Didacna eichwaldi is a brackish-water bivalve mollusc of the family Cardiidae, the cockles. It has an oval-triangular, rather thick, whitish or cream shell, up to in length, with flattened ribs and a distinctly sharp posterior ridge in juvenile individuals, which can be seen on the umbo in adults. The species is endemic to the Caspian Sea. It lives in the middle and southern parts of the lake at depths down to and does not occur in desalinated areas.
Description.
"Didacna eichwaldi" has an oval-triangular, rather thick, convex shell, with a moderately or strongly protruding umbo, 21–34 flattened radial ribs and a distinctly sharp posterior ridge in juvenile individuals, which can be seen on the umbo in adults. The shell length is up to and the height is up to . The coloration is whitish or cream.
Differences from similar species.
"Didacna baeri" has a more rounded oval shell, with a less protruding umbo and a smooth posterior ridge, which is not sharp in juveniles.
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.
Distribution and ecology.
"Didacna eichwaldi" is endemic to the Caspian Sea. It lives in the middle and southern parts of the lake at depths down to and does not occur in desalinated areas.
Fossil record.
"Didacna eichwaldi" is widespread in the Holocene (Neocaspian) deposits on the coasts of the middle and southern parts of the Caspian Sea. The oldest records date to the Late Pleistocene (Upper Khvalynian deposits).
Taxonomy.
The species was first described as "Cardium crassum" by Karl Eichwald in 1829, who never observed living individuals of this bivalve and only found numerous empty shells. In 1838 he transferred it to the newly described genus "Didacna" and in 1841 he published the first illustations of the species, commenting on how it seemingly "died out only a century ago". 
The type locality of the species as indicated by Eichwald is "Caspium mare" (Caspian Sea). The type specimens have not been located.
The original name of this species is a junior homonym of "Cardium crassum" . In 1837 Ivan Andreevich Krynicki introduced a replacement name, "Cardium eichwaldi". Despite this, Eichwald and many subsequent authors referred to the species under the invalid name "Didacna crassa".
Logvinenko and Starobogatov (1969) treated this species as a junior synonym of "D. baeri". This synonymy was followed in catalogues of extant molluscs of Russia and by Kijashko (2013), but was not accepted by some other authors such as Yanina (2005) and Nevesskaja (2007). Wesselingh et al. (2019) and subsequent authors treated these species as distinct due to differences in shell characteristics.
"Didacna pseudoprotracta" is an extinct species described by Livental (1931) from the Middle Pleistocene (Chaudian) deposits of the Chauda Cape (Kerch Peninsula, Crimea) and Guria (Georgia). He noted that the species resembles the extant "Didacna protracta", but is probably more closely related to the extinct "Didacna pseudocrassa". Nevesskaja (1963) regarded "D. pseudoprotracta" as a synonym of "D. crassa", although in her view "D. crassa" was present in the Pleistocene of the Black Sea region and was classified into multiple fossil subspecies (such as "D. crassa guriensis") which are now recognized as distinct species. J. J. ter Poorten (2024) listed "D. pseudoprotracta" as a questionable synonym of "D. eichwaldi".