# Didacna longipes

Didacna longipes is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a cream-colored oval shell, up to 25 mm (0.98 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. 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 m (0 to 98 ft).

## Description

Didacna longipes has a rather thick, moderately convex and almost equilateral oval shell, with a weakly protruding umbo, 26–33 flattened radial ribs, which are often highlighted only by color, and a distinct smooth or sharp posterior ridge, which often becomes almost invisible near the ventral margin. The shell length is up to 25 mm (0.98 in), the height is up to 23 mm (0.91 in), and the convexity is up to 16 mm (0.63 in). The external coloration is cream and the interior is white.

### Differences from other species

Most forms of Didacna baeri have less equilateral shells, which are also slightly less thick and have a smoother posterior ridge.
The juveniles of Didacna barbotdemarnii differ from those of D. longipes by having less thick shells, with less pronounced growth lines, less ribs, a somewhat wider umbo, sharper posterior ridge, narrower anterior margin and a more elongated posterior margin.

## Distribution and ecology

Didacna longipes 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 m (0 to 98 ft). The species avoids areas with salinity of less than 10‰ and often co-occurs with D. barbotdemarnii.
This cockle first appeared in the Caspian Sea during the Holocene.

## Taxonomy

The species was first described as Cardium longipes by Oscar Andreevich Grimm in 1877. The type locality is in the Caspian Sea off Azerbaijan (40°39'N, 50°16'E) at a depth of 11 m (36 ft). The specimens collected by Grimm from this locality are now stored in the Zoological Institute of the Russian Academy of Sciences (ZIN) and one of them has been designated as the lectotype of D. longipes by Logvinenko and Starobogatov (1967).
The taxonomic status of D. longipes was uncertain in the past. For instance, it has been proposed that the species could be a "deformed form" of D. baeri. Logvinenko and Starobogatov (1967) have confirmed that D. longipes is a distinct and valid species, but also noted that Grimm's specimens from the ZIN differ somewhat from the original drawings of the species.
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.