Carolina Marsh Clam
Polymesoda caroliniana (Bosc, 1801)
Family Cyrenidae
Usually found in brasckish water and rare in Southern Florida is the Carolina Marsh Clam.
From my collection (S. Hutchinson Island- January, 2021)
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Carolina Marsh Clam:
A Rather Well-Rounded Clam
By Patricia B. Mitchell.
The Carolina Marsh Clam is broadly ovate to almost circular in shape and may grow to a length of about 2 inches. The fairly thick bivalve shell is inflated, with fine concentric growth lines. A thin shiny brownish or greenish periostracum serves as an “overcoat” for this mollusk. Often the valve halves of Polymesoda Rafinesque, 1820, are somewhat corroded near the umbones (beaks). Underneath the noncalcareous periostracum the shell is dull white. Often the white interior is tinged with pretty purple.
Each of the two valves has three central teeth in the area where the valves join. The right valve has two tiny grooved side teeth on each side; the left valve has one. There is a small, narrow, pointed pallial sinus (a notch at the posterior end of the pallial line where the retracted siphons rest).
Polymesoda caroliniana lives in mud in warm to temperate waters. It may be found in tidal rivers, intertidally, or just below the low tide line. Its habitat (which is expanding) is anywhere from Virginia to northern Florida, and in the Gulf of Mexico to Texas.
The Carolina Marsh Clam was a large part of the diet of the Calusa Indians of southeast North America.
Notes
For more information on the Carolina Marsh Clam see:
R. Tucker Abbott and Percy A. Morris, Shells of the Atlantic & Gulf Coasts & the West Indies.
Harald A. Rehder, National Audubon Society Field Guide to North American Seashells.
Edward E. Ruppert and Richard S. Fox, Seashore Animals of the Southeast.
Classification: Family Corbiculidae; Superfamily Corbiculoidea; Order Veneroida; Subclass Heterodonta.
Scientific nomenclature is subject to change, due to ongoing research. The above classification corresponds to that published by the Conchologists of America, Inc.
Digital formatting is by Jonathan Mitchell.
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Shell of the Week: The Carolina Marsh Clam
Polymesoda caroliniana Bosc, 1801 may grow to 50 mm (about 2 inches.) Its shell is oval, with a sculpture of fine commarginal (“concentric”) ridges. The shell color is brown. It differs from the Southern Marsh Clam, Polymesoda floridana, by having a more rounded shell with truncated posterior end (left end of leftmost views on photo). This is a truly brackish water species, inhabiting the upper reaches of local estuaries (the shell illustrated was collected in the Caloosahatchee River near downtown Fort Myers .)
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Discovery of the Carolina Marsh Clam, Polymesoda caroliniana (Bosc), A Supposed Florida Disjunct Species, in Everglades National Park, Florida
D.C. Tabb
D.R. Moore
Document Type
Article
Abstract
The presence of disjunct species of animals on either side of the Florida peninsula has been reported by a number of authors. The littorinid mollusk, Littorina irrorata Say, which has a range from Massachusetts to the Rio Grande of Texas, except for south Florida, is one such species (Bequaert 1943). The marsh crab, Sesarma cinereum (Bosc), is another example of an animal with a distribution from Virginia to the western Gulf of Campeche except for a break in southern Florida (Rathbun, 1918). Williams (1965) lists 23 species of crustaceans having interrupted distribution at the Florida peninsula. This report on discovery of a breeding population of the Carolina marsh clam, Polymesoda caroliniana (Bosc) in southern Florida supports the contention by Hedgpeth (1953) that at least some, perhaps many, of the disjunct records may be a result of insufficient collecting in south Florida. The Carolina marsh clam has been assumed to be a typical disjunct species since it was described as such by van der Schalie (1933). It was not included in Marine Shells of Southwest Florida by Perry (1940) nor in Florida Marine Shells by Vilas and Vilas (1945). Abbott (1954) apparently knew of no southern Florida material, and recent examination of collections of this species in the E. S. National Museum provided no material south of New Smyrna on the east coast or Fort Myers on the west coast of Florida. Gunter and Hall (1963) found a breeding colony in the St. Lucie River estuary near Fort Pierce, Florida extending the range nearly 275 km farther south along the Florida east coast but gave no details on the size of the population.
The initial discovery of a single valve of the Carolina marsh clam in extreme southern Florida was made by Tabb and Manning (1961) in deltaic muds at the mouth of the East River where it enters Whitewater Bay in Everglades National Park. Since 1962 sufficient discoveries have been made in Everglades National Park to prove the existence of a breeding population occupying two rather different but adjoining habitats over an extensive area of southern coastal marsh.