# Didacna baeri

Didacna baeri is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a whitish or cream-colored broadly-oval or oval-triangular shell, up to 40–50 mm (1.6–2.0 in) in length, with flattened and often brown ribs. The species is endemic to the Caspian Sea, where it lives in the Middle and Southern sub-basins at depths from 0 to 50–60 m (0 to 160–200 ft) and does not occur in desalinated areas. It is named after Baltic German scientist Karl Ernst von Baer.

## Description

Didacna baeri has a rather thick and convex broadly-oval or oval-triangular shell, with a weakly or moderately protruding umbo, 23–35 flattened and often brown radial ribs and a distinct smooth posterior ridge. The shell length is up to 40–50 mm (1.6–2.0 in). The external coloration is whitish or cream, with thin pale yellowish green periostracum. The interior is white, often with a brown stain on the posterior margin.

### Differences from other species

Didacna eichwaldi has a more protruding umbo and its posterior ridge is sharp in juveniles (which can be seen on the umbo in adults).
The shell of Didacna longipes is more equilateral compared to most forms of D. baeri and is also slightly thicker and has a sharper posterior ridge.
Several extinct species are similar to D. baeri. Didacna surachanica usually has a less protruding and wider umbo. Didacna subcatillus differs by a less convex shell with a higher apical angle of the umbo. The shell of Didacna ovatocrassa is, on average, less elongated and less convex.

## Distribution and ecology

Didacna baeri is endemic to the Caspian Sea. It lives in the middle and southern parts of the sea at depths from 0 to 50 m (0 to 160 ft), rarely down to 60 m (200 ft). The species does not occur in desalinated areas.
The amphipod Cardiophilus baeri was first discovered in the mantle cavity of D. baeri.

## Fossil record

Didacna baeri is widespread in the Holocene (Neocaspian) deposits on the coasts of the middle and southern parts of the Caspian Sea. It also rarely occurs in the Late Pleistocene (Upper Khvalynian) deposits. Nevesskaja (2007) hypothesized that the species descended from the Late Pleistocene (Lower Khvalynian) D. subcatillus.

## Taxonomy

The species was first described as Cardium Baeri by Oscar Andreevich Grimm in 1877. He named it after Baltic German scientist Karl Ernst von Baer. The type locality of D. baeri is in the Caspian Sea off Turkmenistan (40°32'N, 52°23'E) at a depth of 37 m (121 ft). Here Grimm saw several hundred living individuals, and one of his specimens, which is now stored in the Zoological Institute of the Russian Academy of Sciences, has been designated as the lectotype of the species by Logvinenko and Starobogatov (1967).
Logvinenko and Starobogatov (1969) synonymized D. eichwaldi with D. baeri. This synonymy is no longer accepted due to morphological differences between the species.
Pravoslavlev (1939) described the variety transmittens from the Pleistocene deposits of the Lake Baskunchak and the Lower Volga near Chyorny Yar, Solyonoye Zaymishche and other places. Svitoch (1967) treated it as a synonym of Didacna subpyramidata. Nevesskaja (2007) listed Pravoslavlev's record of D. baeri in synonymy of Didacna delenda.
Gadzhiev (1968) described shells of D. baeri with a stronger protruding umbo from the Holocene deposits of the Baku Archipelago and Xanlar Island as the variety alata. This name is invalid since it was introduced as variety after 1960. If the name becomes available it would be a junior homonym of Didacna alata.
Didacna alibajramliensis is an extinct species described by Gadzhiev (1966) from Azerbaijan. Nevesskaja (2007) regarded it as a tentative synonym of D. baeri.