Corbicula fluminea (O. F. Müller, 1774)
The common North American bivalve, currently known as Corbicula fluminea is a freshwater clam with a relatively thick, massive shell, compared to most other freshwater bivalves. The shell is triangular to ovate, with a distinct umbo, raised above the dorsal shell margin. The shell hinge has three distinct cardinal teeth, and two lateral teeth. The shell has many concentric ridges, about ~1.5 per mm of shell height. The ratio of shell length to shell height is ~1.06, and shell length to shell width is ~ 1.47. The shell interior is glossy white to pale gray with light blue, rose, or purple highlights. The periostracum in healthy, growing shells is yellow-green, but in old, eroded shells is dark brown and white. While North American populations are hermaphroditic and show little genetic variability, phenotypic variation in shell color and shape is considerable (McMahon 1991, in Thorp and Covich 1991; Lippson and Lippson 1997; Coan et al. 2000). The clams mature at sizes as small as 6.6 mm, but occasionally reach 60 mm in length (McMahon 1983).
The systematics of the genus Corbicula is uncertain. Morton (1986) lumped ~200 synonyms, and then divided Asian populations into two species C. fluminea a freshwater, hermaphroditic clam, and C. fluminalis a dioecious (2 sexes) brackish water clam. He classified North American populations, which are all hermaphroditic, as C. fluminea, the name provisionally used here. Subsequent genetic analysis indicates that most North and South America populations belong to a single genetic lineage, lineage A, with a whitish shell interior (Lee et al. 2005). However, three other morphologically and genetically different forms were also found, lineage B, from the Southwestern US (purple shell interior), lineage C (purple interior, finer shell sculpture), from La Plata, Argentina, and a 4th lineage from Igazu Falls, Brazil (Lee et al. 2005). Forms A and B were found co-occurring in the Illinois River, together with a new form D, possibly an androgenetic hybrid of forms A and B, formed by male sperm fertilizing hermaphroditic clams, with only male chromosomes being retained (Tiemann et al. 2017). Worldwide, some of the names synonymized by Morton (1986) have been revived. Corbicula lineage A from North America appears to be genetically most similar to C. leana from Japan, while lineage B resembles populations of C. fluminea from China and Korea (Siripattrawan et al. 2000; Hedtke et al. 2008). Pigneur et al. (2011) found three morphotypes of Corbicula in Europe: lineage R, sharing haplotypes with the American lineage A and the Japanese C. leana; lineage S, genetically resembling the South American C, but differing in morphology; and a form Rlc, resembling the American lineage B.
The species status of these varying invasive lineages is unclear. All are hermaphroditic, and share androgenesis, in which the offspring retain only male chromosomes, resulting in clonal populations (Hedtke et al. 2008; Tiemann et al. 2017). It is possible that the names will be revised in the future, in which case the most widespread form (American lineage A; European lineage R) will probably be known as C. leana, as suggested by Hedtke et al. (2008). However, for purposes of continuity, we will use the name C. fluminea for lineage A until the change is formally made.
The brooded and pediveliger larvae of C. fluminea are described and illustrated by Nichols and Black (1994) and compared with larvae of the Zebra and Quagga Mussels (Dreissena polymorpha and D. bugensis).
Corbicula fluminalis, described from the Euphrates River, is one of two living Corbicula species recognized by Morton (1986), who synonymized it with C. japonica, which inhabits freshwaters and estuaries, up to 30 PSU salinity. Corbicula fluminalis has been reported from European waters together with C. fluminea. However, the taxonomy of these forms is complex, and not completely resolved, despite recent genetic studies (Pigneur et al. 2011). Corbicula fluminalis has not been found in North America (McMahon, in Thorp and Covich 1991; Lee et al. 2005).
This Japanese diecious brackish-water species was synonymized with the hermaphroditic Middle Eastern C. fluminalis (Morton 1986), but is now regarded as a distinct species. It has not been introduced to North America, to our knowledge.