Theodoxus fluviatilis (Linnaeus, 1758)
Known distribution: the western Palearctic and North America.
Grazer, on rocky substrate in brackish and freshwater streams and rivers, in calcium-rich environments, also in semi-marine intertidal: baltic marshes, sheltered estuaries.
Like many freshwater Nerites, Theodoxus fluviatilis often bears eggs capsules on its shell. Each capsule contains about 200 eggs. In a capsule, the first larva to be born eats the others.
Above, specimens collected on stones in shallow water, in the pool at the exsurgence of Font Estramar, Salses-le-Château, Pyrénées-Orientales, S. France. Altitude ≈ sea level. The water of this cave-river is slightly brackish and warm (about 18°C). Sizes: 5,8-7,5mm.
The variant “saulcyi” is small, black, and is often found in some karst springs (Voulismeni emergence in Agios Nikolaos, Crete; Ágios Ioánnis spring at Agio Pnevma, Central Macedonia; Megapotamos Springs, near the church of Agios Nikolaos Kourtaliotis, south of Rethymnon, Crete; karst spring at Ulupınar Köyü, Akşehir, Konya province, central Anatolia…). Here, a dwarf population found between the warm salt spring and the cold freshwater exsurgence, Lamiña Cave, Camou-Cihigue, Pyrénées-Atlantiques, SW. France. Altitude ≈ 270m. Sizes: 3,5-4,25mm.