Neritina has been a hit lately. There is still a lot of confusion in their differentiation and naming, so we don’t really know which species we have in the aquarium. The first descriptions of neritines in France and Germany refer to Neritina natalensis, but recently we have more and more often seen traders referring to these snails as an Asian species. I will stick to the original name, but take it with a grain of salt. Some species of neritina from Southeast Asia or the Brazilian coast look practically the same or even better. Thus, we also meet with the names Neritina turrita, Neritina zebra, Neritina zic-zac, Clithon sp., Vittina sp. etc.
I’ll just add to all this to reduce confusion – there are most likely not that many different types of neritina, just simply browse any shell collection website and you’ll get a crossing of eyes from the striped and otherwise patterned neritina. There are a huge number of them and they are very similar. Then every aquarist trying to identify their striped additions will come up with their own name. Not to mention that with a change in conditions, there can be a radical change in the pattern on a newly accreted shell, as seen in many of the photos.
For orientation on aquarium invertebrates, I must traditionally recommend the German site Wirbellose, which says, among other things, that “For snails sold as Neritina zebra, this is Neritina natalensis. Neritina zebra is a marine species…”.
However, we can rest assured, the neritina commonly available in our aquarium stores are either Neritina natalensis or another, very similar species. Their breeding requirements are essentially the same. Just be careful when buying unusual species that have a “horned” shell or are simply not zebra but differently coloured. This may be because they are purely marine species that will not last long in a regular aquarium.
But enough confusion among neritina, let’s go to our well-known Neritina natalensis.
Let’s start with a bit of systematics: Neritina natalensis belongs to the gastropods (class Gastropoda), according to the older system it was classified in the subclass Prosobranchia, order Archaeogastropoda. It is now placed in the subclass Orthogastropoda, order Neritopsida.
The family Neritidae, with its centre of gravity in the tropics and subtropics, has most representatives marine, some species live in brackish waters in estuaries and a few are freshwater. However, a link to the marine environment may also be developed in freshwater species, some of which retain a planktonic larval (veliger) developmental stage that develops in the sea.
This is also the case with Neritina natalensis: the species lives in the rivers of southern Africa, where it also mates and lays its eggs. After hatching, the larva, which is less than 1 mm in size, enters the sea with the water current. Only after it has finished its development and transformed into a ‘snail’ does it return to the watercourse, constantly travelling upstream during its lifetime (the closer to the source, the larger the individual can be found).
A close relative of this neritina (the correct Czech name is actually zubovec!) is Theodoxus danubialis, a gastropod living in the Danube basin. It is critically endangered in South Moravia and also rare in Slovakia. In northern Europe, Theodoxus fluviatilis is abundant (used to be in the Elbe River in the Czech Republic, now extinct). Both species are an example of a freshwater gastropod with complete independence from the marine environment – the larva undergoes its development inside the egg case, so it is apparently a direct development – the egg hatches directly into a small snail.
Neritina natalensis grows to about 2 cm in size and is relatively long-lived; under optimal conditions it can live for 4 years or more. It breathes through its gills, so unlike ampullarians it does not need to breathe above the water surface. Nevertheless, it ventures into its vicinity quite often and a must when keeping it in captivity is to seal the area above the surface well; otherwise it can be found outside the aquarium. Such an excursion usually has tragic consequences – the neritina has a very massive shell, resistant to pressure but heavy and fragile. Beware of this when handling snails; they fall to the bottom at high speed, and even if their shell does not shatter when hitting rocks, pieces of the surface “glaze” may chip off and your beauty suddenly looks like it has returned from the battlefield…
The shell closes with a small lid. It can perfectly protect the body of the snail from drying out – it happened to me that I rescued a neritina after a trip behind the furniture after a very long time, it was out of water for at least a week… and after putting it in the aquarium it was splashing on the glass in two hours as if nothing had happened.
The black pattern on the red-brown ground is specific to each individual; you’d be hard pressed to find two neritina with the same pattern. The ulita is protected when it moves by a sort of bar that is wrapped around the base all the way around. This “collar” is almost transparent, with a noticeable tiger-like pattern typical of a neritina’s body. If you look closely at the photos, you can see it in almost every one of them.
Neritina feed on algae – and ONLY algae. They have a radula adapted to scrape them off. Numerous experiments have been conducted to determine their appetite, and the only conclusion is that they consume algae of all kinds, including filamentous. Plant debris or any other food is said to be ignored. They’re obsessed with eating algae that form coatings on rocks and glass. They are less inclined to walk on gravel or climb plants. However, they should never nibble them, they just scrape the algae off the surface. They can handle, for example, sessile algae on anubias leaves. But my neritina also climb on more delicate plants (Micranthemum micranthemoides, Echinodorus tenellus, etc.) – due to their heavy shell they struggle a bit…
I have to admit my experience with neritina and their supposed inertia towards plants: the leaves of Echinodorus tenellus were more than thoroughly scraped by one of them, only a torso of a leaf remained, not even the whole vein. So the claim that Neritina natalensis would rather starve than take on plants should be taken with a grain of salt! However, by that time there was really no trace of algae in the aquarium…
Neritina natalensis finishing the devastation of Echinodorus tenellus leaves. (Photo: Markéta Rejlková) Watching neritina at work is fascinating. If they find rich grazing, they don’t move. Minute by minute they shift and behind them a perfectly clean surface appears – I was surprised to see the original colour of the stones in my aquarium. One of my neritina immediately went to a rock about 10 x 10 cm and stayed on it for 30 hours, during which time it removed even the last remnants of algae. She then walked around it several times, and only when she was satisfied that there was nothing left to eat did she go to explore the aquarium.
It took a few days for the neritina to clear away all the algae buildup in its new home. Then they have to search for food and explore the gravel and plants as well. In contrast to the painstaking systematic work of “grazing”, they are relatively quick in their search for new finds. In doing so, they keep the glass and rocks clean for a long time – so if you want to have an aquarium decoration of white rocks and white gravel, neritina is the ideal solution. Supposedly, one individual is enough for a regular 40-litre aquarium – and you never have to clean the glass again.
An interesting feature of the entire Neritidae family is that they maintain home ranges of sorts. Whether they’re active day or night, they return to the same place each time to rest… They like to stay on rocks and roots, where they also lay their eggs most often. Aha – an important question for every aquarist: will my neritina reproduce as fast as most other snails when I can’t get rid of them? Unfortunately, neritina do not reproduce at all in captivity.
As I wrote before, their larvae develop in the sea. This is somewhat analogous to the development of the shrimp Caridina japonica, which has already been able to reproduce quite successfully in captivity. So where is the problem with neritina?
Neritina natalensis is sex-segregated, fertilization is internal. The female can lay eggs several weeks after she has met the male. Sooner or later, most aquarists will find laid eggs in large numbers scattered individually on rocks, roots, plants or glass in their neritina aquarium. They are tiny whitish formations, somewhat resembling sesame seeds. (In reality, they are capsules full of tiny eggs – in the case of Theodoxus fluviatilis, the first larva to hatch eats all 50-150 of its siblings, and so a single plump little snail emerges from each capsule. In neritina, there is a theoretical chance to get offspring from all eggs.) So far, the reproduction of neritina looks promising, BUT: no one has observed that the eggs hatch into larvae. What is the cause of the failure is unknown. (It should be noted that most of the lucky neritina owners have no idea that instead of snails you have to look for larvae, and larvae of really miniature size…) Nevertheless, this is a challenge to all experimenters!
All the neritina in our aquariums were born in Africa (or elsewhere, if it is not specifically Neritina natalensis). Even considering this, their price ranges from 2-4 euro, in our area from 20 crowns (wholesale) upwards (if you can get them in retail). Some of them pay for the long journey with their lives; there are also reports that half of the neritina newly introduced into the aquarium die. In my case, one out of four died after a few days, and every time I bought more than 3 later, some died within a few days or weeks.
You can usually only admire a conspicuous shell on the neritina. You rarely get a “face to face” look at them and then notice how captivating this snail’s eyes are… not only are they almost human shaped, but they’re even blue.