Didacna parallella
Didacna parallella is a brackish-water bivalve mollusc of the family Cardiidae, the cockles. It has an oval, rounded-trapezoidal or trapezoidal, thin, cream or tan shell, up to in length, with an off-centered umbo and flattened ribs. The species is endemic to the Caspian Sea. It lives in the Southern Caspian and along the western coast of the Middle Caspian at depths between 50 and 85 m.
Description
Didacna parallella has an oval, rounded-trapezoidal or trapezoidal, thin, moderately convex shell, with an anteriorly displaced umbo, 29–42 flattened radial ribs and a posterior ridge, which is marked by a thickened rib. The shell length is up to. The external coloration is cream or tan. The interior is cream, with a reddish brown stain on the posterior margin. The hinge consists of two cardinal teeth in the right valve and one cardinal tooth in the left valve.Distribution and ecology
Didacna parallella is endemic to the Caspian Sea. It lives in the Southern Caspian and along the western coast of the Middle Caspian at depths between 50 and 85 m. Records from depths of in the southern Caspian Sea off Azerbaijan are uncertain.J. J. ter Poorten noted that D. parallella could be extinct due to the lack of recent records of living specimens. F. P. Wesselingh and co-authors have stated that live records are known at least until 1986 and the species is unlikely to be extinct.
Fossil record
Didacna parallella occurs in the Late Pleistocene and Holocene deposits of the Caspian Sea.Taxonomy
The species was described by Russian and Soviet geologist Vladimir Vladimirovich Bogachev in 1932 from fossil shells found in the Late Pleistocene deposits of Qala in Azerbaijan. The type series is lost. In 2007 Lidiya Aleksandrovna Nevesskaja designated one of the specimens illustrated by Bogachev as the lectotype of the species.In 1969 Boris Mikhailovich Logvinenko and Yaroslav Igorevich Starobogatov treated D. parallella as an extant species. This view has been accepted in later works on the molluscs of the present-day Caspian Sea. Nevesskaja, however, considered it an extinct species that only occurs in the Late Pleistocene beds of the sea.
Some authors have used the incorrect spelling Didacna parallela for this species.
Didacna parallella var. borealis is a small-sized variety described by P. V. Fedorov from fossil material in 1953. A. A. Svitoch listed it as a synonym of D. parallella. G. I. Popov classified it as a subspecies of Didacna subcatillus. Nevesskaja treated the variety as a possible synonym of D. parallella.