2018-05-01 / Features

Building on history with clams

By Robert Turpin, Marine Biologist


Robert Turpin Robert Turpin This month’s marine creature feature is the coquina clam, Donax variabilis. The Latin genus name Donax was likely derived from the clam’s two separate tubular siphons, which appear like reeds (plants). These siphons are used to circulate water into and out of the animal for respiration and feeding when the little clam is buried in the sand along the beach. The species name variabilis refers to the variable colors of the external surface of the clams’ shells. These bivalve (two shells) mollusks live in the swash zone of the sandy beaches of the Gulf of Mexico from Florida to Texas. They may be found as far north as the Atlantic coast of New York and as far south as the beaches of the Caribbean.

Coquina clams are found in the intertidal zone — between high tide line and low tide — in summer. They move offshore in winter to warmer conditions at depths to 60 feet. They sometimes occur in dense aggregations of many hundreds of individuals, and can change the appearance of the receding waves as the water flows across the thousands of siphons. When waves are large, coquina clams may be dislodged from the sand and use their muscular “foot” to re-bury as quickly as possible. The molluskan foot has no bones or toes and looks nothing like any vertebrate foot. Nevertheless, many mollusks can completely bury themselves within a few seconds.

As the species name describes, the external surface colors of coquina clams are extremely variable. Digging up a double-handful of sand might reveal dozens of coquina clams. Some individuals might be red or yellow or purple or nearly any color in-between. Some individuals may be almost white or brownish. The inside surfaces of their shells are often blueish or purple. The shells are nearly triangular and are very beautiful and have been used for crafts, jewelry and more.

Clean and dried coquina shells may be purchased for about $10 per pound. Although coquina clams are very small, usually less than 2/3-inch, they are edible. Most recipes recommend thoroughly rinsing and simmering the clams in seasoned water for about 10 minutes. The broth is then carefully poured off to separate the “soup” from the remaining clams and sand in the bottom of the pot.

As with other shellfish, neither coquina clams nor their broth should be consumed when “Red Tide” events affect surrounding waterways. Toxins associated with “Red Tide” and other harmful algal blooms can cause serious illness or death.

Some people may be surprised to learn that collecting live coquina clams is a regulated form of marine life harvest, and may require a fishing license in the state of Florida, unless the individual is exempt from fishing license requirements.

Coquina clams are filter-feeders, straining tiny plankton and other small bits of food from the waves. In turn, coquina clams are eaten by shorebirds and fish, including the Florida pompano. Coquina clams, mole crabs, also known as “sand fleas, and other worms, mollusks and such are a vital ecological link in the coastal ecosystem.

Coquina clams usually live about two to three years and spawn in their first year of life. They have separate sexes (males and females) and produce planktonic larvae. The larvae mature through several larval stages in the open waters, then settle along the beaches as they mature from the planktonic larval phase into the sedentary adult “clam” phase.

This dispersed, open-water larval phase provides important population-level benefits as well as substantial risks to individual clams. Only a fraction of the planktonic larvae are “lucky” enough to return to the beach zone as they metamorphose into the adult life form. The benefit of this life history strategy is the ability to repopulate beaches that experience large-scale mortalities caused by hurricanes and other natural disasters.

Human-induced mass mortalities include beach re-nourishment and oil spills.

Coquina clams and other inhabitants of the intertidal zones of the Gulf of Mexico and tributaries were impacted by oil from the 2010

Deepwater Horizon oil spill. Coquina clams were monitored by researchers and natural resource managers to document the extent and severity of the oil spill.

During previous glacial periods, also known as “ice ages,” the empty shells of millions of coquina clams were deposited in concentrated areas and were stranded as sea levels dropped. Over the ensuing millennia, rain dissolved substantial amounts of shells, encasing dense concentrations of coquina into a form of limestone called coquina rock. This rock has been quarried for hundreds of years in Florida and elsewhere and used as paving and building materials. In 1672, construction began on St. Augustine’s Castillo de San Marcos. Coquina rock was used extensively in the construction of the fort, which survived three days of cannon bombardment. More than three centuries later, the fort continues to demonstrate the strength of the tiny coquina clam.

Growing up on the Florida Panhandle coast, I spent many of my childhood hours digging coquina clam “jewels” and other treasures? along local beaches. My wife and I have been blessed with many hours watching our daughters enjoying those same simple pleasures provided by our coast’s natural resources.

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