Phylum — Mollusca.
Class — Gastropoda.
Superfamily — Neritoidea.
Family — Neritidae.
Genus — Theodoxus.
— Theodoxus danubialis [C. Pfeiffer, 1828] — Danubian nerite. Family Neritidae. Inhabits the Danube and Dniester rivers. The snail lives on rocky substrates and therefore depends on running water. It is distinguished by a large shell – 9-14 mm in diameter. The shell coloration is zig-zag patterned with dark transverse bands on a light brown or cream background. The width of the transverse bands varies. Individuals with almost black shells are sometimes found. The operculum is bright yellow with a brown rim along the edges. The animal’s body is light gray. The antennae are long and pointed.
They prefer rocky substrates and like to stay on large stones in the aquarium. They feed mainly on diatoms. They prefer hard water, rich in the calcium so necessary for building thick shells. They tolerate CO2 injection in the aquarium poorly. Without limestone rocks and algal fouling, they starve and die. They are gonochoric mollusks. The number of eggs in a clutch varies from 30 to 70. The incubation period ranges from four to eight weeks.
— Theodoxus fluviatilis [Linnaeus, 1758] — River nerite, ladybird snail. Family Neritidae. Its habitat range extends across Europe from Scandinavia to the Mediterranean coast. The shell has a hemispherical (semi-ovoid) shape, and its walls are quite thick. The shell has several rapidly expanding whorls. The semicircular aperture of the shell has a protruding and raised upper edge. Males and females do not differ externally.
The shell coloration can vary – there are three main types of coloration, the appearance of which depends on various environmental parameters. The main background of the shell can be black, olive, or even brown. The most common coloration includes white, violet-brown, reddish, or black spots of various shapes, lines, or even zig-zags. Light and dark bands, teardrop-shaped spots often complement the pattern. There are specimens with monochromatic coloration – for example, pure black.
There are several local subspecies. The subspecies Theodoxus fluviatilis littoralis – these snails live in brackish water, making them suitable for marine aquariums. They are slightly smaller in size than their freshwater relatives.
Keeping them presents absolutely no difficulties. They are more active at night than during the day. They usually stay on stones, large plants, tree roots, driftwood, and even on the shells of other, larger species of gastropod mollusks. Favorite locations for Theodoxus in the aquarium are the glass, large stones, filter, thermometer, and less frequently – the substrate and plants. Like all snails, the growth rate directly depends on nutrition, and since the main food source is algal and detrital film, it is desirable to have a part of the glass (e.g., the back glass) left uncleaned in the aquarium.
With an abundance of food, the snail will grow rapidly and reach its maximum size. It should be noted that the main food source in nature for Theodoxus is diatoms, the shells of which they crush by rubbing against a hard substrate before eating them. Obviously, this is why Theodoxus prefer hard substrates, including for reproduction.
In the aquarium, they eat algae and decaying wood, especially if this food is available on a hard surface. They do not respond to supplementary feeding, such as vegetables or dry food. They do not touch live plants. The temperature factor does not play a significant role; the optimal temperature is up to 25°C. Like most snails, they prefer a neutral or alkaline environment. pH 7.5-8.5; dH > 12°; KH > 8°.
Theodoxus scrape algal and organic films from hard substrates, which constitutes their diet. They lay eggs in these same places: mainly on glass, stones, and hard aquarium items. Often females lay eggs on their own shells and on the shells of other snail species. Sexual maturity in nature is reached at the age of 1.5-2 years with a body length of about 7 мм.
In a room aquarium, sexual maturity can be reached (at a temperature of 25°C) even in 4 months. In an aquarium under “high” temperature conditions, Theodoxus fluviatilis lays eggs at regular intervals throughout the year. Usually, the female attaches many egg capsules in close proximity to each other; single capsules are observed less frequently.
The spawn of these snails is completely unlike the eggs of other mollusks. It has the shape of a capsule about a millimeter in size, white in color, and is usually attached by the female to glass or another hard surface. Depending on the ambient temperature, they hatch after 4 to 8 weeks. As they mature, the capsule turns yellow, then brown, and finally a tiny juvenile emerges, also no more than a millimeter in size, with an uncolored translucent shell.
While the shell hardens, the babies hide in the aquarium substrate, usually closer to the walls. The population size of Theodoxus fluviatilis grows quite slowly compared to the growth of other gastropod mollusks most commonly kept in aquariums. Therefore, the problem of overpopulation with snails does not arise so acutely, and there is no need to constantly regulate their numbers. They live up to 3 years.
Identification: River nerite, ladybird snail — Theodoxus fluviatilis.
Tylomelania gemmifera, insulaesacrae, matannensis, patriachalis.
Marisa cornuarietis, Marisa rotula. 
