# Didacna barbotdemarnii

Didacna barbotdemarnii is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a cream-colored broadly oval shell, up to 30 mm (1.2 in) in length, with a weakly protruding umbo and flattened ribs, which are often highlighted only by color. The species is endemic to the Caspian Sea, where it lives in the Southern and Middle Caspian sub-basins and in the southern part of the Northern Caspian at depths from 0 to 30–40 m (0 to 98–131 ft). It is named after Russian geologist Nikolai Pavlovich Barbot-de-Marny.

## Description

Didacna barbotdemarnii has a rather thick and moderately convex broadly oval shell, with a weakly protruding umbo, 19–26 flattened radial ribs, which are often highlighted only by color, and a posterior ridge, which can be smooth or is often marked by a sharp rib. The shell length is up to 30 mm (1.2 in), the height is up to 20 mm (0.79 in), and the convexity is up to 13 mm (0.51 in). The external coloration is cream, with thin yellowish green periostracum. The interior is white, with a brown-red stain on the posterior margin.

### Differences from other species

The juveniles of Didacna longipes differ from those of D. barbotdemarnii by having thicker shells, with more pronounced growth lines, more ribs, a somewhat narrower umbo, less sharp posterior ridge, wider anterior margin and a less elongated posterior margin.
The shells of juvenile Didacna trigonoides are less elongated, more convex and have less ribs.
The extinct Didacna ebersini has a less elongated and slightly more convex shell, with a lower apical angle of the umbo.

## Distribution and ecology

Didacna barbotdemarnii is endemic to the Caspian Sea. It lives in the Southern and Middle Caspian sub-basins and in the southern part of the Northern Caspian at depths from 0 to 30–40 m (0 to 98–131 ft). The species prefers sandy bottoms and often co-occurs with D. longipes.

## Fossil record

Didacna barbotdemarnii occurs in the Holocene (Neocaspian) deposits of the Caspian Sea. Nevesskaja (2007) hypothesized that it descended from the Late Pleistocene D. ebersini.

## Taxonomy

The species was first described as Cardium Barbot-de-Marnii by Oscar Andreevich Grimm in 1877. He named it after Russian geologist Nikolai Pavlovich Barbot-de-Marny. The type locality is in the Caspian Sea off Kazakhstan (44°17'N, 50°22'E) at a depth of 13 m (43 ft). One of Grimm's specimens from this locality is stored in the Zoological Institute of the Russian Academy of Sciences and has been designated as the lectotype of the species by Logvinenko and Starobogatov (1967).
Several authors misspelled the name of this species as Didacna barbotdemarnyi.
Nevesskaja (2007) believed that D. longipes could not be clearly identified from the existing descriptions and described the new species, Didacna carinata. It was described as being similar to D. barbotdemarnii and differing by a more pronounced posterior ridge, more protruding umbo, more convex shell and less developed cardinal teeth. Grimm's drawings of D. longipes were used for the new species, but D. longipes was not explicitly synonymized with it. The holotype of D. carinata is stored in the Paleontological Institute of the Russian Academy of Sciences. Kijashko (2013) reconfirmed the validity of D. longipes, while the taxonomic status of D. carinata is uncertain and it has been treated as a tentative synonym of both D. longipes and D. barbotdemarnii.