Growing up, I visited East and Gulf Cost beaches frequently. There, I’d find shells from Donax variabilis, aka coquina. I loved coquina because their shells looked like sunrises and sunsets—and because these tiny bivalves, a type of clam, would wash up on the surf and then hurriedly dig beneath the wet sand. Coquina are small—you can easily scoop dozens into your hand if you find a colony.
Coquina live in an ecotone (a place where two different environmental conditions meet), the intertidal zone. Ecotones tend to have an abundance of life, because they hold characteristics of the two types of ecosystems. To get an idea of what this means—especially if you aren’t familiar with the term ecotone, I think it’s helpful to consider Rachel Carson’s description of the intertidal zone. In The Edge of the Sea, she described the shoreline as having “a dual nature, changing with the swing of the tides, belonging now to the land, now to the sea…only the most hardy and adaptable can survive in a region so mutable, yet the area between the tide lines is crowded with plants and animals.”
Indeed, for anyone whose spent any amount of time at the beach, you know that the places where the waves wash ashore can be quite a violent place—and that depending on the weather and the season, high tides can be extremely high and low tides extremely low.
Coquina are fairly hardy, and like other bivalves, are filter feeders. Filter feeders provide a valuable ecosystem service by cleaning the water. They can live several years in the wild, but will only last a couple of days in still water. They feed on unicellular algae, plankton, and detritus, using one of the two valves that protrudes from their shell. The other valve is used to get rid of waste.
They are consumed fish, shorebirds, humans, and some predatory snails. They use their muscular foot to burrow into the sand, which allows them to escape predation and keeps them from being swept away in retreating waves—although there’s evidence that they rely on waves to move them along a shoreline, rather than remaining sedentary like oysters or mussels.
This movement, and burrowing, is part of what I loved about them. As a child, I would wait for them to wash in on the tide, and then plunge my hand into the loose wet sand. I loved the sensation of them burrowing against my hand. It was in this way that I also first learned about the mole crabs (Emerita talpoida), a small crab without pinchers that also burrows into the sand and are frequently found on the same sandy beaches as coquina. I’d use what I had (a Frisbee, a bucket, just my hands) to scoop coquina, and sometimes mole crabs, into my hands, to marvel at the particulars of their evolution.
Part of what I loved about coquina was their place in the beauty of the world. Their brightly colored shells added beauty to the beach—especially beaches with a lot of bleached shells—and I remember walking along the coast a few days after a major storm, and how sometimes hundreds of empty coquina shells would roll in the edge of the surf, and how they gently clicked against each other.
However, while still quite common and not a species of concern, coquina do face environmental pressures. Sea-level rise, increasing ocean acidification, and beach erosion can all impact these little invertebrates. They may be especially at risk where damming of rivers has led to a decrease in fresh sand deposition or where beach re-nourishment projects, which add sand back to beaches, occur because they can get buried under tons of sand. Given their role in the ecosystem as filter feeder and as a food source, it may be important to monitor their presence (or absence) on a beach, especially one that is experiencing beach erosion.
Donax appears around the world, in places where there is sandy surface, and are generally an indicator of good beach health (although since they live in colonies, the absence of a visible Donax community isn’t necessarily an indicator of poor beach health—just the absence of a colony where you happen to be searching).
I’ve never eaten coquina, but I’ve heard they make a delicious green broth. If you’ve tried that, we’d love to hear from you on Facebook!