Adacna laeviuscula

Adacna laeviuscula is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has an irregularly oval, thin and semitranslucent white shell, up to 40–50 mm (1.6–2.0 in) in length, with a narrower anterior end, and flattened ribs. The species is endemic to the Caspian Sea, where it lives at depths from 0 to 30–60 m (0 to 98–197 ft) and burrows into soft sediments.

Adacna laeviuscula
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Cardiida
Family:	Cardiidae
Genus:	Adacna
Species:	A. laeviuscula
Binomial name
Adacna laeviuscula (Eichwald, 1829)
Synonyms
Pholadomya caspica Agassiz, 1842

Description

Adacna laeviuscula has an irregularly oval, thin, compressed, semitranslucent shell, with a narrower anterior margin, an anteriorly displaced umbo, 20–29 flattened radial ribs, which are wider on the posterior half, and a wide and deep pallial sinus, which extends up to half of the shell length or is slightly deeper. The shell length is up to 40–50 mm (1.6–2.0 in). The valves are strongly gaping at the anterior and posterior margins. The coloration is white, with very thin light yellowish periostracum. The hinge may have no teeth or a reduced cardinal tooth can be present in the right valve.^[1][2][3][4]

The siphons of this species are twice as long as its shell.^[5]

Differences from similar species

Smaller specimens of A. laeviuscula from the Northern Caspian Sea differ from Adacna vitrea glabra by more developed ribs.^[2]

Adacna fragilis, which inhabits lakes of the Danube Delta, estuaries of the Black Sea and the Taganrog Bay of the Sea of Azov, is a smaller sized species, with a more symmetrical shell, the pallial sinus of which does not extend up to half of the shell length. It also has slightly more ribs which are usually more pronounced on the middle part of the shell.^[4][6]

Monodacna semipellucida has a broadly oval shell, with a shallower pallial sinus and reduced cardinal teeth in both valves.^[7][8]

Distribution and ecology

Adacna laeviuscula is endemic to the Caspian Sea.^[9] It lives at a salinity of at least 4‰^[2] at depths from 0 to 30 m (0 to 98 ft), rarely down to 60 m (197 ft).^[10] The species burrows into soft sediments.^[11] Individuals from the Northern Caspian Sea in waters with salinity levels of 4–9‰ are smaller than those living in the central and southern parts of the sea at salinities of 12–14‰.^[5]

Despite earlier studies, Starobogatov et al. (2004) have reported that A. laeviuscula occurs at depths between 30 and 60 m (98 and 197 ft).^[11] However, fresh shells of this species with paired valves often wash up on beaches which indicates that it lives in shallow foreshore habitats.^[12]

Fossil record

Fossilized shells of Adacna laeviuscula occur in deposits of the Caspian Sea Basin since its Khazarian stage,^[13] which began 125,000–80,000 years ago during the Pleistocene.^[14] It has also been found in the Late Pleistocene of the Manych Depression where a strait connecting the Caspian and the Azov-Black Sea basins has repeatedly formed in the past.^[15] Nevesskaja et al. (2001) hypothesized that the species could be a descendant of the extinct Monodacna praelaeviuscula from deposits of the Apsheronian Basin,^[16] which existed on the territory of the modern Caspian Sea from 1,8 or 2,1 million to 880,000–750,000 years ago.^[14]

Taxonomy

Drawings of Glycymeris laeviuscula from Eichwald's publication (1829). This specimen is now recognized as the lectotype of the species.

The species was first described as Glycymeris laeviuscula by Karl Eichwald in 1829.^[17] In 1838 he transferred it into the newly described genus Adacna.^[18] Subsequently, Vest (1875) designated A. laeviuscula as the type species of its genus.^[19][20]

Eichwald initially reported Glycymeris laeviuscula from "the southern border of the Caspian Sea in bight of Astrabad" (Gorgan, Iran). However, one of the two shells collected by him in the Bay of Baku (Azerbaijan) was found to be identical to the original drawings of the species, apart from the fact that they depict a left valve while the actual specimen is a right valve. This specimen is now considered to be the lectotype of A. laeviuscula, and according to Article 76.2 of the International Code of Zoological Nomenclature, the Bay of Baku becomes the type locality of the species. The lectotype as well as the second specimen from this locality (the paralectotype) are both stored in the Zoological Institute of the Russian Academy of Sciences.^[9][21]

Jean Louis Rodolphe Agassiz, who was possibly unaware of Eichwald's works, redescribed the species as Pholadomya caspica in 1842.^[22] This name is now regarded as a synonym of A. laeviuscula.^[21]

Adacna fragilis from the Black Sea Basin has sometimes been treated as a variety^[5] or a subspecies of A. laeviuscula.^[2][10] It was once again recognized as a distinct species by Starobogatov et al. (2004),^[11] but some later authors believed that it could be a synonym of A. laeviuscula and its taxonomic status was listed as uncertain by Wesselingh et al. (2019).^[12] More detailed studies have shown that A. fragilis is a distinct species from the Caspian A. laeviuscula as they differ by shell characteristics and salinity preferences.^[6]