Adacna fragilis

Adacna fragilis is a brackish-water cockle, a bivalve mollusc of the family Cardiidae. It has an oval, thin, white, pale yellow or brownish shell, up to 20–33 mm (0.79–1.30 in) in length, with irregularly placed ribs. The species is endemic to the Black Sea Basin, where it lives in shallow inshore habitats in estuaries and limans and burrows deep into soft sediments. It is native to some of the lakes of the Danube Delta, the Kuchurgan, Dniester and Dnieper-Bug limans as well as the Taganrog Bay of the Sea of Azov. By the 21st century the species have severely declined in some of these areas due to human activity, becoming locally extinct in the Razelm-Sinoe Lake complex, the Kuchurgan Liman and, possibly, in the Dnieper-Bug Liman. Large populations of A. fragilis still occur in the Taganrog Bay.

Adacna fragilis has an oval, thin, compressed shell, with a slightly anteriorly displaced umbo, about 23–32 irregularly placed radial ribs, which are usually more pronounced on the middle part of the shell, and a deep pallial sinus, which does not reach the vertical midline of the shell. The shell length is up to 20–33 mm (0.79–1.30 in). The valves are slightly gaping at the anterior and posterior margins. The coloration is white, pale yellow or brownish, with very thin yellowish gray-tan periostracum. The hinge has no teeth.

Fully extended siphons of this species are significantly longer than its shell. The foot is weakly developed.

Adacna vitrea has much weaker developed ribs and its coloration is often partially pinkish or purple.

Adacna laeviuscula from the Caspian Sea is a larger species, with a less symmetrical shell, slightly less ribs and a pallial sinus which extends up to half of the shell length.

Adacna fragilis is endemic to the Black Sea Basin. It lives in shallow inshore habitats in estuaries and limans and burrows deep into soft sediments. The species is native to lakes Kagul, Yalpuh and Katlabukh, the Razelm-Sinoe Lake complex (Danube Delta), the Kuchurgan, Dniester and Dnieper-Bug limans as well as the Taganrog Bay of the Sea of Azov.

In the Razelm-Sinoe Lake complex, the Dniester and Dnieper-Bug limans A. fragilis have formed relatively species-poor communities together with the cockles Monodacna colorata and Hypanis plicata. However, by the 21st century these communities have disappeared as populations of A. fragilis and H. plicata have largely declined due to human activity and the latter species possibly became extinct in the Black Sea Basin. Both species used to be common in the Razelm-Sinoe Lake complex but rerouting of the Danube and closing of the Black Sea inlets in the second half of the 20th century caused the disappearance of most brackish water molluscs except for M. colorata. Reports of A. fragilis from this area in 2002–2004 are unconfirmed and later records in 2007–2008 are misidentifications of M. colorata. In the Kuchurgan Liman A. fragilis was still recorded in 1966–1970 but disappeared as the liman was turned into a cooling pond in 1981–1984. In the Dniester Liman the species declined due to salinity regime changes caused by human activity. For similar reasons, it became very rare or locally extinct in the Dnieper-Bug Liman. Large populations of A. fragilis still occur in the Taganrog Bay.

A. fragilis (together with M. colorata) became abundant in the Sasyk Lake, which was previously inhabited by marine communities but was transformed into a brackish water reservoir after getting connected to the Danube, via a canal, in 1978. The species was also reported from the Kremenchuk Reservoir of the Dnieper River.

Although the conservation status of A. fragilis has not been assessed by the International Union for Conservation of Nature, the species has been included in the Red Data Book of Ukraine under the Vulnerable category (as Hypanis laeviuscula) and in the Red Book of Moldova under the Critically Endangered category (as H. laeviuscula fragilis).

Adacna fragilis was first described from the Dniester Liman and the Katlabukh Lake by a Russian geologist and paleontologist Konstantin Osipovich Milaschewitsch in 1908. The type specimens have not been traced.

Zhadin (1952) treated this species as a variety of A. laeviuscula, while Scarlato and Starobogatov (1972) regarded it as a subspecies (as Hypanis laeviuscula fragilis). Starobogatov et al. (2004) have once again recognized A. fragilis as a distinct species, but 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). Studies by Gogaladze et al. (2021) have shown that A. fragilis is a distinct species from the Caspian A. laeviuscula as they differ by shell characteristics and salinity preferences.

• ter Poorten, J. J. (2024). A taxonomic iconography of living Cardiidae. Harxheim: ConchBooks. ISBN 978-3-948603-48-9.