because we are
an overlap area of a northern cold-water fauna and a southern sub-tropical
fauna. Often a shell bed will contain a bunch of broken pieces, so it’s a
little disappointing, but if you examine some of those fragments, you might
discover that some of those pieces have interesting “stories.”
It was the original snail (gastropod) or clam-like (bivalve) mollusk that
made the hard calcium carbonate shell, using dissolved calcium and carbon
dioxide that it absorbed from the seawater. But sometimes even after the
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animal dies and the shell is left behind, the empty shell might still have an
interesting role to fill in its ecosystem. It’s these clues of how an empty shell
was used by other animals that I find intriguing and, more often than not,
entices me to pick up and examine yet another shell or fragment.
Lots of marine animals require a hard substrate to settle onto in order to
grow. We are familiar with how Oysters and Barnacles have this requirement.
For very small animals that need a hard structure to settle on, a Mollusk
shell, dead or alive, might be big enough. Good examples are some of the
fan or feather-duster worms that settle onto old shells and build tunnel-like
tubes out of sand grains or out of hard calcium carbonate. It’s not uncommon
to find these old worm tubes on the inside or outside of shells.
But one of my favorite examples is what a certain family of Sponges do
to shells. I know you’ve seen the result, but maybe didn’t know the cause.
There is a family of sponges known as the Boring Sponges. They aren’t
large, upright and colorful as are some other types of sponges, but instead
are small and cryptic. They are called Boring Sponges because they have
the ability to settle onto shells and dissolve into the hard calcium carbonate
and hollow out inside layers of the shell in order to create space to live
inside the hollows. As it grows and spreads, it also dissolves small pin-holes
through to the surface of the shell so that it can grow into these holes and
extend slightly out into the water so that it can draw in seawater for food and
oxygen. So the soft Boring Sponge essentially makes a nice, hard, protective
habitat for itself out of an old shell. And then when the Boring Sponge dies
and disappears, what is left is a shell that is hollowed out and full of pinholes
all over its surface. I bet you’ve seen these before. You will find all sizes
of shells and fragments that have these small holes. You just never know
what you will see on Tybee’s beach!
Dr. Joe Richardson is a retired marine science professor with 35+ years of
research and teaching experience along GA and the southeastern coast and
Bahamas. Besides research, he conducts Tybee Beach Ecology Trips year
round (www.TybeeBeachEcology.com) and frequently posts pictures of what
they are finding on his Tybee Beach Ecology Trips Facebook page.