By Tom Baird
Ever notice how the beach changes from summer to winter? We are now in the “winter beach” and this is the time when shelling is best.
Throughout the summer, sand is deposited on the beaches and the dunes build. The beach becomes higher and wider. In winter, changes in wave height and direction cause sand to be pulled offshore from the beach and deposited in protective offshore sandbars. The winter beach is narrower and lower. While this phenomenon is more extreme on the Atlantic and Pacific coasts, nevertheless our Panhandle beaches also go through this cycle.
Because sand is pulled off the beach in winter, shells that were buried all year are now exposed. Both tourists and residents can be seen carrying buckets or bags of shells to take home. The most commonly collected shells are the giant cockles and white angelwings, with the occasional empty shell of a large left-handed whelk.
The giant cockle or heart cockle (Dinocardium robustum) is familiar to most residents. It is that big – up to five inches long - tan or cream colored bivalve with parallel ribs with brown patches. It occurs around Florida wherever one finds their preferred habitat - sandy shallows off beaches. Therefore, the big bend area, with its lack of wave energy, and therefore lack of beaches, does not offer suitable habitat for the giant cockle. The giant cockles found on our Gulf beaches is a more colorful variety than their Atlantic cousin and is classified as a subspecies, Dinocardium robustum vanhyningi, often called Van Hyning’s cockle.
These bivalves are filter feeders that use their long muscular foot to dig down into the sand and extend their siphons into the water column to pump in suspended food particles. The cardium part of theirgenus name, Dinocardium, may remind you of heart. To see why they are sometimes called heart cockles, find two shells of about equal size and place their edges together. Now turn them to the side and look at the shells with their edge facing you. The profile from the side resembles a Valentine heart. The fully grown shells are also about the size of a human heart.
You probably remember the old song about Molly Malone singing, “cockles and mussels, alive, alive oh!” As Molly was pushing her wheelbarrow, the cockles she was selling were not our Van Hyning’s cockle, but the common European cockle, Cerastoderma edule, or any of several small cockle-like clam species that are harvested using rakes at low tide. Cockles are still available at seaside stalls in the United Kingdom. Our own giant cockles are edible also and the reddish flesh makes fine chowder. However, few people go to the trouble to harvest our giant cockles. For a bit of trivia, cockles are even mentioned in the Magna Carta. That historic document granted English subjects the right to collect up to eight pounds of cockles from the shore without hindrance or confiscation.
Our beaches have several other smaller shells that are classified in the same family as the giant cockle. One can easily find shells of the Atlantic strawberry cockle (Americardia media), spiny papercockle (Papyridea soleniformis), Florida pricklycockle (Trachycardium egmontianum), and even the smooth-shelled common egg cockle (Laevicardium laevigatum).
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.