Theodoxus danubialis (Pfeiffer, 1828)
Donau and its northern tributaries from the upper part, its southern tributaries in the former Yugoslavia (Sava river), east to the delta in the Black Sea; also in Dalmatia, along the Cetina river, where it is found together with Th. fluviatilis; also present in N. Italy (northern half of the Po drainage basin – rivers in the Friuli-Venezia Giulia: Piave, Tagliamento, lower Isonzo). Micrograzer. On and under stones in streams, rivers and lakes. Original taxon: Nerita danubialis.
 
Notice the faded colour of the operculum. Donau river bank, Visegrád, Szentendrei járás, N. Hungary. 7-8,3mm.
« The Donau-Schwimmschnecke: shell ovate-elliptic, arched above, flat below, strong, smooth, somewhat shiny, and adorned with black or dark brown acute-angled tortuous lines, sometimes uniting. The spire has 2-3 whorls: the last one very large, bulbous, the others very small and scarcely protruding as a blunt tip. The aperture is crescent-shaped, white, its outer margin sharp, its columellar margin smooth, advanced towards the mouth and narrowing. » – C. Pfeiffer: Naturgeschichte deutscher Land- und Süsswasser- Mollusken vol. III, Weimar 1828, p.48.

Pattern variation in specimens from the lower Donau area, in Tulcea Judeţ, E. Romania. Left: Dunărea river bank, Tulcea. 9,5mm. Right: Lacul Babadag, south of Tulcea. 9,2mm.
In danubialis the apophysis is less protruding than in fluviatilis but, in compensation, there is a conspicuous hook-shaped pseudoapophysis on the base (in fluviatilis, this extension is, at most, vestigial). – Upper Zlatna Panega river, Obshtina Yablanitsa, Oblast Lovetch, N. Bulgaria. 8,85-8,9mm.
V. Gashtarov legit (BG). Благодаря Виктор!
Dobra river. Under the old austro-hungarian bridge, Novigrad na Dobri, Karlovac Comitat, north Croatia. 9,4-10,2mm.
The Dobra river at Novigrad. Original picture provided by R. Crnković (HR) – (CC BY-NC-SA).
Same spot. 10,1mm.
Notice the well developed pseudoapophysis.
On Kolpa (Kupa) river bank, Kupa, Primorje-Gorski Kotar Comitat, NW. Croatia. 10,2-11,5mm.
A densely patterned population, almost all black, has been found in the Lajta river, north of Fertõ Lake, Gyõr-Moson-Sopron Comitat, NW. Hungary. 7,4-8,5mm. These specimens do not seem to belong to prevostianus but, as it can be extrapolated from the relationship between prevostianus and danubialis, it is not unwise to imagine the existence of intergrades in the morphology. Original pictures provided by S. Pavlidis (GR) – (CC BY-NC-SA).
A difference between danubialis and fluviatilis:
in danubialis, the blotches are darker than the background (specimen D) while they are paler in fluviatilis (specimens F). In addition, danubialis is more globose. Original image provided by R. Crnković (HR) – (CC BY-NC-SA).
A possible southern population inhabits the shore of Trichonida Lake, Aetolia-Acarnania, W. Greece. Here, a specimen of 7,6mm. The pattern and the operculum are those of danubialis, as well as the rounded shape.

Same spot. 9-9,25mm.
Falls of Agia Sofía, upon the northern shore of Trichonida Lake, Aetolia-Acarnania, W. Greece. 6,8mm. There are noticeable differences between these specimens from the river and those living along the lake, downstream.
Same spot. 4,2mm. Apophysis of the operculum:
left = from outside; center = inside; right = from columella.
Young specimens from a karst spring between Missolónghi and Antírrio, SW. side of Événos drainage basin, Aetolia-Acarnania. 4,6-4,8mm. The usual zigzag pattern of danubialis becomes here a kind of dark mesh with pale holes.

— go to part 2 or back to Neritininae —