Didacna eichwaldi

Didacna eichwaldi has an oval-triangular, thick, convex shell, with a moderately or strongly protruding umbo, 21–34 flattened radial ribs and a distinctly sharp posterior ridge in juveniles, which becomes smoother with age. The shell length is up to 40–55 mm (1.6–2.2 in), the height is up to 30–45 mm (1.2–1.8 in). The coloration is whitish or cream. The hinge consists of two cardinal teeth in the right valve and one cardinal tooth in the left valve. The juveniles sometimes have weakly developed lateral teeth.

Similar species. Didacna baeri has a more rounded oval shell, with a less protruding umbo and a smooth posterior ridge. Several fossil species resemble D. eichwaldi. Didacna nalivkini has a less elongated shell, with a less distinct posterior ridge on the umbo. Didacna eulachia differs by a more elongated and less equilateral shell, with a larger number of ribs. Didacna ovatocrassa has a less protruding umbo and a less distinct posterior ridge. The shell of Didacna subcrassa is less elongated.

Didacna eichwaldi is endemic to the Caspian Sea. It lives in the middle and southern parts of the sea at depths down to 35 m (115 ft) and does not occur in desalinated areas.