# Didacna trigonoides

Didacna trigonoides is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a whitish or cream oval-triangular shell, up to 40–65 mm (1.6–2.6 in) in length, with tan or reddish brown flattened ribs. The species is endemic to the Caspian Sea. It is a sedentary filter feeder that lives at depths from 3 to 60 m (9.8–196.9 ft) and feeds on suspended algae and mollusc larvae.

## Description

Didacna trigonoides has a rather thick and convex oval-triangular shell, with a strongly protruding umbo and 20–30 tan or reddish brown flattened radial ribs. The shell length is up to 40–65 mm (1.6–2.6 in), the height is up to 40 mm (1.6 in), and the convexity is up to 30–33 mm (1.2–1.3 in). The juveniles have a distinct and often double ridged posterior keel which becomes weaker with age. The coloration is whitish or cream, with thin greyish brown to orange-brown periostracum on the exterior and a posterior reddish brown stain on the interior. The right valve has a poorly developed posterior lateral tooth.

### Differences from other species

Didacna pyramidata has a less convex shell, with a less protruding umbo, more ribs and a stronger lateral tooth.
Shell of the extinct Didacna praetrigonoides has more ribs and a less pronounced triangular shape.
The extinct Didacna subpyramidata has a less elongated shell with a less protruding umbo.

## Distribution

D. trigonoides is endemic to the Caspian Sea. It occurs in all parts of the sea and is the only species of its genus that is widespread in the Northern Caspian.

## Ecology

The species is a sedentary filter feeder that lives on sandy, shelly and mixed hard substrates at depths from 3 to 60 m (9.8–196.9 ft). It favors waters with salinity of 7–12‰, can tolerate salinity levels of 4–7‰ and 12–15‰, while levels of 2,3‰ and over 15‰ are lethal to it.
The adults of D. trigonoides burrow halfway into the sediment while the juveniles fully bury themselves, leaving a small part of their shell on the surface. In this position the short and immobile siphons of this cockle are always pointed upwards. It feeds on suspended algae but larger specimens can also consume mollusc larvae.

## Fossil record

D. trigonoides is widespread in the Holocene deposits of the Caspian Sea. Nevesskaja (2007) hypothesized that it descended from the Late Pleistocene D. praetrigonoides.

## Taxonomy

The species was first described as Cardium trigonoides by Peter Simon Pallas in 1771 who called it "the most numerous shell of the Caspian Sea" but found no living specimens. In 1838 Karl Eichwald transferred it to the newly described genus Didacna and considered it an extinct species. Subsequently, D. trigonoides has been designated as the type species of its genus by Stoliczka (1870).
The type locality of D. trigonoides as indicated by Pallas is "Maris Caspii" (Caspian Sea). The original type specimens are lost. One specimen from the Paleontological Institute of the Russian Academy of Sciences, collected by Nicolai Ivanovich Andrusov on the beach of Chechen Island, was designated as the neotype of this species by Nevesskaja (2007).
Svitoch (1967) described two fossil subspecies of D. trigonoides: D. trigonoides chasarica from the Middle Pleistocene and D. trigonoides chvalynica from the Late Pleistocene. Yanina (2005) synonymized the latter subspecies with the extinct Didacna ebersini while Nevesskaja (2007) treated it as a tentative synonym of D. ebersini and D. praetrigonoides praetrigonoides.

### Synonyms

In 1791 Johann Friedrich Gmelin described the species Cardium trilaterum from the Caspian Sea. It is now considered to be a synonym of D. trigonoides.
Didacna trigonoides tuzetae is a subspecies described by Tadjalli-Pour (1977) from Iran. J. J. ter Poorten (2024) regarded it as a questionable synonym of D. trigonoides.