Cockles and angelwings

Published: Monday, December 23, 2013 at 11:29 AM.

Much sought after by collectors this time of year is the delicate bright white shell of the angelwing (Cyrtopleura costata). More common on the Atlantic or southwest coasts of Florida, nevertheless angelwings are commonly found on Cape San Blas beaches. To be so fragile and elegant, it seems surprising that these bivalves live burrowed into muddy clay deposits offshore. They can also be found in St. Joseph Bay where there are muddy clay bottoms. They need the stiff muds for support because the muscles holding the two shells together are quite weak. In fact, this clam cannot fully close its shell making it susceptible to extreme environmental changes.

With its muscular foot, the animal digs a burrow in the stiff muds and projects two siphons just above the surface of the substrate. Once in place it pumps water through the siphons to filter suspended food particles. In life, the burrows and gaping valves of this animal provide a refuge for several species of small mud crabs. Even the abandoned burrows provide a home for other sea creatures long after the angelwing has departed.

Since these clams are adept at boring into stiff clays, it is not surprising that they are closely related to shipworms, which are not worms at all but boring clams. You have probably inspected their tunnels winding through pieces of driftwood thrown up on the beach.

More commonly found on Panhandle beaches are the Campeche angelwings (Pholas campechiensis). Occasionally, one finds the little false angelwing (Petricolaria pholadiformis). While true angelwings can grow a shell almost six inches long, false angelwing shells only reach a two inch maximum.

So whether you are a shell collector or just like to look for pieces of polished beach glass, the “winter beach” is a great time to stroll our beaches.

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. 

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