Freshwater Mussels of Florida

Family Mactridae – Surfclams

Additional Bivalves in Inland Waters 408 Family Mactridae – Surfclams Nine genera and 10 species of the family Mactridae occur in the United States (Turgeon et al. 1998). They are primarily inhabitants of estuarine and marine environments, but Rangia cuneata is occasionally found in freshwater. Rangia cuneata (Sowerby 1831) Atlantic Rangia Description Length to 85 mm; shell thick, triangular to oval; umbo broad, inflated, extending well above shell margin, curved anteriorly; posterior ridge often well developed; posterior slope usually steep; periostracum thin, yellowish brown to grayish brown; hinge plate moderately wide; anterior and posterior lateral teeth well developed; hinge ligament fully internal, positioned in chondrophore; pallial line with moderately deep pallial sinus; nacre white to bluish white (Andrews 1971, 1981; Abbott 1974) (Figure 12.5). Figure 12.5. Rangia cuneata – Length 50 mm, UF 360321. Choctawhatchee River at junction with Sister River, Walton County, Florida, March 2000. Illustration by Susan Trammell. Range Rangia cuneata occurs on the Atlantic Coast from New York to Palm Beach County , Florida, and on the Gulf Coast from Mexico to Lee County, Florida (Woodburn 1962; Gunter and Hall 1965; Abbott 1974). Remarks In Florida Rangia cuneata occurs in tidally influenced reaches of larger streams but has been found in the St. Johns River upstream of Lake Harney, Seminole and Volusia counties. The species inhabits soft mud and sand sediments in low salinity (0–18 ppt), shallow, brackish water, and freshwater habitats. Unlike Polymesoda caroliniana, R. cuneata may also be found among roots in vegetated habitats. In tidal estuaries of Additional Bivalves in Inland Waters 409 southwest Florida, R. cuneata occurred at salinities of 1–16 ppt and was most abundant at 4–7 ppt (Montagna et al. 2008). It was not collected from the Atlantic Coast north of Georgia prior to the 1950s. Its occurrence in the vicinity of Chesapeake Bay has been attributed to accidental introduction during oyster transplants (Pfitzenmeyer and Drobeck 1964). Family Sphaeriidae – Fingernailclams, Peaclams, and Pillclams The family Sphaeriidae includes small (length usually less than 25 mm), thinshelled bivalves that are widely distributed across North America and occur in a variety of natural and man-made habitats, including wetlands, creeks, rivers, ponds, lakes, roadside ditches, canals, and reservoirs. In the United States and Canada, the family is represented by four genera (Eupera, Musculium, Pisidium, and Sphaerium) and 39 species (Mackie 2007). All genera and 12 species have been reported from Florida (Table 12.1). Species descriptions presented herein are focused upon external shell characters useful for differentiation of taxa. Detailed sphaeriid accounts, including species descriptions , life history, and ecological information, are presented by Herrington (1962) and Mackie (2007). A comprehensive morphologically based treatment of Sphaeriidae was presented by Mackie (2007). However, the work was completed several years before funding was procured for its publication and does not include some of the more recent works addressing taxonomic and phylogenetic relationships. In 2003 most taxa of the Sphaeriidae were included in a molecular genetic analysis using mitochondrial and nuclear genes (Lee and O’Foighil 2003). Results of the molecular analysis did not recover the currently recognized genera Musculium and Sphaerium as monophyletic clades. The conflict between the morphological and phylogenetic studies has resulted in taxonomic uncertainty (Lee 2004). Generic names herein follow the traditional taxonomy using shell morphology and soft anatomy (Mackie 2007), but it should be pointed out that these are likely to change in the future based on phylogenetic studies. The Sphaeriidae are characterized by small size, yellowish to brown periostracum, two cardinal teeth in the left valve (only one in Eupera cubensis), one cardinal tooth in the right valve, and nonserrated lateral teeth located anterior and posterior of the cardinal teeth. The ligament holding the two valves together is short and not well developed. The pallial line and muscle scars are indistinct. Three of the four sphaeriid genera can usually be distinguished by the position of the umbo (Figure 12.6). The umbo is anterior in Eupera, posterior in Pisidium, and central in both Sphaerium and Musculium. Distinguishing Musculium and Sphaerium can be difficult, but Musculium has a very narrow hinge plate beneath the umbo and in Sphaerium it is two or more times wider than the thickened end of the cardinal tooth on the right valve. Taxonomic keys useful for identifying species of Sphaeriidae are included in Herrington (1962), Burch (1972), Heard (1979a), and Mackie (2007). ...