Nerites (here Theodoxus euxinus) are primordial gastropods.
Pictures: © Alexander Mrkvicka, Vienna (mrkvicka.at).
Family: Neritidae Lamarck 1809
Source: Mollbase on http://www.mollbase.de/list/.
Systematics of Gastropoda: Clade Neritimorpha: Neritidae.
Helicina clappi (Helicinidae), a terrestrial relative of nerites
from Florida. Picture: Robert Pilla (Jacksonville Shell Club).
Nerites are an archaic family of gastropods with a compact thick-walled shell and a calciferous shell lid (operculum). The live in the sea, as well as in fresh water - according to new scientific discoveries, nerites have transited from the sea to the fresh water no less than six times (Holthuis 1995). There are two terrestrial families, the Helicinidae and the Hydrocenidae, with since the Devonian period, 400 million years ago can be proved (see also: Geological Timeline). Even today there are several recent groups living in the transition zone between sea and freshwater (which means in brackish water), as well as in the region between sea and land
Ira Richling: The Land Snail Family Helicinidae.
A nerite's shell is thick-walled and shaped half-round like a boat. The columellar side of the aperture characteristically forms an even plate stretching over the parietal side of the aperture. Nerites, as already mentioned, have an operculum, which usually inside has two appendages (apophyses), the longer one of which is called the rib, the shorter one the peg. Form and colour of the operculum are an important character of identification in the nerites, which can be very variable concerning their colouring.
A view on the animal, the shell lid (operculum) and the charac-
teristic columellar shield.
Nerites, like other prosobranch shells, have separate sexes. The egg capsules are laid on all available hard substrates, preferably, though, on other nerites' shells. Each egg capsule contains 30 to 70 eggs, only one of which will be a juvenile snail after 4 to 8 weeks, depending on the environmental conditions. All the others are for nutrition.
Nerites live on algae they graze off stones and water plants.
Among nerites there are also species kept in aquariums, such as racing snails (for example the zebra racing snail, Neritina natalensis from east and south Africa). Those as well like to leave the water and are well liked as algae grazers.
In Europe there are only fresh water nerites. In Germany there are three species, with one additional species living in thermal springs in Austria. In the meantime, further species have been introduced to the river Danube from the east, such as Theodoxus euxinus from the Ukraine, nowadays found in the "Neue Donau".
Gemeine Kahnschnecke (Theodoxus fluviatilis) auf einer Maler-
muschel (Unio pictorum).
Picture: © Alexander Mrkvicka, Vienna (mrkvicka.at).
The common or river nerite (Theodoxus fluviatilis) is very variable concerning its colouring. Usually it displays a dark red or violet brownish network pattern on lighter ground. The operculum has but a broad rib, no peg.
Theodoxus fluviatilis lives in the intermediary and lower parts of rivers, but also in lakes, where it usually appears in large numbers. A small darker subspecies, Theodoxus fluviatilis littoralis, lives in the brackish waters of the Baltic Sea river mouths. According to Zettler et al. (2004) it is stated, however, that "no significant differences in shell form, operculum or radula justified the separation as a subspecies." (Glöer, Zettler (2005), translated).
The common nerite is distributed in temperate and warm temperate Europe, between southern Sweden and Italy, from Spain to Finland, from Ireland to Russia as far as Asia Minor. It is not found, however, in the Alps and their northern foot hills. Also, the species can be found in the Iranian highlands.
In the north German lowland plain the species is especially common in Mecklenburg-West Pomerania, but elsewhere the populations are in a strong decline: In the rest of Germany Theodoxus fluviatilis lives only in the rivers Rhine, Moselle, Main, Neckar and Franconian Saale.
From Hungary the common nerite has been spreading upstream the river Danube into Slovakia (KOŠEL 2004), and so is also encountered near Vienna.
river nerite (Theodoxus fluviatilis). (In German)
Mollusc of the year 2004: Common river nerite (Theodoxus fluviatilis). (In German)
Schultz, H.; Schultz, O.: (2001): Erstnachweis der Gemeinen Kahnschnecke (Theodoxus fluviatilis) in Österreich. Ann. Nat. Hist. Mus. Wien 103B, 231 - 241. (PDF).
Zettler, M. L.; Frankowski, J.; Bochert, R.; Röhner, M.: (2004): Morphological and ecological features of Theodoxus fluviatilis (Linnaeus, 1758) from Baltic brackish water and German freshwater populations. Journal of Conchology 38: 303-316. (PDF)
Glöer, P; Zettler, M. L. (2005): Kommentierte Artenliste der Süßwassermollusken Deutschlands. Malak. Abh. 3 - 26, p. 14. (PDF).
Widely and narrowly striped specimens of Theodoxus danubia-
lis. Picture: © Alexander Mrkvicka, Vienna (mrkvicka.at).
The Danube nerite is recognized by its shell with transversal dark zic-zac stripes on lighter, usually yellowish ground. In the east it may also be plain black. The operculum is pale yellow, the peg is plate-like inside a groove. Compared to Theodoxus fluviatilis the shell of Theodoxus danubialis is narrower and higher.
In Germany Theodoxus danubialis only is present in the river Danube from Kelheim in Bavaria downstream and in the lower tributaries, as well as in the lakes and rivers of the southern Alps until the Iseo lake in Lombardy. While there are only relictary populations in the German Danube (near Bad Abbach), the Danube nerites is believed disappeared in the Austrian main parts of the river, there are only relict populations in tributaries, such as Perschling and Leitha in Lower Austria.
Danube nerite (Theodoxus danubialis), black specimen with
egg capsules. Pictures: © Alexander Mrkvicka, Vienna.
The Danube nerite prefers clean streaming water rich in oxygen and with stony ground. Where such waters are dammed, the populations usually disappear.
Because Theodoxus danubialis reacts much more sensibly to changes in water quality, than Theodoxus fluviatilis, the species in the meantime is listed on the red lists of Germany, Austria and the Czech Republic as critically endangered (CR), in Slovenia it is classified as vulnerable (VU).
Alexandra Behrendt: Danube nerite (Theodoxus danubialis) (In German).
Striped nerite (Theodoxus transversalis).
Picture: © Alexander Mrkvicka, Vienna (mrkvicka.at).
The striped nerite's shell neither shows a network pattern nor a transverse stripe pattern, but several fine spiral lines which add up to form three longitudinal bands, which are specially well visible in weathered specimens against the lead grey to yellowish ground colour. The operculum has an orange colour and a strong spirally twisted peg.
Similar to Theodoxus danubialis, Theodoxus transversalis likewise prefers clean streaming water rich in oxygen with stony ground. While the species was present in the German Danube as far upstream as Donauwörth and Ingolstadt, today there are only populations left from Kelheim downstream and relictary populations in the Alz river. In Austria the striped nerite could well be extinct already, during a survey near Vienna in 2007 and 2008 the species could not be found anymore.
Nerites and pitch snails in a thermal spring in Lower Austria.
Thermal spring nerite (Theodoxus prevostianus).
Pictures: © Alexander Mrkvicka, Vienna (mrkvicka.at).
In the river Tisza and the downstream parts of the Danube in Romania and Bulgaria Theodoxus transversalis still lives, also in the tributaries where the water still is good enough. W. Fischer et al. (2009) report a population of the species from the Dojran lake in Macedonia.
Theodoxus transversalis species homepage.
Fischer, W.; M. Duda, M; Reischütz, A. (2009): Beiträge zur Molluskenfauna Österreichs XVI. Anmerkungen zur Süßwassermolluskenfauna Wiens. - N. Erste Vorarlb. Mal. Ges. 16: 5 - 19. (PDF).
Fischer, W.; Reischütz, A.; Reischütz, P. L. (2009): Theodoxus transversalis (C. Pfeiffer 1828) im Dorjansee (Mazedonien). N. Erste Vorarlb. Mal. Ges. 16: 345 - 346.
Thermal spring snail (Bythinella pareyssii)
on a nerite less than 2 cm in size itself.
Picture: © Alexander Mrkvicka, Vienna.
The thermal spring nerite, usually is one colour black to dark violet, the shell surface is finely striated. The operculum form resembles that of Theodoxus danubialis, but the central field is dark grey with orange coloured rims.
Theodoxus prevostianus is a very rare relic, probably from one of the interglacial periods, when the climate in what is today Vienna was much warmer. Since then those snail groups have survived only in some thermal springs with a constant warm temperature of about 24 °C in Lower Austria (Bad Vöslau and Bad Fischau) and in Hungary.
On the initiative of HR Dr. Oliver Paget of the Vienna Museum of Natural History, the Hansy stream, of which the thermal spring in Bad Vöslau is a part, was declared a natural treasure in 1979. Apart from Theodoxus prevostianus there are two more species of thermal spring gastropods living there, the thermal spring pitch snail (Esperiana daudebartii) and the tiny thermal spring snail (Bythinella pareyssii).
The so-called "Schnecken-Salettl" is a pavilion under the supervision of the Vienna Museum of Natural History, informing visitors about the little known, but special, resort patients.
Schneckenreservat Bad Vöslau. (In German)
More recent publications point towards a much closer relationship between Theodoxus danubialis and Theodoxus prevostianus, than previously assumed. Presumably a common ancestral group of both modern species, probably Theodoxus prevostianus from the Pleistocene, had spread out from the lower Danube river area. Genetic divergence developed by geographic variation and genetic divergence in the same locality (allopatric divergence). In this context Theodoxus prevostianus is assumed to be a non-monophyletic group closely related to Theodoxus danubialis, which latter species on the other hand is noticeably different between Danube river populations and their conspecifics from Northern Italy.
|Bunje, P. (2007): Fluvial range expansion, allopatry, and parallel evolution in a Danubian snail lineage (Neritidae: Theodoxus). Biol. J. Linn. Soc., 2007, 90, 603–617 (PDF).|
|Feher, Z.; Zettler, M. L.; Bozso, M.; Szabo, K. (2009): An attempt to reveal the systematic relationship between Theodoxus prevostianus (C. Pfeiffer, 1828) and Theodoxus danubialis (C. Pfeiffer, 1828) (Mollusca, Gastropoda, Neritidae). Mollusca 27 (2) (PDF).|
Martin Kohl: Fresh Water Molluscan Shells: Neritidae.