# Didacna protracta

Didacna protracta is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a whitish rounded-trapezoidal shell, up to 42 mm (1.7 in) in length, with a small umbo and flattened ribs. The species is endemic to the Caspian Sea. It lives in the middle and southern parts of the sea at depths between 25 and 85 m (82–279 ft), rarely deeper.

## Description

Didacna protracta has a rather thick and moderately convex rounded-trapezoidal shell, with a small umbo, 30–40 flattened radial ribs, dense growth lines on the ventral margin and a smooth posterior ridge, which is more distinct near the umbo. The shell length is up to 42 mm (1.7 in). The coloration is whitish, with thin pale yellowish gray periostracum.

### Differences from other species

Didacna profundicola has a smaller shell, up to 15 mm (0.59 in) in length.
The extinct Didacna catillus has a less equilateral and usually more convex shell, with a wider posterior slope and without the characteristic dense growth lines on the ventral margin.

## Distribution and ecology

Didacna protracta is endemic to the Caspian Sea. It lives in the middle and southern parts of the sea at depths between 25 and 85 m (82–279 ft), rarely deeper. The species does not occur in waters with salinity of less than 5‰.

## Fossil record

Didacna protracta is widespread in the Late Pleistocene (Lower Khvalynian) deposits of the northern Caspian Region. It is less common in the Upper Khvalynian deposits and also occurs in the Holocene (Neocaspian) deposits.

## Taxonomy

The species was first described as Adacna protracta by Karl Eichwald in 1841 who found its fossilized shells in the surroundings of the Lake Elton on the Caspian Lowland (the type locality of the species). One of the Eichwald's specimens is stored in the Geological Cabinet of the Saint Petersburg University. The type series is lost. One specimen from the Paleontological Institute of the Russian Academy of Sciences (PIN), collected from the Late Pleistocene (Lower Khvalynian) deposits in the surroundings of the Lake Elton, was designated as the neotype of this species by Nevesskaja (2007).
Earlier in 1829, Eichwald described the fossil species Cardium protractum. Some authors believed that it is identical to A. protracta. However, the type locality of C. protractum, "Hab. calcem Poczaiowensem", corresponds to Staryi Pochaiv, which is situated in Western Ukraine. Therefore, C. protractum and A. protracta are not considered conspecific.
O. A. Grimm (1877) referred to his specimens of D. protracta and D. profundicola as Cardium catillus, a species described by Eichwald from fossil shells under the name Monodacna catillus. N. I. Andrusov (1910) have shown that A. protracta is not conspecific with M. catillus and assigned both species to the genus Didacna.
Logvinenko and Starobogatov (1966) described the section Protodidacna which included D. protracta as the type species as well as D. profundicola. J. J. ter Poorten (2024) listed Protodidacna as a tentative synonym of the genus Didacna.
Eight varieties of D. protracta were recognized by Andrusov (1910). Logvinenko and Starobogatov (1969) merged the varieties submedia, media and grimmi into the subspecies D. protracta submedia which was distinguished from the nominotypical subspecies (D. protracta protracta) by living in the deeper parts of the Caspian Sea and having slightly larger shells, with a straight or slightly concave ventral margin, smoother ribs and more pronounced growth lines. These subspecies are no longer recognized and the differences between them are explained by allometric growth.
Since the species was first described from fossil material, Glazunova (1971) gave the name D. protracta protracta to the Late Pleistocene shells and redescribed "D. protracta protracta" from the Holocene deposits and the modern Caspian Sea as D. protracta novocaspica (while still recognizing D. protracta submedia). The subspecies was distinguished from the Late Pleistocene D. protracta protracta by a less pronounced posterior slope. The type locality of D. protracta novocaspica is the eastern shelf of the Middle Caspian Sea near the Cape Peschanyy (Kazakhstan). The holotype is stored in PIN.
Nevesskaja (2007) recognized two newly described subspecies: the Late Pleistocene (Lower Khvalynian) D. protracta mangyschlakensis and the Holocene D. protracta raricostata. The former subspecies differs by a larger and less equilateral shell, while the latter has a lower apical angle of the umbo and less ribs. The holotypes are stored in PIN. The name D. protracta mangyschlakensis is a homonym of the simultaneously published name Didacna subcrassa mangyschlakensis and D. protracta raricostata is a junior homonym of Didacna raricostata.