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Hypanis plicata
Species of brackish-water bivalve From Wikipedia, the free encyclopedia
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Hypanis plicata, the folded lagoon cockle,[1][2] is a brackish-water bivalve mollusc of the family Cardiidae and is the only extant member of the genus Hypanis. It has an elongated oval or rounded-trapezoidal, rather thin and semitranslucent shell, up to 30–45 mm (1.2–1.8 in) in length, with irregularly placed sharp triangular ribs. The species is widely distributed throughout the Caspian Sea where it lives at depths down to 30 m (98 ft), less often down to 50–70 m (160–230 ft). Originally, it was also present in the northwestern Black Sea, where it inhabited the Razelm-Sinoe Lake complex, the Dniester Liman and the Dnieper-Bug Liman, but by the 21st century these populations have largely declined due to human activity and are possibly extinct. H. plicata is a filter feeder that burrows into soft sediments or bores into hard clay, leaving its long, fused siphons on the surface. It is considered edible. The larvae, juveniles and adults are likely eaten by certain fishes.
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Description
Hypanis plicata has an elongated oval or rounded-trapezoidal, rather thin, compressed, semitranslucent shell, with a low, anteriorly displaced umbo, 30–41 irregularly placed sharp triangular radial ribs, which sometimes bear scales on the anterior margin, and a deep pallial sinus, which extends up to 1/3 of the shell length.[4][5][6][7] The shell length is up to 30–45 mm (1.2–1.8 in). The valves are slightly gaping at the anterior and posterior margins. The coloration is white, with thin, semitranslucent light grayish periostracum and sometimes with cream, yellowish or pink bands near the margins.[7] The hinge may have no teeth or a reduced cardinal tooth can be present in the right valve.[5][8]
The siphons of this species are fused together and are longer than its shell when fully extended. The foot is rather short.[7]
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Distribution
Hypanis plicata is widely distributed throughout the Caspian Sea.[4][9] Originally, it was also present in the northwestern Black Sea,[5] where it occurred in the Razelm-Sinoe Lake complex, the Dniester Liman and in the Dnieper-Bug Liman[10] from Nova Odesa to Mykolaiv.[11] By the 21st century, however, the Black Sea populations have largely declined due to human activity.[10] The species used to be common in the Razelm-Sinoe Lake complex but disappeared due to rerouting of the Danube and closing of the Black Sea inlets in the second half of the 20th century.[10] The last time H. plicata was seen alive in this locality was in 2004,[10] while a 2007 record[12] is a misidentification of Monodacna colorata.[13] The Dniester Liman and the Dnieper-Bug Liman populations have declined due to salinity regime changes and are possibly extinct.[10]
In 2005 several empty valves and a single individual with a damaged soft body were found in the lower Don River near the Porechny Island at a depth of 2.5 m (8 ft 2 in). These specimens were introduced from the Caspian Sea, but it is uncertain whether the species is able to survive in freshwater conditions of the Don.[14]
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Ecology
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Perspective
In the Caspian Sea Hypanis plicata lives in areas with salinity of at least 4‰ at depths down to 30 m (98 ft),[4] less often down to 50–70 m (160–230 ft).[5] In the Dnieper-Bug Liman it was restricted to well-oxygenated silty sediments, where it lived at salinities of 0.7–2‰ at depths from 0.5 to 10 m (1 ft 8 in to 32 ft 10 in) and was abundant and most common at depths of 2.0–3.5 m (6 ft 7 in – 11 ft 6 in).[15] The species is a filter feeder that burrows into soft sediments and leaves its long siphons on the surface.[16][17] It can also bore deep into hard clay similarly to bivalves of the family Pholadidae.[5][17][18] Reproduction takes place in the warm period. Fertilization is external and the eggs are released in batches.[17]
H. plicata often lives together with bivalves of the genera Dreissena and Adacna.[17] In the Razelm-Sinoe Lake complex, the Dniester and Dnieper-Bug limans it have formed relatively species-poor communities together with the cockles Monodacna colorata and Adacna fragilis, but these communities have disappeared by the 21st century due to the severe decline of H. plicata and A. fragilis in these habitats.[10]
Fossil record
Hypanis plicata occurs in the Pleistocene deposits of Azerbaijan, Turkmenistan and the eastern Sea of Azov Region (southern Russia) corresponding to the Apsheronian Basin[19][20][21] which existed on the territory of the modern Caspian Sea from 1,8 or 2,1 million to 880,000–750,000 years ago.[22] In the Black Sea Basin the species is known to have occurred since at least the Middle Pleistocene (Chaudian stage of the Black Sea).[22]
Specimens tentatively classified as H. plicata have been found in the Late Pleistocene deposits of the mostly dry Konya Basin in south-eastern Turkey, which was previously occupied by a large lake. In this locality the species is found together with the extinct cockles Monodacna pseudocolorata and Adacna yaninae.[3]
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Uses
The species is considered edible.[2][23] The larvae, juveniles and adults are likely eaten by the Caspian roach (Rutilus caspicus), common bream (Abramis brama) and gobies.[17] Markovsky (1954) has proposed to introduce H. plicata into reservoirs of large rivers as an additional food source for fish, although it is unknown if its larvae can survive in fresh water.[16]
Conservation
While the conservation status of Hypanis plicata has not been assessed by the International Union for Conservation of Nature,[24] the species has been included in the Red Data Book of Ukraine under the Vulnerable category.[25]
Taxonomy
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Perspective
The species was first described as Glycymeris plicata by Karl Eichwald in 1829.[26] The type locality is the southern coast of the Caspian Sea in bight of Astrabad (Gorgan, Iran).[7] The lectotype was selected from a series of possible syntypes which were collected by Eichwald from the type locality and are kept in the Zoological Institute of the Russian Academy of Sciences.[17]
The genus Hypanis was introduced in 1832 by Christian Heinrich Pander in Ménétries' publication and only included Hypanis plicata,[27] making it the type species of the genus by monotypy.[28] Hypanis lacked a diagnosis and only a reference to Eichwald's description of G. plicata was provided.[29]
In 1838 Eichwald included G. plicata in the newly described genus Adacna.[30] Jean Louis Rodolphe Agassiz was possibly unaware of Eichwald's works[31][32] since he redescribed the species as Pholadomya crispa in 1842.[33] Alexander von Middendorff (1849) established the synonymy of Ph. crispa with H. plicata[34] but still included the species in Pholadomya.[31][32] Some authors have also placed it in the genera Cardium and Didacna.[32] Most researchers retained the species in Adacna,[19][20] while often also treating Hypanis as its subgenus.[35][36] Nevesskaja (1963) recognized Hypanis as a distinct genus.[37] Logvinenko and Starobogatov (1969) regarded Adacna and Monodacna as subgenera of Hypanis, with H. plicata being included in the subgenus Hypanis s.str.[4] Currently, Adacna and Monodacna are considered to be distinct genera and H. plicata remains to be the only extant member of its genus.[3][38]
Synonyms
As mentioned above, the name Pholadomya crispa is considered to be a synonym of H. plicata.[28]
Eichwald (1838) classified the Caspian Sea and the Dniester Liman populations of H. plicata as the same species and treated the Dniester population as the variety major.[30] Milaschewitsch (1916) described the Black Sea populations as the species Adacna relicta and indicated that it lives in the Dniester Liman and, possibly, in the Berezan Liman.[39] Zhadin (1952) regarded A. relicta as a variety of H. plicata,[36] while Nevesskaja (1965)[40] as well as Scarlato and Starobogatov (1972) recognized it as the subspecies H. plicata relicta.[5] Starobogatov et al. (2004) have once again considered it to be a distinct species, H. relicta,[41] but this view was not supported by Kijashko (2013), who retained it as a subspecies since the only distinguishing feature of H. plicata relicta indicated by Nevesskaja (1965) was the deeper pallial sinus.[8] Wesselingh et al. (2019) synonymized A. relicta with H. plicata and pointed out that molecular studies would be necessary to clarify the taxonomic status of the Black Sea populations.[38]
Adacna grimmi is a name used by Nicolai Ivanovich Andrusov (1923) to describe a species from the Apsheronian deposits and a modern specimen from the Caspian Sea figured by Oscar Andreevich Grimm (1877).[35][42] This species was believed to differ from H. plicata by more widely spaced ribs.[35] A. grimmi has been recognized by some subsequent authors,[20][36] but was synonymized with H. plicata by Logvinenko and Starobogatov (1969).[4]
Borcea (1926) described the variety Adacna relicta var. dolosmiana from the Lake Golovita (Razelm-Sinoe Lake complex) and distinguished it by a less elongated or almost circular shell.[43] Scarlato and Starobogatov (1972) treated it as a distinct species, H. dolosmiana.[44] Starobogatov et al. (2004) have proposed that it should be moved to a new genus.[41] Wesselingh et al. (2019) synonymized the variety with H. plicata.[38]
Hypanis plicata golbargae is a subspecies described by Tadjalli-Pour (1977) from Iran.[45] J. J. ter Poorten (2024) regarded it as a synonym of H. plicata.[7]
Hypanis regularis is a name introduced by Starobogatov et al. (2004) which was once again used to describe Grimm's figure of the Caspian H. plicata.[41] Since this specimen was already redescribed by Andrusov in 1923 this makes H. regularis a synonym of A. grimmi, which in turn is a synonym of H. plicata.[8]
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