Didacna

Didacna is a genus of brackish-water bivalve molluscs of the cockle family (Cardiidae) characterized by rounded-triangular, oval or trapezoidal shells with flattened ribs. It includes about 90 accepted species, 9 of which are extant and endemic to the Caspian Sea. The genus presumably originated in the Black Sea and the Caspian Sea basins during the Pleistocene around 900,000 years ago. However, several fossil species from older deposits outside of those basins have also sometimes been classified within this genus. These cockles are sedentary filter feeders that bury into the sediment and feed on suspended detritus, microscopic algae and mollusc larvae. They are a food source for some fish species.

== Description == Members of the genus Didacna have solid rounded-triangular, oval or trapezoidal shells, with flattened radial ribs on the exterior and grooves corresponding to the ribs on the interior. A distinct posterior ridge is present and is sometimes marked by a stronger developed rib. The pallial line is continuous and there is no pallial sinus. The hinge of each valve has two cardinal teeth, one of which may be reduced. The lateral teeth are poorly developed or absent.