Corbicula fluminalis
Corbicula fluminalis is a species of freshwater clam, an aquatic bivalve mollusk in the family Cyrenidae. This species is distinct from, but often confused with, the rather similar species "Corbicula fluminea". Even though both species are native to Asia, they are both present as introduced species in the United States and Europe, and they are both commonly known as "Asian clams". Discriminating the two species. Two species are present in introduced populations, "C. fluminea" and "C. fluminalis". However, the two species are often confused. The scientific names themselves are also sometimes confused in the literature (e.g. by being  called "Corbicula fluminata"). Care needs to be taken in order to properly distinguish the two species. The ratio of width and height in "C. fluminea" is on average 1.1. In "C. fluminalis" it is smaller (0.97); still, there is much variation and considerable overlap in shape. Most easily, the two species can be distinguished by the density of the ribs on the shell; "C. fluminea" has 7 to 14 ribs per cm, "C. fluminalis" 13 to 28. This character is already clearly recognizable (albeit only by direct comparison) in very small (5 mm diameter) specimens. In addition, when viewed from  the side (looking at the opening between the shells), "C. fluminalis" is rounder, almost heart-shaped, while "C. fluminea" has a slightly flatter shape, like a teardrop with a notched broad end. Small specimens of "C. fluminalis" are almost spherical, while those of "C. fluminea" are decidedly flattened. All these differences except the rib number are a consequence of "C. fluminalis" having a markedly more swollen, pointed and protruding umbo.