Even though Theodoxus snails (also known as River Nerites or Ladybug snails) are pretty common in Europe, they are one of the rarest snail species you can find on the U.S. pet trade market. This is, in my opinion, a huge loss for this hobby.
Although Theodoxus species are not very common in the aquarium trade, they are one of the best snails for the brackish or freshwater tanks you can wish for. Why? Because they are peaceful, always hungry for the algae, and relatively easy to care for.
In addition, unlike Nerite snails, Theodoxus species can breed in captivity but they will not overcrowd your tank! It makes them some of the most underrated snails in the aquarium hobby.
I need to start off by saying that knowledge of these snails is scanty. Basically, it is going to be one of the first guides about this species. Nonetheless, in this guide, I have done my best to gather all information regarding Theodoxus snails including their behavior, life span, growth, feeding, handling, embryonic development, etc.
Quick Notes about Theodoxus Snails
|Other Names||River Nerites or Ladybug snails|
|Scientific Name||Theodoxus sp.|
|Tank size (optimal)||10 gallons (~40 liters)|
|Keeping||Easy to medium|
|Breeding||Easy to medium|
|Size||0.5 inches (1.3 cm)|
|Optimal Temperature||18 – 26 °C (64 – 79 °F).|
|Optimal PH||7.0 – 8.5|
|Optimal GH||12 – 30|
|Optimal KH||6 – 16|
|Nitrate||Less than 40 ppm|
|Life span||up to 3 years|
|Color Form||Light and yellow to dark with dots or stripes|
Natural Habitat of Theodoxus Snails
From an evolutionary point of view, the origin of the snail has been assumed to be the brackish Black Sea. However, about 400 000 years ago, they started to adapt to the freshwater environment. Nowadays Theodoxus snails are the most widely distributed gastropods in this freshwater genus of the family Neritidae.
The distribution range of the most widespread species of Theodoxus covers the area of Europe from Scandinavia to Greece and Crete Island, and from Ireland and the Iberian Peninsula to the Sea of Azov Sea and the Baltic States. This species is also recorded in Asia, i.e. Anatolia and Iran.
Although there are large areas within Europe that are uninhabited, particularly in mountain ranges.
Note: According to IUCN database, because of its tolerance to organic pollution, it is currently regarded as invasive in the Danube River basin.
These snails are typically found in hardwater and calcium-rich rivers, streams, and lakes as well as in brackish water in the coastal regions of Europe and southwest Asia.
Theodoxus snails live in colonies on and under rocky limestone, associated with many aquatic plant species.
Description of Theodoxus Snails
These are small snails up 6–9 mm (0.35 inches), exceptionally up to 13 mm (0.5 inches) in width. They are often described as being dime-sized or smaller.
In general, their shell consists of 3–4 whorls and it has an elongate ovoid form, with only a slightly prominent spire.
Depending on the species, the coloring of the shells is very variable. For example:
- In Theodoxus fluviatilis (River Nerites), the shells are usually black to various brighter with darker zigzag stripes often with triangular whitish spots.
- Theodoxus danubialis snails (Danube nerite) have transversal dark zic-zac stripes on the 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.
- Theodoxus transversalis snails (Striped Nerite) often have several fine spiral lines that add up to form three longitudinal bands. The operculum has an orange color and a strong spirally twisted peg.
- Theodoxus pallasi(Black sea Nerite) may have a black or gray-yellow shell with white to yellow spots or dark stripes.
In addition, the coloration also depends on the age of the snail. When they are young, they are more whitish and become black to the end of their life.
The life span ranges from 2 – 3 years and greatly depends on the environment temperature.
Taxonomy Problems of Theodoxus snails
Although there are at least 34 described species of the genus Theodoxus, some scientists believe that the real diversity is much lower and recognize only a few valid species.
The point is that, as shown by several studies the shell coloration and its patterns are very plastic in all species of the genus Theodoxus and could be influenced by factors like ionic composition of water, type of substratum, and nutrition of individuals of this species in various habitats.
For example, the shells of the specimens of Theodoxus fluviatilis from Northern Europe are ornamented with a pattern of white drop-like spots on a dark or red background. In the specimens from South France and Spain are ornamented with a pattern of zigzag stripes, while in the specimens from Balkan all combinations of white drop-like spots and zigzag stripes were observed. The animals from lacustrine habitats show dark or light bands on the shell.
Basically, if you are not a biologist, the only way to say that you have Theodoxus fluviatilis species is to look at the operculum. Their operculum does not have variability. The operculum in Theodoxus fluviatilis is reddish with a blood-red border.
Note: Another way is to look at the difference in the rib pit, however, I seriously doubt that ordinary hobbyists can do that. Theodoxus fluviatilis can be distinguished from other species (like Theodoxus transversalis, T. danubialis and T. prevostianus.) by having a rib pit, formed by the rib and the rib shield. The rib shield, and consequently a rib pit are lacking in other species.
In summary, biologists concluded that in the case that coloration and patterns of the shells cannot prove certain identification.
Behavior of Theodoxus Snails
All Theodoxus snails are non-aggressive and docile by nature.
They are primarily crepuscular or nocturnal. However, it does not mean that you will never see them in a day time. They are simply less active during this time.
The nocturnal behavior is not uncommon for grazing invertebrates and is mainly associated with attempting to avoid visual predators whilst feeding.
Interesting fact: On a 24-h cycle, dusk is a period in which algae had maximum nutrients at the end of the photosynthetic period corresponding to profitable conditions for grazers to feed at the end of the light period.
Feeding Theodoxus Snails
The species are herbivores and detritivores.
According to the study, the main food sources of Theodoxus snails are diatoms (65%), detritus (30%), and green algae (5%).
They will also appreciate algae wafers (check out the price on Amazon). However, keep in mind that because of their small size, it can be better to crush the wafer into dust and spread it around the tank.
|Do not forget that calcium plays a huge role in snails. Calcium is needed for several vital life processes in snails, the most obvious being the formation of shells. According to the study, the borderline between soft and hard water is a calcium concentration of 20 mg/l.|
You can read some of my related articles (the principle is the same with snails):
Theodoxus Snails and Feeding Problems
Once they finish all the algae in the tank, they may start starving. So, you will have to provide them with more algae.
Do not worry, you will not have to spoil your tank, there is another way.
We can grow algae for them and the easiest way to do that is to use rocks in a separate container. It is very simple!
- You need some kind of transparent container (any large bottle, spare tank, etc.).
- Fill it with water. Use the water that comes from water changes.
- Put there a lot of small rocks like marble chips and ceramic filter media (The rocks should be clean and aquarium safe, of course).
- Leave it under the strongest lighting you can find. Ideally – 24/7.
- Use any fertilizer to grow plants in a tank.
- Using an airstone will boost algae growth as well.
- Once you see that rocks are turning green, take a few a place them in the tank to feed the snails.
- Return the rocks to the container when they are clean.
Are Theodoxus Snails Plant Safe?
Unfortunately, people sometimes confuse grazing on with eating the plant. Even if you see them eating a “healthy” plant, it is not as it seems. It means that the plant is already dying, although, it has not shown yet.
Caring and Keeping Theodoxus Snails
Like most aquatic snails, Theodoxus snails are usually straight forward and easy to care for. However, for optimal results, here are some handy tips that you should follow in order to keep them in an aquarium.
Second, wild-caught snails do not adapt well. However, before they die, they often manage to leave a few eggs and these babies will easily adapt to your tank.
Because of their small size, there are no minimum requirements. Even a 2.5 gallon (10 liters) tank will be good enough for a few Theodoxus snails.
The only problems with small tanks:
- It can be difficult to keep your water parameters stable.
- It can be hard for the snails to find enough food and you can have feeding problems.
Theodoxus Snails: Brackish or Freshwater
The genus Theodoxus is a freshwater and brackish group of gastropods native to Europe and the Mediterranean region.
These snails can do well in freshwater environments, provided the water bodies are large enough, suitable habitats are available and calcium values are not too low.
Although previous studies have revealed that individuals from both habitats are not distinguishable by morphological characters, freshwater and the brackish group will not adapt to different levels of salinity equally well. That may be based on subtle genetic differences.
This information can be useful for those who are planning to keep Theodoxus snails in marine tanks. Basically, in this case, you will have to find snails that come from the marine environment.
According to the experiment, animals of the freshwater population tolerated media with a salinity up to 16% but did not well in media with higher salt concentrations. When transferred to containers containing medium with a salinity of 32%, the animals kept their operculum closed for approximately 8 hours. In addition, up to 10% of these animals died within 48 h.
Another problem of putting a brackish group of snails into freshwater or vice versa is the fact that salinity influences the change in the gut microbiome of snails. As a result, snails have a hard time eating because they have not developed special bacterial in their stomach.
Temperature: Theodoxus snails will thrive in a range of 18 – 26 C (64 – 79 F).
pH: Optimal water pH should be provided for this species in the range of 7.0 – 8.5.
Hardness: Theodoxus snails will appreciate hard water KH > 6 and GH > 12. In soft water, these snails will lose coloration within a few months. Even though they will live, this is a clear sign that the water does not suit them.
Theodoxus snails do not really need light. Therefore, lighting should be adapted to the needs of plants and algae in the tank.
For more information, you can read my “Advanced Guide to Planted Tank Lighting”.
Even though Theodoxus snails can be kept in any tank with any substrate, I would still recommend smooth gravel over any other type.
- Their natural environment is composed of smaller rocks, sand, gravel, and lots of boulders.
- The main reason – it is easier to grow algae on them!
There are no special requirements. As long as you have got the filter that works great with the size of the tank you have got you will be fine.
However, because of their small size, you will need to think of their safety. One of the problems with them is that they might eventually clog a filter intake or other parts of the filter.
The point is that these snails will crawl everywhere looking for food, and in some cases can travel across the filter and get stuck in the slats. In some cases, they are unable to get free and can end with serious injuries or even death.
So, if you have a hang on the back filter, it’s important to cover the filter with a sponge filter for this reason and eliminate the possibility.
Aeration and Flow
During my research, I have found some conflicting information regarding this subject.
According to one study, Theodoxus snails need high levels of oxygen to thrive. Because they are morphologically suited to a life in streams with a swift current and in the wave zone of lakes.
Another study states, that these snails can tolerate moderate organic pollution and very low oxygen contents (even below 2 mg/l) but do not endure too long periods of drought or ice.
This is not necessary but your snails will appreciate all types of leaves, rocks, woods, PVC pipes, etc. in your tank. It will give them some places to hide. In addition, they provide a lot of surface area for algae and biofilm growth.
You can read more about it in my articles:
Important: Before putting them into your tank do not forget to carefully acclimate them (read more about it) as all invertebrates. Do it very slowly. In general, 2 – 3 hours will be good enough.
Be careful with chemicals like copper (read more). Snails do not tolerate copper-based medications.
Basic Tank Equipment (links to check the price on Amazon)
Sexing Theodoxus Snails
Males and females can be distinguished by external features. However, it is close to impossible for an ordinary hobbyist.
In females, the border of the rip shield of the operculum is straight, but curved in males.
Breeding Theodoxus Snails
Theodoxus snails reach maturity when they are about 1 – 1.5 years old at a shell length of 5.5–5.7 mm (0.2 inches).
Females lay the capsules in small clusters of 4–5, which are attached to any hard substrates, including on other snails.
In nature, the estimated number of capsules per female approximates 30 – 40 capsules during summer and about 20 capsules in autumn.
In freshwater, each egg capsule usually contains from 100 to 200 eggs. When the population lives in brackish water, it usually contains 55 – 80 eggs. The color of the capsules changes with time. The fresh ones are almost white. Then they turn yellow to brown.
Depending on the temperature, it usually takes from 1 – 8 months to hatch. For example:
- 25 C (77 F) – 30 days
- 20 C (68 F) – 65 days
- 10 C (50 F) – 180 days. Overwinter capsules (almost dark) – up to 8 months. It is assumed that the development of egg stops at temperatures below 10 C (50 F).
From each capsule only one juvenile hatches while the other eggs will serve as nourishment.
Newly hatched baby Theodoxus snails are about 0.1 mm (0.04 inches) in size and resemble the adults in shell shape. They do not stay on the substrate and prefer to bury into the substrate feeding on microorganisms they find there.
Juveniles will usually appear on the surface once they are 2 months old. They eat the same food just like the parents.
If you want to breed these snails, it is absolutely important to have lots of food in the tank. Otherwise, they will not have enough energy for that.
Theodoxus Snails and Suitable Tankmates
Theodoxus snails are absolutely harmless creatures. They will be a great choice for any peaceful community tank.
Concerning the fish species and some natural enemies, these snails should not be kept with Botia lohacata, Cichlids, Loaches, and Goldfish.
Shrimp species that prefer slightly alkaline water will be the best choice to keep with Theodoxus snails. For example, Vampire shrimp, Bamboo Shrimp, Amano Shrimp, Ghost shrimp, Cherry shrimp, Blue tiger shrimp, Snowball shrimp, Caridina cf. babaulti, Blue Velvet Shrimp, Malawa Shrimp, etc.
Theodoxus snail’s tank mates can also include other snails like Black Devil Snails, Brotia Pagodula snails, Rabbit snails, Nerite Snails, Mystery Snails, Ramshorn snail, Malaysian Trumpet snails, and others of their kind.
I have seen some posts where people report that it is possible to keep Theodoxus snails with Assassin snails. Those people say that Theodoxus snails attach to the surface so hard that it does not allow Assassin snail to attack.
Well, this is not true! Unfortunately, Assassin snails will easily overpower Theodoxus snails.
Also, keep these snails away from all types of Crayfish (even Dwarf Mexican crayfish), most types of freshwater crabs, and even some types of predatory shrimp such as Macrobrachium family. All of them can be very aggressive towards the snails.
African Dwarf Frogs
African dwarf frogs will not be good tankmates for small Theodoxus snails. There is a chance that they will try to swallow them.
Theodoxus snails are excellent scavengers and easy to care for. They are absolutely harmless creatures and can be safely kept in planted tanks.
Their small size allows to keep them even in nano tanks. The only problem is that they are pretty rare in the pet trade.
|Theodoxus snails (River Nerites)|
|Tolerate freshwater and brackishwater||They are small snails if you want to keep them in a display tank|
|Amazing algae eaters||Require hard water|
|Can reproduce in freshwater (but slowly)|
|Do not eat plants|
- Life history, growth and production of Theodoxus fluviatilis in Lake Esrom, Denmark. Limnologica 36 (2006) 26–41.
- Acute toxicity of some heavy metals to the fresh water snail, Theodoxus niloticus (Reeve, 1856). Egyptian Journal of Aquatic Research 44 (2018) 83–87.
- The morphological plasticity of Theodoxus fluviatilis (Linnaeus, 1758) (Mollusca: Gastropoda: Neritidae). Ecologica Montenegrina., 2 (2), 2015, 88-92.
- The distribution of the gastropods Theodoxus fluviatilis (L.) and Potamopyrgus antipodarum (Gray) in lakes on the Åland Islands, southwestern Finland, BOREAL ENVIRONMENT RESEARCH 5: 187–195, 2000.
- Blackwell Publishing, Ltd.Pan-European phylogeography of the aquatic snail Theodoxus fluviatilis (Gastropoda: Neritidae). Molecular Ecology (2005) 14, 4323–4340.
- Differences in osmotolerance in freshwater and brackish water populations of Theodoxus fluviatilis (Gastropoda: Neritidae) are associated with differential protein expression. Journal of Comparative Physiology, 2009.
- Extremely low genetic diversity in the endangered striped nerite, Theodoxus transversalis (Mollusca, Gastropoda, Neritidae) – A result of ancestral or recent effects? North-Western Journal of Zoology. 2012.
- Kivistik C, Knobloch J, Käiro K, Tammert H, Kisand V, Hildebrandt J-P and Herlemann DPR (2020) Impact of Salinity on the Gastrointestinal Bacterial Community of Theodoxus fluviatilis. Front. Microbiol. 11:683. doi: 10.3389/fmicb.2020.00683.
- Theodoxus fluviatilis’ re-establishment in the River Rhine: a native relict or a cryptic invader? Conservation Genetics. 2015.
- Distribution of the species of Theodoxus Montfort, 1810 (Gastropoda: Neritidae) in Serbia: an Overview. Zoogeography and Faunistics, 66 (4), 2014: 477-484.