Adacna minima is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has a suboval or subrhomboidal, thin and semitranslucent whitish shell, up to in length, with flattened and sometimes almost invisible ribs. The species is native to the Caspian Sea where it lives at depths from 0 to 35 m (0 to 115 ft) in its central and southern parts and rarely occurs in the eastern portion of the Northern Caspian. It was previously distributed throughout the entire Aral Sea where it became locally extinct after 1977 as the lake was shrinking and became more saline due to human activity.
Description.
"Adacna minima" has a suboval or subrhomboidal, thin, compressed, semitranslucent whitish shell, with flattened and sometimes almost invisible radial ribs and a very shallow, wide pallial sinus. The shell length is up to . The valves are moderately gaping posteriorly. The hinge has a relatively well developed cardinal tooth in the left valve and a reduced cardinal tooth in the right valve.
Distribution and ecology.
"Adacna minima" is native to the Caspian Sea where it lives at depths from 0 to 35 m (0 to 115 ft) in its central and southern parts and rarely occurs in the eastern portion of the Northern Caspian.
Additionally, the species used to be distributed throughout the entire Aral Sea where it lived at depths from 0 to over 10–12 m (0 to 33–39 ft). By 1965, as the lake began to shrink and become more saline due to human activity, most specimens of "A. minima" had deformed shells. It was last seen in the Aral Sea in 1977 and has not been recorded there in the following years.
Fossil record.
"Adacna minima" is abundant in the Late Pleistocene deposits of the lower Volga River corresponding to the Hyrcanian stage of the Caspian Sea which began 107,000 ± 7,000 years ago. It also occurs in the Holocene deposits of the Aral Sea.
Taxonomy.
The species was first described from the Northern Caspian Sea and the Aral Sea by a Russian zoologist Alexey Alexandrovich Ostroumov in 1907. The type specimens have not been traced.
Zhadin (1952) did not list "A. minima" as a distinct species, but used its name for a variety of "Adacna vitrea" ("A. vitrea" var. "minima") and indicated that it lives along the northern shore of the Caspian Sea and in the Aral Sea. Later authors have treated "A. vitrea" and "A. minima" as distinct species both of which inhabit the Caspian Sea and were previously present in the Aral Sea. The Northern Caspian form of "A. vitrea" from desalinated areas near the Volga and the Ural rivers is now known under the name "A. vitrea glabra".
Logvinenko and Starobogatov (1967) described the Caspian population of "A. minima" as the subspecies "A. minima ostroumovi", with the holotype being collected near the Ogurchinskiy Island. Later, Starobogatov (1974) recognized two subspecies of "A. minima" in the Aral Sea: "A. minima minima" and the newly described "A. minima sidorovi". The former occurred at depths of over , while the latter lived in the coastal zone at depths of up to and differs by more visible ribs (the lectotype was found on the western coast of the Aral Sea near the Kara-Kibir well). Andreeva and Andreev (2003) regarded "A. minima sidorovi" as a distinct species (as "Hypanis sidorovi") and believed that it could be distinguished from "A. minima" and "A. vitrea" by the shape of the ligament. The type specimens of "A. minima ostroumovi" and "A. minima sidorovi" are kept in the Zoological Institute of the Russian Academy of Sciences. Wesselingh et al. (2019) questioned the validity of these subspecies and synonymized them with "A. minima", while Vinarski et al. (2024) listed them as valid. Molecular studies of these taxa would be difficult as the species is likely extinct in the Aral Sea.