Theodoxus fluviatilis

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Theodoxus fluviatilis, commonly called the river nerite, is a small snail that lives in fresh water and brackish water. It has a gill and an operculum (a little lid) and belongs to the Neritidae family. It is one of the most widespread nerites, found from Europe to Central Asia.

What it looks like
- The shell is small and thick, usually 5–13 mm across. It has 3–3.5 whorls and is often heavily patterned. Color and pattern can vary a lot depending on where it lives.
- The body is light yellow with a dark head. The shell’s external color patterns are very plastic, so individuals can look quite different.

Where it lives
- It grows on stones in rivers, streams, and lakes, and it can also live in brackish water at river mouths and estuaries.
- It tolerates mild pollution and thrives in rocky, hard-bottom habitats, from shallow water to a few meters deep.
- Brackish-water populations can handle higher salinity than freshwater ones.

What it eats
- It mainly grazes on biofilms and diatoms that grow on rocks. It can also eat detritus and sometimes some cyanobacteria and algae.

Reproduction and life cycle
- The snails are dioecious, meaning there are separate males and females.
- Females lay eggs in capsules on stones. Each capsule contains many eggs, but usually only one snail hatches from each capsule.
- In freshwater, capsules may have about 100–200 eggs; in brackish water, about 55–80 eggs.
- Fresh capsules are about 1 mm in diameter. Development and hatching depend on temperature and can take weeks to months.
- Snails reach sexual maturity in less than a year, and their typical lifespan is 2–3 years.

Distribution and status
- The species is widespread across Europe and into western Asia and parts of Africa. It is the northernmost Neritidae in some areas and is found in places like the UK, Ireland, the Netherlands, Belgium, France, Switzerland, Spain, Italy, and the Balkans, among others.
- It is the type species of its genus, Theodoxus.
- Some populations have declined because of river engineering and pollution, but others are stable or expanding, especially in cleaner waters. The Rhine in Germany nearly lost the species in the 1970s due to pollution but recolonized after water quality improved and again in the late 2000s, likely via the Main–Danube Canal.
- It can be found in a wide range of habitats, including lakes and rivers, and even in some karst springs.

How to tell it apart
- The shell is distinctive for its variability in color and pattern, but it can be distinguished from close relatives by its shell shape, the D-shaped operculum with a rib, and the presence of a rib pit on the operculum.
- The animal’s soft parts are typical of Neritidae, with a light body and a dark head.

Ecology and importance
- The river nerite is an indicator species for river health in some regions because it tolerates a range of conditions and responds to changes in water quality.
- It can reach very high local densities in favorable brackish waters, highlighting its ecological success in diverse environments.