Invasion History

First Non-native North American Tidal Record: 1952
First Non-native West Coast Tidal Record:
First Non-native East/Gulf Coast Tidal Record: 1952

General Invasion History:

Cyrenoida floridana was first described from Florida by Dall in 1889, and was believed to range from Brunswick, Georgia to the Everglades and around to Charlotte Harbor, Florida on the Gulf coast (Dall 1896). Subsequently, it was found in Bay St. Louis, Mississippi (Bishop and Hackney 1987), and in Laguna Chica and Grande in Veracruz, Mexico (Garcia-Cubas et al. 1992). In 1952-53, C. floridana was collected in Chesapeake and Delaware Bays (Morrison 1954), a northward range extension of ~900 km. Kat (1982), studied the reproductive cycles of C. floridana and concluded that this clam had been introduced to Chesapeake and Delaware Bays, on the basis of its apparently maladaptive reproductive cycle, late discovery, and disjunct distribution.

North American Invasion History:

Invasion History on the East Coast:

In 1952-1954, J. P. E. Morrison collected C. floridana from locations in upper and lower Chesapeake Bay, from Deale, Maryland (MD) and the Chester River south to the Rappahannock River, and the Eastern Shore of Virginia (VA) (Morrison 1954; Wass 1972). It was also found in the Delaware Bay, at Bombay Hook National Wildlife Refuge, and Cumberland County, New Jersey (NJ) (Morrison 1954; US National Museum of Natural History collections). In 1975 (this clam was found in Canary Creek Marsh, near Lewes, Delaware, adjacent to a canal connecting Rehoboth and lower Delaware Bays (Leathem et al. 1974). Subsequently, it was found in marshes of the Mullica River, entering Great Bay, NJ in 1998 (Angradi et al. 2001) and in marshes in Chincoteague Bay (MD-VA) in 2000 (Prezant et al. 2002).

The northern populations of C. floridana appeared to be disjunct. However, C. floridana was collected subsequently near Beaufort, North Carolina (NC) in 1966 (United States National Museum of Natural History Collections). Because of their small size, and their presence among decaying marsh vegetation, Morrison (1954) mentioned that 'it is easy to understand why they are so easily overlooked and why they are so uncommon in shell collections'. Kat (1982), studying a Delaware population, concluded that maladaptive features of this species reproductive cycle (starting an unsuccessful period of gamogenesis in the fall, as well as high winter mortality) suggested that this clam was a recent immigrant to the mid-Atlantic region. Kat (1982) suggested that 'This relatively recent range expansion seems coincident with, and probably can be attributed to, the construction of the Intracoastal Waterway. This series of canals probably also provides avenues of dispersal for juveniles to peripheral areas subsequent to local extinctions caused by periods of severe climate.' We consider the northern (New Jersey-Virginia) populations to be likely introductions. Possible vectors include dry ballast of sailing ships (some were active in coastal trade into the 1930s), dredges used in building the coastal canals, or rafted marsh grass or mud on the decks of barges or coastal ships. Ballast water or fouling is unlikely, since this clam lacks planktonic larvae, or byssus threads to attach to a hull.


Cyrenoida floridana is a small (13-19 mm) bivalve. It has a rounded, oval shell, with the length about 1.1-1.2 X the height, and a low, but distinct umbo. The shell is thin, very delicate, whitish, and translucent, with a thin easily removed periostracum. The surface is smooth, or has fine growth lines. The interior margin is smooth or polished. The pallial line is indistinct and often broken. The hinge has two cardinal teeth, but no laterals. The ligament is external, short, and brownish. This clam burrows in the peat and decaying vegetation of the upper intertidal zone of marshes (Dall 1896; Abbott 1974).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Bivalvia
Subclass:   Heterodonta
Order:   Veneroida
Superfamily:   Lucinoidea
Family:   Cyrenoididae
Genus:   Cyrenoida
Species:   floridana


Cyrenella floridana (Dall, 1896)

Potentially Misidentified Species

Polymesoda caroliniana
Carolina Marsh Clam- This clam is also found in brackish marsh habitats, but is larger, with a thick, strong, brownish shell.



Cyrenoida floridana is a hermaphroditic, viviparous bivalve. It broods its eggs and larvae, releasing shelled, crawling juveniles (Kat 1982). In Mississippi, C. floridana bred year-round, but had peak recruitment in summer (Bishop and Hackney 1987). In Delaware, breeding success was low, with one generation being produced in summer, and a second brood being aborted in the fall. This and high winter mortality suggests that this clam was poorly adapted to the mid-Atlantic region (Kat 1982).

In Canary Creek Marsh (Lewes, Deleware), Cyrenoida floridana is most abundant in the portion of the upper intertidal zone, dominated by the grass Distichlis spicata (Salt Grass), but is also found among Spartina patens (Salt Marsh Hay), dwarf Spartina alterniflora (Smooth Cordgrass), Juncus roemerianus (Black Rush) and Phragmites australis (Leathem et al. 1974; Bishop and Hackney 1987; Angradi et al. 2001). These plants inhabit zones of the marsh above the daily mean high-tide line (Lippson and Lippson 1997). Cyrenoida floridana inhabits the sediments about 1 cm deep, with decaying plant matter around and between the roots of the plants. However, the sediments inhabited by C. floridana have high water content (Leathem et al. 1974; Angradi et al. 2001). Cyrenoida floridana is frequent in brackish portions of estuaries, and lives in habitats where it is likely to be exposed to fresh water during heavy rainfall (Morrison 1954; Bishop and Hackney 1987; Angradi et al. 2001). This clam is a deposit feeder, feeding on the rich organic detritus accumulating between marsh plants (Leathem et al. 1974).


Detritus; Benthic microalgae

Trophic Status:

Deposit Feeder



General HabitatSalt-brackish marshNone
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeMid IntertidalNone
Tidal RangeHigh IntertidalNone
Vertical HabitatEndobenthicNone

Tolerances and Life History Parameters

Minimum Salinity (‰)0Likely minimum salinity during rainy periods in brackish marshes.
Maximum Salinity (‰)38Typical Gulf of Mexico salinity
Minimum Length (mm)13Minimum adult size, Kat 1982
Maximum Length (mm)19Kat 1982
Broad Temperature RangeNoneWarm temperate-Subtropical
Broad Salinity RangeNoneOligohaline-Euhaline

General Impacts

There are no reported impacts for C. floridana in its native or introduced range. 

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 1896 Native Estab
CAR-VII Cape Hatteras to Mid-East Florida 1896 Native Estab
NA-ET3 Cape Cod to Cape Hatteras 1952 Def Estab
M130 Chesapeake Bay 1952 Def Estab
M090 Delaware Bay 1954 Def Estab
M080 New Jersey Inland Bays 1998 Def Estab
M120 Chincoteague Bay 2000 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Abbott, R. Tucker (1974) American Seashells, Van Nostrand Reinhold, New York. Pp. <missing location>

Angradi, T.R.; Hagan, S. M.; Able, K. W. (2001) Vegetation type and the intertidal macroinvertebrate fauna of the brackish marsh: Phragmites vs. Spartina., Wetlands 21(1): 75-92

Bishop, T. Dale; Hackney, Courtney T. (1987) A comparative study of the mollusc communities of two oligohaline intertidal marshes: Spatial and temporal distribution of abundance and biomass, Estuaries 10(2): 141-152

Dall, W. H. (1896) On the American species of Cyrenoidea, Nautilus 10: 51-52

Dall, William Healey (1889) A preliminary catalogue of the shell-bearing marine mollusks and brachiopods of the south-eastern coast of the United States, Bulletin of the United States National Museum 37: 1-221

García-Cubas, Antonio; Reguero, Martha; Elizarrarás, Rafael (1992) [Mollusks from Chica-Grande lagoon system, Veracruz, Mexico: systematics and ecology], Anales Del Instituto de Ciencias Del Mar y Limnologia 19(1): 71-101

Kat, Pieter W. (1982) Reproduction in a peripheral population of Cyrenoida floridana (Bivalvia: Cyrenoididae), Malacologia 23(1): 47-54

Leathem, Wayne; Kinner, Pete; Maurer, Don (1974) Northern range extension of the Florida marsh clam (Cyrenoida floridana) (Superfamily Cyrenoidacea), Nautilus 90(3): 93-95

Lee, Harry 2001-2015 Harry Lee's Florida Mollusca Checklists. <missing URL>

Lippson, Alice Jane; Lippson, Robert L. (1997) <missing title>, Johns Hopkins University Press, Baltimore. Pp. <missing location>

Morrison, J.P.E. (1954) Some zoogeographic problems among brackish water mollusks, The American Malacological Union Annual Report 20: 7-10

Morrison, J.P.E. (1970) 9. Brackish water mollusks, Malacologia 10(1): 55

Posey, Martin H.; Alphin, Troy D.; Meyer, David L.; Johnson, John M. (2003) Benthic communities of common reed Phragmites australis and marsh cordgrass Spartina alterniflora marshes in Chesapeake Bay., Marine Ecology Progress Series 261: 51-61

Prezant, Robert; Counts, Clement L.; Chapman, Eric J. (2002) Mollusca of Assateague Island, Maryland and Virginia: additions to the fauna, range extensions, and gigantism., Veliger 45: 337-355

U.S. National Museum of Natural History 2002-2021 Invertebrate Zoology Collections Database.

Wass, Melvin L. (1972) A Checklist of the Biota of Lower Chesapeake Bay, None <missing volume>: <missing location>