Didacna trigonoides is a brackish-water bivalve mollusc of the family Cardiidae, the cockles. It has an oval-triangular, rather thick, whitish or cream shell, up to in length, with flattened tan or reddish brown 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 an oval-triangular, rather thick, convex shell, with a strongly protruding umbo, 20–30 flattened tan or reddish brown radial ribs and a posterior keel, which is distinct and often double ridged in juveniles, but becomes weaker with age. The shell length is up to . The external coloration is whitish or cream, with thin greyish brown to orange-brown periostracum. The interior is white, with a reddish brown stain on the posterior margin. The right valve has a poorly developed posterior lateral tooth.
Differences from similar species.
"Didacna barbotdemarnii" differs from juvenile "D. trigonoides" by a more elongated and less convex shell, with slightly more ribs.
"Didacna pyramidata" has a less inflated shell, with a lower umbo, more ribs and a stronger lateral tooth.
"Didacna praetrigonoides" is a possibly extinct species mostly known from the Late Pleistocene deposits of the Caspian Sea, although its shells sometimes wash up on beaches. It differs from "D. trigonoides" by a larger number of ribs and a less pronounced triangular shape.
The fossil species "Didacna subpyramidata" has a less elongated shell and a lower umbo.
Distribution.
"Didacna trigonoides" is endemic to the Caspian Sea. It occurs in all parts of the lake and is the only species of its genus that is widespread in the Northern Caspian.
Ecology.
"Didacna trigonoides" 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 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 bivalve are always pointed upwards. It feeds on suspended algae, but larger specimens can also consume mollusc larvae.
Fossil record.
"Didacna trigonoides" is widespread in the Holocene deposits of the Caspian Sea. Nevesskaja (2007) hypothesized that the species 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". 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. Wesselingh et al. (2019) synonymized it with "D. trigonoides".