# Didacna eichwaldi

Didacna eichwaldi is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a whitish or cream oval-triangular shell, up to 40–55 mm (1.6–2.2 in) in length, with flattened ribs and a posterior ridge which is sharp on the umbo. The species is endemic to the Caspian Sea. It lives in the middle and southern parts of the sea at depths down to 35 m (110 ft) and does not occur in desalinated areas.

## Description

Didacna eichwaldi has a rather thick and convex oval-triangular shell, with a moderately or strongly protruding umbo and 21–34 flattened radial ribs. The shell length is up to 40–55 mm (1.6–2.2 in) and the height is up to 30–45 mm (1.2–1.8 in). The posterior ridge is sharp in juveniles (which can be seen on the umbo in adults) and becomes smoother with age. The coloration is whitish or cream.

### Differences from other species

Didacna baeri has a more rounded oval shell, with a less protruding umbo and a smooth posterior ridge (not sharp in juveniles).
Several extinct species resemble D. eichwaldi. Didacna nalivkini has a less elongated shell with a less distinct posterior ridge on the umbo. The shell of Didacna eulachia is more elongated, less equilateral and has more ribs. Didacna ovatocrassa has a less protruding umbo and a less distinct posterior ridge. Didacna subcrassa has a less elongated shell.

## Distribution and ecology

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 (110 ft) 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

This cockle was first described as Cardium crassum by Karl Eichwald in 1829 who treated it as an extinct species. In 1838 he transferred it to the newly described genus Didacna. In 1841 he published the first drawings of the species and mentioned 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 Gmelin, 1791. 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) synonymized this species with D. baeri. This synonymy is no longer accepted due to morphological differences between the species.
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 not related to it. Nevesskaja (1963) regarded D. pseudoprotracta as a synonym of D. eichwaldi, as she believed that the latter species is widespread throughout the Pleistocene deposits of the Black Sea region and attributed these records to multiple subspecies which are now recognized as separate species. J. J. ter Poorten (2024) listed D. pseudoprotracta as a questionable synonym of D. eichwaldi.