The weather is getting warmer and with that warm water temperatures. Life in the bay is getting active again. Soon, we’ll all be anticipating scallop season and the hunt for those delicious little bivalves - bay scallops (Argopecten irradians). But did you know that there is another bivalve in St. Joseph Bay and surrounding Gulf waters whose muscle meat tastes exactly like a scallop? I can vouch for it. So why aren’t we out there collecting that mollusk too? The short answer is that they are not as easy to get.
That tasty but hard to collect mollusk is the pen shell. Pen shells are those dark or olive brown triangular, wedge-shaped shells with the slight iridescence inside the shell. They are often found washed up on the beach. They grow fairly large, up to 12 inches. There are several species of pen shells or pen clams in the gulf. One often found in St. Joseph Bay is the Sawtooth Pen Shell (Atrina serrata). Pen shells dig down into the sand pointed end first until only the upper fringe of the shell is above the substrate. There they spin a byssus or byssal threads from a gland on the foot, and attach themselves to hard objects beneath the surface such as a piece of limerock or a deeply buried old shell. Once in place, they can be easy to miss, especially if they are buried on turtle grass flats. A good place to find living specimens is Eagle Harbor or the flats east of Black’s Island. Look carefully while snorkeling and you’ll see the upper shells just above the sand or move your hand gently along the bottom until you feel the hard shell.
Once anchored in place, the pen shell settles down to the life of a filter feeder, pumping in water and extracting plankton and other organic matter from the seawater. Because of their relatively stable life strategy, over time, other organisms have taken advantage of the shelter provided by the interior of the pen shell. Little commensal crabs of the genus Pinnotheres and pairs of commensal shrimp of the genus Neopontonides can usually be found living inside the space created by the two valves of the pen shell.
Pen shells have several unusual features. They have two adductor muscles to close their shells, but they are asymmetrical, one is very small and the other quite large. Instead of having a hinge attaching the two shells, like a clam or oyster, pen shells actually bend the shell to close the two halves. While rare, pen shells will also secrete a black pearl. Like oysters and a few other bivalves, pen shells will secrete some of the material that forms the mantle to cover an irritant. Sand, a bit of debris, or food can become irritating and the mollusk seals it off. Because the interior of the pen shell is dark, it produces a black pearl.
Various species of pen shell are found throughout the world, and they are harvested for food in Japan and various Polynesian Islands. Perhaps one of the most unusual uses of pen shell was the harvesting to obtain the byssal threads they secrete to attach themselves to the substrate. These fine threads, called sea silk, were woven into fabrics in ancient times. Mainly obtained from the Mediterranean pen shell, Pinna nobilis, fabrics of this material were highly prized in ancient Egypt, Greece, Persia and Rome because of the golden color of the processed thread. The practice of using sea silk continued up into the early 20th century, when destruction of grass beds and pollution caused a decline in Mediterranean pen shell populations. The craft of using byssal threads for cloth is still carried on by a few artisan women in Sardinia. There is also evidence that the ancient Chinese also used pen shell byssal threads to weave cloth.
So while pen shells are edible, their value lies in their role as filter feeders, helping to keep bay waters clear and by providing a habitat for other species. Once the pen shell is pulled up its chances of reestablishing itself are small because the digging foot has become reduced by their sessile existence. The dark fragile shells are better collected by walking the beach than disrupting a living organism on the off chance you’ll find a black pearl. I’ll stick with collecting scallops.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.