Adacna fragilis

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 {{cvt|20–33|mm}}. 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.{{cite book |last1=Scarlato |first1=O. A. |last2=Starobogatov |first2=Y. I. |date=1972 |chapter=Klass dvustvorchatyye mollyuski — Bivalvia |trans-chapter=Class bivalve molluscs – Bivalvia |title=Opredelitel' fauny Chernogo i Azovskogo morey|trans-title=Guide to the fauna of the Black Sea and the Sea of Azov |editor-last1=Mordukhai-Boltovskoy |editor-first1=F. D. |volume=3 |location=Kiev |publisher=Naukova Dumka |page=216 |language=ru}}{{sfn|ter Poorten|2024|p=152}}

Fully extended siphons of this species are significantly longer than its shell. The foot is weakly developed.{{sfn|ter Poorten|2024|p=152}}

Adacna vitrea has much weaker developed ribs and its coloration is often partially pinkish or purple.{{sfn|ter Poorten|2024|p=152}}

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.{{sfn|ter Poorten|2024|p=152}}

File:Taganrog Bay Semibalki.jpg is one of the only habitats where Adacna fragilis still occurs in large numbers.]]

Adacna fragilis is endemic to the Black Sea Basin.{{sfn|ter Poorten|2024|p=152}} It lives in shallow inshore habitats in estuaries and limans and burrows deep into soft sediments.{{sfn|ter Poorten|2024|p=152}}{{cite book|last1=Starobogatov|first1=Y. I.|last2=Bogatov|first2=V. V.|last3=Prozorova|first3=L. A.|last4=Saenko|first4=E. M.|date=2004|chapter=Molluscs|title=Key to Freshwater Invertebrates of Russia and Adjacent Lands|editor-last1=Tsalolikhin|editor-first1=S. J.|volume=6. Molluscs, Polychaetes, Nemerteans|location=Saint-Petersburg|publisher=Nauka|page=49|isbn=5-02-026204-8|language=ru}} The species is native to lakes Kagul, Yalpuh and Katlabukh,{{cite book|last=Markovsky|first=Y. M.|date=1955|title=Fauna bespozvonochnykh nizov'ev rek Ukrainy, usloviya yeyo sushchestvovaniya i puti ispol'zovaniya. Chast' 3. Vodoemy Kilijskoy delty Dunaya|trans-title=The fauna of invertebrates of the lower river streams of Ukraine, its life conditions and ways of utilization. Part 3. The basin of the Chilia Delta of the Danube|location=Kyiv|publisher=AN SSSR|page=57|language=ru}} the Razelm-Sinoe Lake complex (Danube Delta),{{cite journal |last1=Gogaladze |first1=A. |last2=Son |first2=M. O. |last3=Lattuada |first3=M. |last4=Anistratenko |first4=V. V. |last5=Syomin |first5=V. L. |last6=Pavel |first6=A. B. |last7=Popa |first7=O. P. |last8=Popa |first8=L. O. |last9=ter Poorten |first9=J. J. |last10=Biesmeijer |first10=J. C. |last11=Raes |first11=N. |last12=Wilke |first12=T. |last13=Sands |first13=A. F. |last14=Trichkova |first14=T. |last15=Hubenov |first15=Z. K. |last16=Vinarski |first16=M. V. |last17=Anistratenko |first17=O. Y. |last18=Alexenko |first18=T. L. |last19=Wesselingh |first19=F. P. |date=2021 |title=Decline of unique Pontocaspian biodiversity in the Black Sea Basin: A review |journal=Ecology and Evolution |volume=11 |issue=19 |pages=12923–12947 |url=https://onlinelibrary.wiley.com/doi/10.1002/ece3.8022 |doi=10.1002/ece3.8022 |doi-access=free|pmid=34646444 |pmc=8495785 |bibcode=2021EcoEv..1112923G |hdl=1887/3238855 |hdl-access=free }} the Kuchurgan,{{cite book|last=Markovsky|first=Y. M.|date=1953|title=Fauna bespozvonochnykh nizov'ev rek Ukrainy, usloviya yeyo sushchestvovaniya i puti ispol'zovaniya. Chast' 1. Vodoemy delty Dnestra i Dnestrovskiy liman|trans-title=Fauna of invertebrates of the lower river streams of Ukraine, its life conditions and ways of utilization. Part 1. The basin of the Dniester Delta and the Dniester Liman|location=Kyiv|publisher=AN SSSR|page=29|language=ru}} 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{{cite journal |last1=Begun |first1=T. |last2=Teaca |first2=A. |last3=Muresan|first3=M. |last4=Pavel|first4=A. B. |date=2020 |title=Current state of the mollusc populations in the Razim-Sinoe Lagoon system |url=https://animalsciencejournal.usamv.ro/pdf/2020/issue_1/Art81.pdf |journal=Scientific Papers. Series D. Animal Science |volume=63 |number=1 |pages=553–561 |url-status=live |archive-url=https://web.archive.org/web/20220617013009/https://animalsciencejournal.usamv.ro/pdf/2020/issue_1/Art81.pdf |archive-date=17 June 2022}} are unconfirmed{{sfn|ter Poorten|2024|p=152}} and later records in 2007–2008{{cite journal |last1=Popa |first1=O. P. |last2=Sárkány-Kiss |first2=A. |last3=Kelemen |first3=B. |last4=Iorgu |first4=E. I. |last5=Murariu |first5=D. |last6=Popa |first6=L. O. |date=2009 |title=Contributions to the knowledge of the present Limnocardiinae fauna (Mollusca: Bivalvia) from Romania |journal=Travaux du Muséum National d'Histoire Naturelle "Grigore Antipa" |volume=52 |pages=7–15}} are misidentifications of M. colorata.{{sfn|ter Poorten|2024|p=142}} 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.{{cite book |last=Filipenko |first=S. |date=2023 |title=Zoobentos Dubossarskogo i Kuchurganskogo vodokhranilishch |trans-title=Zoobenthos of Dubasari and Kuchurgan reservoirs |url=https://eco-tiras.org/books/Filipenco-book-Benthos-Dubasari%20and%20Cuciurgan%202023.pdf |location=Chișinău |publisher=self-published |pages=50–55 |isbn=978-9975-3610-1-9 |language=ru |url-status=live |archive-url=https://web.archive.org/web/20250108174620/https://eco-tiras.org/books/Filipenco-book-Benthos-Dubasari%20and%20Cuciurgan%202023.pdf |archive-date=8 January 2025}} 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.{{cite journal |last1=Khalaim |first1=A. A. |last2=Son |first2=M. O. |date=2016 |title=Biological and ecological characteristics of Hypanis laeviuscula fragilis (Milachevitch, 1908) (Mollusca, Cardiidae) from Sasyk Reservoir |url=https://dspace.uzhnu.edu.ua/jspui/bitstream/lib/23407/1/%d0%91%d0%98%d0%9e%d0%9b%d0%9e%d0%93%d0%9e-%d0%ad%d0%9a%d0%9e%d0%9b%d0%9e%d0%93%d0%98%d0%a7%d0%95%d0%a1%d0%9a%d0%90%d0%af%20%d0%a5%d0%90%d0%a0%d0%90%d0%9a%d0%a2%d0%95%d0%a0%d0%98%d0%a1%d0%a2%d0%98%d0%9a%d0%90.pdf |journal=Uzhgorod University Scientific Bulletin: Series: Biology |volume=41 |pages=59–63 |language=ru |url-status=live |archive-url=https://web.archive.org/web/20250428223723/https://dspace.uzhnu.edu.ua/jspui/bitstream/lib/23407/1/%D0%91%D0%98%D0%9E%D0%9B%D0%9E%D0%93%D0%9E-%D0%AD%D0%9A%D0%9E%D0%9B%D0%9E%D0%93%D0%98%D0%A7%D0%95%D0%A1%D0%9A%D0%90%D0%AF%20%D0%A5%D0%90%D0%A0%D0%90%D0%9A%D0%A2%D0%95%D0%A0%D0%98%D0%A1%D0%A2%D0%98%D0%9A%D0%90.pdf |archive-date=28 April 2025}} The species was also reported from the Kremenchuk Reservoir of the Dnieper River.{{cite journal |last=Munasypova-Motyash |first=I. A. |date=2006 |title=O sovremennoy faune dvustvorchatykh mollyuskov podsemeistva Limnocardiinae (Bivalvia, Cardiidae) Severo-Zapadnogo Prichernomorya |trans-title=On the recent fauna of subfamily Limnocardiinae (Bivalvia, Cardiidae) in North-Western shore of Black Sea |url=https://www.v-zool.kiev.ua/pdfs/2006/1/03_Munasipova.pdf |journal=Vestnik Zoologii |volume=40 |number=1 |pages=41–48 |language=ru |url-status=live |archive-url=https://web.archive.org/web/20240703222305/https://www.v-zool.kiev.ua/pdfs/2006/1/03_Munasipova.pdf |archive-date=3 July 2024}}

Although the conservation status of A. fragilis has not been assessed by the International Union for Conservation of Nature,{{cite book|last1=Vinarski|first1=M. V.|last2=Kantor|first2=Y. I.|date=2016|title=Analytical catalogue of fresh and brackish water molluscs of Russia and adjacent countries|location=Moscow|publisher=A. N. Severtsov Institute of Ecology and Evolution of Russian Academy of Sciences|page=64|isbn=978-5-9908840-7-6}} the species has been included in the Red Data Book of Ukraine under the Vulnerable category (as Hypanis laeviuscula){{cite book|last=Munasypova-Motyash|first=I. A.|date=2009|chapter=Hipanis leviuskula Hypanis laeviuscula|title=Chervona knyha Ukrayiny. Tvarynnyy svit|trans-title=Red Data Book of Ukraine. Animals|editor-last=Akimov|editor-first=I. A.|edition=Third|location=Kyiv|publisher=Globalconsulting|page=306|isbn=978-966-97059-0-7|language=uk}} and in the Red Book of Moldova under the Critically Endangered category (as H. laeviuscula fragilis).{{cite book|last1=Toderas|first1=I.|last2=Munjiu|first2=O.|last3=Subernetki|first3=I.|date=2015|chapter=Hypanis laeviuscula fragilis (Milasch., 1908) – Hipanis-neted-fragil|title=The Red Book of the Republic of Moldova|url=https://zoology.md/sites/default/files/inline-files/Cartea%20Rosie_Partea%20II%20Animale_1.pdf|edition=Third|publisher=Ştiinţa|page=467|isbn=978-9975-67-998-5|url-status=live |archive-url=https://web.archive.org/web/20220614000221/https://zoology.md/sites/default/files/inline-files/Cartea%20Rosie_Partea%20II%20Animale_1.pdf |archive-date=14 June 2022}}

Adacna fragilis was first described from the Dniester Liman and the Katlabukh Lake by a Russian geologist and paleontologist Konstantin Osipovich Milaschewitsch in 1908.{{cite journal|last=Milaschewitsch |first=K. O.|date=1908 |title=Mollyuski, sobrannyye vo vremya ekskursii S.A. Zernova na minonostse № 264 na r. Dunay s 28 iyunya po 3 iyulya 1907 g. |trans-title=Molluscs collected during the excursion of S.A. Zernov on the torpedo-boat No. 264 on the Danube River from June 28 to July 3, 1907 |journal=Bulletin de l'Académie Impériale des Sciences de St.-Pétersbourg |volume=2|issue=12|series=6 |pages=991–996|url=https://www.mathnet.ru/links/91aa4ef1285f597c46827c681c8ad930/im7445.pdf|language=ru}} The type specimens have not been traced.

Zhadin (1952) treated this species as a variety of A. laeviuscula,{{cite book |last=Zhadin |first=V. I. | date=1952 |title=Mollyuski presnykh i solonovatykh vod SSSR |trans-title=Molluscs of Fresh and Brackish Waters of the USSR |url=https://www.zin.ru/publications/fauna_keys/keys_46_zhadin_1952.pdf |series=Opredeliteli po faune SSSR, izdavayemyye Zoologicheskim institutom AN |volume=46 |location=Moscow–Leningrad |publisher=The USSR Academy of Sciences Press |page=354 |language=ru |url-status=live |archive-url=https://web.archive.org/web/20241202230923/https://www.zin.ru/publications/fauna_keys/keys_46_zhadin_1952.pdf |archive-date=2 December 2024}} 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).{{cite journal|last1=Wesselingh|first1=F. P.|last2=Neubauer|first2=T. A.|last3=Anistratenko|first3=V. V.|last4=Vinarski|first4=M.|last5=Yanina|first5=T.|last6=ter Poorten|first6=J. J.|last7=Kijashko|first7=P.|last8=Albrecht|first8=C.|last9=Anistratenko|first9=O. Y.|last10=D'Hont|first10=A.|last11=Frolov|first11=P.|last12=Gándara|first12=A. M.|last13=Gittenberger|first13=A.|last14=Gogaladze|first14=A.|last15=Karpinsky|first15=M.|last16=Lattuada|first16=M.|last17=Popa|first17=L.|last18=Sands|first18=A. F.|last19=van de Velde|first19=S.|last20=Vandendorpe|first20=J.|last21=Wilke|first21=T.|date=2019|title=Mollusc species from the Pontocaspian region – an expert opinion list|url=https://zookeys.pensoft.net/article/31365|journal=ZooKeys|issue=827|pages=31–124|doi=10.3897/zookeys.827.31365|doi-access=free|pmid=31114425 |pmc=6472301 |bibcode=2019ZooK..827...31W}} 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.

{{Reflist}}

• {{cite book|last=ter Poorten|first=J. J.|date=2024|title=A taxonomic iconography of living Cardiidae|location=Harxheim|publisher=ConchBooks|isbn=978-3-948603-48-9}}

{{Taxonbar|from1=Q105973138}}

Category:Cardiidae

Category:Molluscs of the Black Sea

Category:Fauna of the Sea of Azov

Category:Bivalves described in 1908