Theodoxus is a genus of nerites, small water snails with an operculum, some of which live in freshwater, and some in both freshwater and brackish water, aquatic gastropod mollusks in the family Neritidae, the nerites.
The distribution of the genus Theodoxus includes Europe and northern Africa and also extends east to southern Iran. No other species within Neritidae have sympatrical distribution with Theodoxus. The distribution of the genus Theodoxus is an exception within Neritidae, because Neritidae live primarily in the southern hemisphere. Species within Theodoxus are the only Neritidae snails, that live in temperate climate.
Bunje & Lindberg (2007) presented the first phylogenetic hypothesis of the clade Theodoxus. The evolution of the genus Theodoxus was affected by separating of Mediterranean sea and Paratethys sea in Miocene.
Description
The shell in this genus is semiovular with a flat apertural plain. There is no umbilicus. The columella and inner whorls are dissolved.
Species in the genus Theodoxus are highly variable in size, in color pattern of the periostracum, in details of the operculum and in the radula, and all these factors can make identification to species level very challenging.
Ecology
These animals are live on stones, and often also under stones, in up to 5-6 m depth or deeper, feeding on algal covers. Theodoxus needs rough surfaces in order to be able to digest its food, so a stony substrate is necessary. Green algae are not consumed; Theodoxus has no cellulases. These snails lay egg capsules containing 30-70 eggs each, usually on the shells of other Theodoxus animals; only one juvenile grows, the other eggs serve as food.
Species
There were no systematic review of the genus Theodoxus as of 2007. Bunje (2004) noted at least 34 extant taxa at species level in the genus Theodoxus. IUCN Red List (2015) provided conservation status for 23 species of Theodoxus.
Species within the genus Theodoxus include:
Subgenus Brusinaella Andrusov, 1912
† Theodoxus petasatus (Seninski, 1905)
Subgenus Calvertia Bourguignat, 1880
Subgenus Neritonyx Andrusov, 1912
† Theodoxus unguiculatus (Seninski, 1905)
Subgenus Ninniopsis Tomlin, 1930
† Theodoxus colchicus (Andrusov, 1912)
Subgenus Neritaea Roth, 1855
Theodoxus anatolicus (Récluz, 1841)
† Theodoxus groyanus (Férussac, 1823)
Theodoxus jordani (Sowerby)
† Theodoxus micans (Gaudry & Fischer in Gaudry, 1867)
