Common mud snails (Bithynia tentaculata).
Picture: Eric Walravens, (Source).
The snails referred to here as mud snails (a somewhat ambiguous common name, as it also describes Viviparidae and Hydrobiidae), are freshwater snails of medium size with a characteristic shell calcareous lid (operculum) with concentric growth lines.
Mud snails have a broad foot with a rounded tail tip. The head ends in a characteristic short snout resembling a proboscis with a rounded end. A mud snail's tentacles are very long and the eyes are placed at the outer sides of the tentacles' bases.
Originally, Bithyniidae were distributed over all continents except the Americas, especially in South Asia and tropical Africa. Only Bithynia tentaculata has been introduced in America. In Central Europe there are three species of the genus, of which two are also found on the British Isles, and three or four more are found in South Europe.
Mud snails are a part of the Rissooidea superfamily, they are related to the tiny spring snails (Amnicolidae) and mud snails (Hydrobiidae).
Systematics of Gastropoda: Clade Caenogastropoda: Bithyniidae.
Common Bithynia - Bithynia tentaculata (Linnaeus 1758)
Alternative vernacular (common) names: Mud Bithynia or Faucet Snail.
Description: The common Bithynia has a horn-coloured yellowish shell, which often is covered in a blackish or brownish layer of algae. The shell is ventricose and conical, the whorls only slightly rounded and separated by a clear suture.
Common Bithynia (Bithynia tentaculata) with the characteristic
aperture outline and a concentric operculum.
Picture: © Alexander Mrkvicka, Vienna.
Aperture and shell lid (Operculum) end in an angular corner at the upper side, which is a possibility of distinguishing Bithynia tentaculata and Bithynia leachi, in which the aperture and the operculum are rounded, as well as from Bithynia troschelii or transsilvanica, in which the apertural corner's angle is more obtuse.
The snail's body is coloured black to dark brown and displays roundish golden yellow to orange spots. The head is quite small and is outweighed by the proboscis-like snout. The tentacles of the common Bithynia are very long and pointed. The tentacles bases, where the eyes are placed, are thickened.
Common Bithynia (Bithynia tentaculata) and Eastern Bithynia (Bithynia troschelii resp.
transsilvanica) in comparison. Pictures: Jiří Novák, (biolib.cz).
Bithynia tentaculata feeds on decaying plant parts and detritus, but mainly, it feeds on food particles it filters from the breathing water, just as a river mud snail (Viviparus) does.
Before laying its 20 - 40 singular eggs, the female Bithynia first cleans the surface of stones, mussels or plant parts of algae. Then it arranges the eggs into a band, using its foot.
Dimensions: H: 7 - 13 mm; W: 5 - 9 mm; N: 5 - 6. (Abbreviations).
Habitat and Distribution: The common Bithynia lives in moving and stagnant waters in a depth of up to 45 m, but it is rarely found below 15 m. Bithynias also can survive in temporarily dry water bodies, such as small ponds and canals.
Bithynia tentaculata prefers waters rich in vegetation with muddy ground and a high content of oxygen and calcium carbonate - in Portugal it not even appears in water on granite ground. In streaming water the common Bithynia usually stays on the lee side of stones, where the water is calmer. Where it dies appear, it usually does so in large numbers. The snail also lives between banks of zebra mussels (Dreissena polymorpha). Common Bithynias, favourable environmental conditions prevailing, can grow as old as three years.
Common Bithynia (Bithynia tentaculata) opening its operculum. Pictures: Eric Walravens, (Source).
Picture right: 1a, b: Tentacles; 2: Proboscis; 3: Penis?; 4: Operculum; 5: Foot; 6: Pallial cavity. Inset image: Eye.
Bithynia tentaculata is a very common snail species, and will remain common in a favourable environment. Towards the environmental conditions, on the other hand, Bithynia tentaculata is quite tolerant: It bears with low, as well as high, pH values, sodium and potassium concentrations because of overfertilisation, and even low water oxygen content. Actually, where Bithynias are abundant and more sensitive species lack, Bithynias are taken as an indicator for eutrophic conditions.
Bithynia tentaculata also is quite tolerant towards salinity: In brackish waters it can survive a salt content of 0.12%. On the Ĺland Islands in the Baltic Sea, common Bithynias could be found in brackish water bays with a salinity of up to 0.074% and a pH value between 7.6 and 9.7 (moderately basic).
The common Bithynia's distribution area covers Europe and West Asia as far as western Siberia and Kashmir, but the species dies not live in the north of Scandinavia and in Greece. In North America is has been introduced and is found in the Great Lakes area, in the Hudson River and the Canadian St. Lawrence river. In Switzerland, the common Bithynia lives in suitable waters in altitudes of up to 1400 m MSL.
In the north of Germany, its distribution area changes into that of Leach's Bithynia (Bithynia leachi), in the east into that of the Eastern Bithynia (Bithynia troschelii or Bithynia transsilvanica). Both can be distinguished externally from Bithynia tentaculata by the form of the aperture and the umbilicus (see above). Only the common Bithynia and Leach's Bithynia also are found on the British Isles.