Theodoxus fluviatilis, common name the river nerite, is a species of small freshwater and brackish water snail with a gill and an operculum, an aquatic gastropod mollusk in the family Neritidae, the nerites.
This widely distributed neritid snail species occurs from Europe to Central Asia. It has a thick shell with a calcified operculum. The coloration pattern on the shell is very variable. Theodoxus fluviatilis lives in freshwater and in brackish water, in rivers and lakes on stones. It feeds mainly by grazing on biofilms and diatoms.
Some of the populations of this species are spreading, and these can reach densities up to thousands of snails per square meter. Females lay egg capsules, each of which contains a large number of eggs, but only one snail hatches from the capsule. The snails reach sexual maturity in a year, and the total lifespan is 2 or 3 years.
Taxonomy Theodoxus fluviatilis was originally described under the name Nerita fluviatilis by Carl Linnaeus in 1758. Linnaeus' original text (the type description) in Latin was very short, and reads as follows:
Which means in English: "Nerita fluviatilis, number 632: the shell is wrinkled, there are no teeth in the aperture. It inhabits rivers in Europe." Later, this species was moved to the genus Theodoxus Montfort, 1810. Theodoxus fluviatilis is in fact the type species of the genus Theodoxus. Anistratenko and colleagues designated the lectotype for Theodoxus fluviatilis in 1999 Anistratenko O. Yu., Starobogatov Ya. I. & Anistratenko V. V. (1999). "Моллюски рода Theodoxus (Gastropoda, Pectinibranchia, Neritidae) Азово-Черноморского бассейна. Molluscs of the genus Theodoxus (Gastropoda, Pectinibranchia, Neritidae) from the Black and the Azov Seas basin". Vestnik Zoologii 33(3): 11–19. (an English translation was published by Anistratenko in 2005).
Subspecies Several subspecies of Theodoxus fluviatilis were described and (inconsistently) recognized by various authors: Theodoxus fluviatilis fluviatilis (Linnaeus, 1789)"Theodoxus (Theodoxus) fluviatilis". Fauna Europaea, last update 27 January 2011, accessed 12 April 2011. – was described from a freshwater environment Theodoxus fluviatilis fluviatilis f. fontinalis Brard, 1815 – is sometimes considered as a synonym of Theodoxus fluviatilis Theodoxus fluviatilis littoralis (Linnaeus, 1789) – was described from brackish water by Linnaeus as a separate species, originally named Nerita littoralis. A study by Zettler (2008) proved that its status as a subspecies is unjustified, being regarded as a synonym of Theodoxus fluviatilis. Although these so-called forms (form fluviatilis and form littoralis) differ in morphology, ecology, reproductive strategy and behaviour, they are probably just ecomorphs. Theodoxus fluviatilis sardous (Menke, 1830) Theodoxus fluviatilis subthermalis Issel, 1865 – or Theodoxus subthermalis (Bourguignat in Issel, 1865) Theodoxus fluviatilis thermalis (Dupuy, 1851) Theodoxus fluviatilis transversetaeniatus A. J. Wagner, 1928 Theodoxus fluviatilis dalmaticus Sowerby – in Lake Ohrid Theodoxus fluviatilis euxinus (Clessin, 1885) – has been considered to be a subspecies (see Theodoxus euxinus)
Bunje (2005) does not consider Theodoxus velox Anistratenko, 1999 to be a distinct species from Theodoxus fluviatilis.
Cladogram A cladogram shows the phylogenic relationships within the genus Theodoxus:
This cladogram shows that the sister group to clade B is clade C. They split in 5–11.5 Ma, when Lake Pannon existed. Theodoxus species living in brackish water include Theodoxus fluviatilis and Theodoxus jordani, but they are apparently not closely related.
Distribution The exact type locality for this species is unknown, but it is probably the Main river in Southern Germany. Glöer (2002) considered the type locality sensu Linnaeus as "Habitat in fluviis, Upsaliae ad molendinam Ulvam & alibi", but this would suggest a brackish water environment. The distribution of this species was considered to be European, Lisický M. J. (1991). Mollusca Slovenska [The Slovak molluscs]. VEDA vydavateľstvo Slovenskej akadémie vied, Bratislava, 344 pp. but in reality the species occurs in the western to central Palaearctic. Its occurrence is scattered throughout Europe and in Western Asia except for the Alps and the regions immediately north of the Alps. This species does not live in Norway or Siberia. Theodoxus fluviatilis has the most widespread distribution of all of the species in the genus Theodoxus. PDF. It is in fact one of the most widely distributed species in the entire family Neritidae.
This species is threatened mainly by river engineering and water pollution in densely populated regions. The species' population trend is overall stable, but is declining in some areas (Germany), while in other areas it is expanding (for example in the Danube river). In the Rhine river during the 1970s, Theodoxus fluviatilis came close to local extinction because of water pollution. Subsequently, the water quality improved for more than two decades, leading to a recovery. Even so, the species became extinct in the Rhine for an unknown reason in the late 1990s. Since 2006, Theodoxus fluviatilis recolonized the Rhine, probably via ship transport through the Main-Danube Canal. An analysis based on cytochrome-c oxidase I (COI) gene has shown that the recolonization probably originated in the Danube.
Europe
The species occurs widely in Western Europe, and it is also widespread in the north of Ireland, living in 10% of Irish streams and rivers. It lives in Great Britain, including the Orkney Islands, as well as in the Netherlands,Theodoxus fluviatilis, accessed 25 September 2008. Belgium, Luxembourg, Liechtenstein, and Monaco. It also is found in France and Switzerland, where it is considered to be critically endangered. More to the south, it occurs in Spain and Portugal, although the species is restricted to karst springs in Central Portugal.
In central Europe, this species has been recently introduced in the Austrian Danube, where it was first recorded in Tulln, Lower Austria in 2001. Schultz H. & Schultz O. (2001). "Erstnachweis der Gemeinen Kahnschnecke, Theodoxus fluviatilis (LINNAEUS, 1758) in Österreich (Gastropoda: Neritidae)". Annalen des Naturhistorischen Museums in Wien 103B: 231–241. PDF. In the Czech Republic, it is now extinct in Bohemia;Red List of the molluscs (Mollusca) of the Czech Republic the only findings were in the Elbe river near Litoměřice in 1917, and the most recent findings of empty shells took place in 1943. Theodoxus fluviatilis also occurs in Poland, in Slovakia where it is non-indigenous since 2002, and in Hungary. Zettler (2008) provided a detailed bibliography of the distribution of T. fluviatilis in Germany. The indigenous distribution of T. fluviatilis included all of the large rivers: Rhine, Main, Moselle, Neckar, Weser, Elbe and Oder. However, this species is now highly endangered in Germany (Stark gefährdet).Glöer P. & Meier-Brook C. (2003). Süsswassermollusken. DJN, pp. 134, pages 29, 108, .
In Northern Europe, this species is found in Denmark, in Sweden as far north as 58° N. It can also be found on the coasts of Finland, in Åland, and is known to be found alive there since 1994. No other Theodoxus species reaches the Baltic Sea. It has the northernmost distribution of the genus Theodoxus and it is also the northernmost species of all Neritidae. In Eastern Europe this snail occurs in Estonia, Lithuania, and Latvia, as well as Belarus,Nagorskaya L., Moroz M., Laeno T., Veznovetz V., Pillot H. M., Dijkstra K. D. B. & Reemer M. (2002). "Macrofauna in floodplain pools and dead branches of the Pripyat river, Belarus". The Institute of Zoology NAS Belarus, 158 pp., page 56. and in Russia from western Russia to Caucasus. Since 1997 it has been found in the Gulf of Odesa, Ukraine. Butenko O. (Бутенко О. И.) (2001). "Моллюски рода Theodoxus (Gastropada, Neritidae) в Одесском заливе Черного моря. [Mollusks of genus Theodoxus (Gastropoda, Neritidae) in the Odesa Bay (the Black Sea)]". Ekologiya Morya 58: 27–28. PDF . In Ukraine and in Crimea it is non-indigenous, and was first recorded in the area in 1955. It also occurs in Moldova. In Southern Europe, Theodoxus fluviatilis lives in Albania, Bosnia and Herzegovina, Romania, Bulgaria, Slovenia, and Croatia. In Macedonia and Albania it occurs in Lake Ohrid (which spans the border of the two countries) as the subspecies Theodoxus fluviatilis dalmaticus. It is found on the mainland of Greece and also on Crete."Picture summary of Theodoxus-fluviatilis_04.jpg". AnimalBase, last modified 20 June 2008, accessed 13 April 2011. It is known to occur in the mainland of Italy and also in Sardinia. It occurs in Montenegro, and in Serbia.
Asia and Africa In Asia, Theodoxus fluviatilis is found in Turkey. It can also be found in Iran, in the provinces of Kerman, Gilan, Mazandaran, Fars, Hormozgan, Lorestan and Khorasan. However, until 2012, all the records from Iran were listed as Theodoxus doriae. In Africa this species occurs in Algeria, and possibly (or probably) in Morocco, where there are records which some authors consider to be reliable. However, instead of one species, Theodoxus fluviatilis, Brown (1994) recognized three species in northwestern Africa: Theodoxus numidicus, Theodoxus maresi, Theodoxus meridionalis.Brown D. S. (1994). Freshwater Snails of Africa and their Medical Importance. Taylor & Francis. .
Prehistoric biogeography Shells of Theodoxus fluviatilis have been found in an Upper Paleolithic archaeological site in the cave Caldeirão, Pedreira (Tomar), Tomar Municipality, Portugal, Callapez P. (2003). "Moluscos marinhos e fluviais do Paleolítico superior da Gruta do Caldeirão (Tomar, Portugal): evidências de ordem sistemática, paleobiológica e paleobiogeográfica". Revista Portuguesa de Arqueologia 6(1): 5–15. PDF. and also in a site from about 6000 years B.P. of Litorina age on the Åland Islands. Shells from the Late Neolithic have been found in Divoká Šárka, Czech Republic. Sobotka M. (1945). "První nález zubatky říční v Čechách". Příroda, Brno, 37(5): 162. Ložek V. (1964). "Quartärmollusken der Tschechoslowakei". Rozpravy Ústředního ústavu geologického, Prague, 31: 374 pp., page 155. Bunje (2005) hypothetized that the ancestral range of Theodoxus fluviatilis was the Ponto-Pannonian region (southern Ukraine, Romania and Hungary). Bunje suggested that the species first colonized northern Italy, Greece and Turkey; in the second phase it colonized Spain, France and Germany; and finally in the Holocene it colonized the British Isles, Sweden and the Baltic Sea. In 2002, German malacologist Peter Glöer summarized the distribution of this species during the Pleistocene and Holocene epochs.
Description The shell of Theodoxus fluviatilis is somewhat depressed (with an usually low spire), strongly calcified, and has 3–3.5 whorls (including the protoconch).Theodoxus fluviatilis. Marine Species Identification Portal, accessed 11 April 2011. Larger specimens are usually eroded. The width of an adult shell is usually 5–9 mm, but can reach up to 11–13 mm. The height of the shell is 4–6.5 mm, or up to 7 mm. These mean values vary among populations depending on the environment: the maximum width of the shell of brackish water populations is 9.3 mm. Brackish water shells are somewhat shorter, reaching up to 5.8 mm, and the maximum weight of the shell is 124 mg. In freshwater populations, the maximum recorded shell width is 13.1 mm, and maximum height is 9.3 mm. The maximum weight of freshwater shells is 343 mg.