Coquina (Donax variabilis) is a beautiful clam common on the beaches in eastern United States and the Gulf of Mexico. The clam varies in color and can be found in different shades of pink, yellow, purple and blue. Some have sunburst patterns, and some don’t. You can find coquina in the swash zone — the intertidal section of beach where waves swash back and forth. There coquina live their lives eating algae and detritus. They have one straw-like siphon, which sucks in water with food, and a second siphon that spits out water minus the food. The clams have a muscular foot that digs into the sand and keeps its owner from being washed away. By raising its foot, coquina can ride the waves further up the beach or back towards the ocean, all the while staying in the swash zone where the eating is good.
That’s coquina the clam, but there’s also coquina the rock, and it’s not a complete coincidence that the clam and rock share the same name. Coquina clamshells plus the hard bits of other marine invertebrates that lived thousands of years ago form coquina rock. When coquina clams die, their shells sink to the bottom. These shells accumulated under water for thousands of years. During the last ice age, which ended around 20,000 years ago, sea level around the globe was almost 400’ lower than it is today. Layers of shell several feet thick were exposed to the air, covered with dirt and vegetation, and rained on. Rain forms a weak acid as it percolates through soil and vegetation. This acid dissolved some of the shells and worked like glue to cement shell fragments together and form the soft and crumbly coquina rock we find on the beach today.
Spaniards started building the Castillo de San Marcos in St. Augustine in 1672 from coquina rock. They weren’t sure how coquina would compare to the harder rocks they were used to building with in their native Spain. The Castillo was bombarded by English cannonballs in 1702 and again in 1740. The soft coquina absorbed the blows, and the fort was merely dented rather than razed. Structures made of hard rock can shatter when barraged by cannonballs. Thus the Spanish held onto the fort and Florida until 1763 when the State was ceded to Britain by treaty, not by cannonball.
If you’re interested, pick up coquina clams at the beach for a closer look. You’ll see the colors, maybe the leg, and the beginnings of the material that so masterly dealt with British cannonballs. And if at the same time you see plastic, pick that up too. Grab a bag from the BlueTube at your beach, pick up plastic and throw it away. Coquina belongs. Plastic doesn’t.