Coquina Shells, Beauty and Oddity In the Swash Zone
(For those who love pictures, there are some at the end of this post (!) that illustrate a few things from this post)
Living Jewels. That’s the phrase that played in my mind as I spent time studying these tiny creatures. Sometimes bright and obvious along the shoreline, a wave would wash over them and their presence became a lot more subtle. It took me a long time to truly discover them that day, but after I did the rest of my time was pretty much tied up in observing them.
Their name “Coquina” is actually a term based in geology (the study of earth’s physical nature - or “rocks”, to over simplify) (1). Mention the word “coquina” to a geologist and their mind will most likely think of a type of rock called limestone, which is made up of shells. Limestone is abundant in Florida, and coquina limestone (rock formed by mollusk shells cemented together) was the first mineral used by Spanish Settlers to build forts in their new world (2).
Anyway! I believe this species’ name to be Donax Variabilis, which is described as a small, triangular or wedged shaped shell that can vary greatly in color, a description that matches the organisms I found. Coquina clams are filter feeders, feeding on tiny particles suspended in the water. In turn coquina clams are a source of food for various birds, crabs, and fish (3).
This beach on this day had many birds present, including a few species of gulls, sandpipers, plovers, terns, and pelicans (that remained offshore). Though the coquina were abundant up and down the shore, I didn’t observe any birds interested in them as sources of food. I did see the birds pursuing small fish near the edge of the water and food that could be gleaned (or stolen) from humans.
Known to be very sensitive to environmental changes, biologists will use coquina clams as what is called an “Indicator Species” (3).
Like a doctor who checks lymph nodes for clues of possible problems within a body, biologists will monitor or check indicator species for possible problems within an environment. In the case of coquina clams, biologists may monitor their species to check the health of their beach habitat (3).
I grew up along Florida’s southeast coast and loved going to the beach throughout the year, but I had never seen these animals before. I’ve also visited the Gulf coast of Florida on several occasions and never came across them there either. This is surprising after reading how widespread this species is along the coastlines of the United States.
These animals are found in what is called the “swash zone”, I came across these in the swash zone on a Florida Gulf Coast beach in Pinellas County.
I spent a long time that day sitting in the swash zone, watching how the play of waves affected these creatures. I noticed how it seemed that the clams would go through different periods. For instance, there would be a period of many cycles of waves - where a wave would advance and recede, continually washing over and back on the clams I observed - where the coquina showed no signs of life, they appeared to be just shells.
Then there would be a cycle of washing waves where the coquina were extremely active, a wave would pull out off the sand and all the clams would begin to move, and I could see their almost transparent siphons and foot reach out of their bivalve shells as they all worked to bury themselves beneath the sand.
I do not have any more information at this time for possible reasons for these observed “periods” of inactivity and activity - in fact for all I know these observations may be an anomaly as I only observed these coquina shells in one area for a few hours over the period of one day.
Factors that I would want to take note of if I were to observe them again might be time of day, high tide vs. low tide, and the look of the seawater.
Coquina caught my eye because of their bright and colorful beauty in the clear cool water, but their beauty isn’t what kept me observing them for the rest of the day. It was the little thrill of discovering that they were living, an abundant innumerable host of tiny strange creatures along the water’s edge. Seeing their vitality and their strangeness only added to their beauty.
Beauty catches the eye in nature, and I think perhaps also in science. I believe that allowing ourselves to keep that love of beauty, even cultivating and sharpening that love (without allowing it to rule us) opens us up to more discoveries, more breakthroughs, and more love for our work.
Jewels may catch the eye, but the living will excite the biologist infinitely more.
Works Cited
Britannica, Southern Coquina Clam
2. Schmidt, Walter, et al. The Limestone, Dolomite, and Coquina Resources of Florida Florida Department of Natural Resources, 1979 - https://web.archive.org/web/20210525070347id_/http://publicfiles.dep.state.fl.us/FGS/FGS_Publications/RI/RI/RI88.pdf
3. Florida Fish and Wildlife Conservation Commission - https://myfwc.com/research/saltwater/mollusc/other-molluscs/coquina-clams/
Coquina Donax variabilis
