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Coquina Clam

Donax variabilis
 
Description
Coquina are of the mollusc class which includes bivalves such as clams, cockles, mussels, and oysters to name a few. They are characterized by having two-halved (vavled) shells. Coquina clams can grow up to 1 inch (2.5 cm) in length. They are more abundant in the smaller sizes, generally around ½ inch. They are about ½ the length in width, at the widest point, which is at the hinge. Their bivalve shells form into a wedge (triangular) shape (Scienceray). The color of Coquina shells can vary from pastels to very vibrant hues. In addition, the pattern of radial bands and stripes compliments their uniqueness.
Image: The colorful beauty of Coquina clam shells
 
 
Image: Colorful Coquina Clams
   
Image: Shores of southeastern N.C. where Coquinas may be found
Image: L.Baker, Wrightsville Beach, NC, 8/2/2011
Image: Unique patterns of Coquinas

Taxonomy
 

Kingdom

Animalia

Phylum

Mollusca

Class

Bivalvia

Order

Veneroida

Family

Donacidae

Genus

Donax

Species

D. variabilis

Common Names: Bean clams, Butterfly Shell Clams, Wedge Shells, and Pompano
 
                                 Image: Donax variabilis shell, opened on sand
Image: L. Baker, Kure Beach, NC, 6/17/2012

Distribution
 
The coquina clam ranges from Virginia, down the Atlantic coast, through the Gulf of Mexico and into Texas (South Carolina D.N.R.). These clams live in the tidal zones of sandy beaches. They rely on the swash tides to move them up and down the beach and for food and nutrients as well. Coquinas are most active suring the warm summer months when movement is easy and the warm Gulf Stream provides an abundance of energy.
Image: Where Coquina clam may be found

 

        
Above Images: Coquina clam habitat in the tidal zone on sandy beaches. As Coquina clams dig in the sand, tiny holes are visible marking their location in the sand. Here, they wait for the next wave to feed.
Photos: L. Baker, Kure Beach, NC, 6/15/2012
 
Evolution
 
Since the Coquina lives in a variable environment and dynamic beach community, the clam has made several evolutionary adaptations to acclimatize to this region including filter feeding and mastering the concept of thixotropy.
  • Filter Feeding - A form of food procurement in which food particles or small organisms are randomly strained from water. In bivalves such as the clam, the gills, larger than necessary for respiration, also function to strain suspended material out of the water. Hairlike filaments called cilia produce a water current over the gills, and other cilia move the trapped food particles along the gill face and into food grooves (Encyclopedia Britannica).                                 
 
Image: The anatomy of a bivalve.                                                             Image: The incurrent siphon (right) and excurrent siphon (left)
 
Animals that filter-feed are not only getting the nutrients they need to survive, they are also helping to clean up the environment. The term "filter-feeding" is just that - using a filter to acquire plankton for food. Coquinas use the incurrent siphon to allow water and nutrients to enter the clam's system. After food has been taken up, the clam send the used water out of the excurrent siphon which drains the clam. In addition to feeding, the filtration process filters out the water that enters the Coquina's gills. As water passes through the clam, pollutants are also filtered before the water is siphoned out back into the sea. Because of this process of pollution dilution via filter-feeding, clams and other bivalves are a very important indicator species to environments. Over time, bivalves can help clean up large amounts of sand simply by filtration.
  • Thixotropy - A beach ecology concept that if water sand is covered with water and is agitated, it mobilizes along the surf zone. A common critter of the surf zone is the variable coquina, Donax variabilis, which burrows in the thixotropic sands by extending its foot into the sand and rapidly disappearing (Loyola University, Thomas).
   

Coquina Clam Movement


Friends and Foes
 
The Coquina clam is central to many food webs in coastal regions. The clam eats several different things that float in the surf and the clam falls prey to many predators that live along the coast.
Prey of Coquina include:
  • phytoplankton, bacteria, other organic suspended material in the surf
Predators of Coquina include:
  • Ghost crabs, Moon Snail, Olive Snail, Sanpipers and Piping Plovers (shorebirds), Pompano and Whiting (fish), and Humans
         Cragen letem - Donax vittatus (+ Euspira catena) (Gwylan) Tags: sea beach wales necklace cymru shell snail seashell seashore cymraeg mor gwynedd mr traeth predation catena donax arfordir cragen malwen bywydgwyllt naturewales donaxvittatus euspira morol cragendorch cragenletem wedgeshell naturcymru cragenfor cragenfr wildlifewales seashorewales bywydgwylltcymru bydnatur
Images: Predators of Coquina include the Red Knot, Moon Snail, and Whimbrel
 
Environmental Impacts
Coquina clams are threatened by several human influences including coastal development, implementation of hard structures such as rock jetties, and excess pollution. These little clams rely on the moving sands in the surf to live and survive and as long as people try to adjust how the sand naturally moves with jetties and groins, the lives of these creatures is threatened as well. Coquinas are a very good indicator species of healthy environments. When together, they form fairly large colonies up to hundreds of individuals. In large groups, they can move many volumes of sand and water and filter out polluted particles. They may be small, but these bivalves are a very important and essential part to all beach communities.
Conservation 
One issues that Coquina face is the process of beach renourishment. Many beaches have to be renourished because the natural supply of sand from rivers and other sources from the mountains have been impeded by dams and reservoirs. If eroding beaches are constantly being renourished, coquina clams, as well as other sand dwelling invertebrates, such as mole crabs, could become buried under the extra sediment. Coquina clams can usually recover their populations if a beach has not been renourished in a year or two.

Unique Geology
Coquina clams are the individual creatures that live and die in beach environments. Their lifespans are fairly short ranging from several months up to a year, with some individuals lasting a little longer. High predation and difficult living conditions keep Coquina populations in check, seasonally. Once deceased, the clam shells are opened, discarded and left to erosion processes along the coast. Coquina shells were abundant enough throughout history to have formed the framework for a loosely cemented limestone common in Florida and along the eastern coast of the U.S., coquina limestone (burrow, T. Martin). The Coquina Outcrop, located at Fort Fisher N.C., is one of these limestone areas that serves as its own unique habitat for many tidal sea creatures.

Images: The above series of photos show the progression of how Coquina limestone is formed over time from one individual coquina shell to eventually an entire tidal ecosystem on the beach. Note the size of the coquina shell for detail.
Images: L. Baker, Kure Beach, NC, 6/17/2012
 
 
References
 
Bivalve. Encyclopedia Britannica, 2010. Web. 18 June 2012. http://www.britannica.com.sg/zoology/bivalve-357370.html.
 
"Coquina Clams." Scienceray. N.p., 16 Aug. 2009. Web. 17 June 2012. http://scienceray.com/biology/marine-biology/coquina-clams/.
 
DeLancey, Larry. "Coquina Clam." Donax Variabilis. South Carolina Department of Natural Resources, n.d. Web. 17 June 2012. http://www.dnr.sc.gov/cwcs/pdf/Coquinaclam.pdf.
 
Martin, T. "Life Traces of the Georgia Coast." Burrow. N.p., 4 Mar. 2012. Web. 18 June 2012. http://www.georgialifetraces.com/tag/burrow/.
 
Mitchell, Patricia B. "Coquina: Cathcing "Butterflies"" Coquina (Donax Variabilis): Candidate for Shell Soup. N.p., n.d. Web. 18 June 2012. http://www.mitchellspublications.com/guides/shells/articles/0021/.
 
"Thixotropy: An Important Concept in Beach Ecology." Center for Environmental Communication. Loyola University New Orleans, 18 Aug. 2009. Web. 18 June 2012. http://www.loyno.edu/lucec/natural-history-writings/thixotropy-important-concept-beach-ecology.
  
 Page Developed By: Lindsey Baker
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