page_title: Theodoxus fluviatilis
section_title: Description
hierarchical_section_title: Theodoxus fluviatilis / Description
caption_attribution_description: English: Photo of an outer side of an operculum of Theodoxus fluviatilis. The length of the operculum is 4 mm and the width of the operculum is 3 mm including the rib.
context_page_description: 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.
context_section_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). 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.
  The exterior of the shell is basically whitish or yellowish, with a net-like dark reddish or violet pattern. This pattern is very variable (depending on environmental factors), sometimes partly presenting bands, and even occasionally being evenly dark. The shell is very variable in color and color patterns, showing great polymorphism. Shell coloration and patterns are very plastic in all species of the genus Theodoxus and these qualities may be influenced by factors like ionic composition of water, type of substratum and nutrition of individuals in various habitats. Zettler and colleagues (2004) showed that in the outer coastal waters of the Baltic Sea, the nearly black and often corroded shell form of Theodoxus fluviatilis is predominant, whereas in the inner (sheltered) parts of coastal waters, yellowish-green forms prevail. Glöer and Pešić (2015) observed that specimens from a darker stony substrate were black or dark brown. Shells of specimens of Theodoxus fluviatilis from Northern Europe are ornamented with a pattern of white, drop-like spots on a dark or red background. Specimens from South France and Spain are ornamented with a pattern of zigzag stripes, while specimens from the Balkans show all possible combinations of white drop-like spots and zigzag stripes. Animals from lacustrine habitats show dark or light bands on the shell.
  Images showing variability in the color patterns of shells of Theodoxus fluviatilis:
  The shell shape of Theodoxus fluviatilis is similar to that of Theodoxus transversalis. The shell shape of Theodoxus danubialis is more spherical. The shape of the aperture of Theodoxus prevostianus is usually descending. However, all of these species display a large morphological plasticity, which makes them difficult to differentiate. The overall outline of the shell is still used for species identification in recent malacological literature. Though the coloration and patterns of the shells cannot be relied upon to identify specimens, opercular characters can be used for a proper identification of Theodoxus fluviatilis. The calcified operculum of T. fluviatilis is D-shaped, light reddish with a red margin, bearing a broad rib (also called a ridge) on its inner surface. The columellar muscle is attached to the rib. The rib is long and thin, attenuated at the base, while the callus is thin; a peg is lacking. The characteristic features of the operculum are already visible in juveniles. There is sexual dimorphism on the border of the rib shield of the operculum, which is straight in females, but curved in males.
  Aberrations in the shape of operculum have been observed. In a specimen from Vouvant in France, and another from a spring near Bar in Montenegro, a double rib was present, but the rib shield was reduced; in a specimen from Ohrid Lake, only the rib shield was reduced. Theodoxus fluviatilis can be distinguished from the other three mentioned species by having a rib pit, which is formed by the rib and the rib shield. The rib shield, and consequently a rib pit, are lacking in Theodoxus transversalis, Theodoxus danubialis and Theodoxus prevostianus. These three species differs in having, in addition to a rib, a peg, which is absent in T. fluviatilis. The visible soft parts of the animal are light yellow with a black head. The tentacles are greyish and long. The eyes are large and blac
caption: The outer side of an operculum.
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