1. Do Snails Add to Their Shells Instead of Shedding Them?
When a snail hatches from its egg, it already carries a small shell on its back. The shell has few coils, but its shape is exactly like that of an adult snail [4]. Snails do not shed their shells to put on larger ones. Instead, they grow by adding to the shell as they get bigger [1].
The material for this growth comes from the mantle, a membrane that covers the outside of the snail’s body. Calcium carbonate hardens within a base of organic matter, such as proteins, to form the layers of the shell [2]. This new material is added to the side near the opening of the shell.
How long does this addition continue? In many species, it stops when the snail reaches adulthood. At that moment, a mark appears on the edge of the opening [1].
2. How a Flared Shell Edge Shows a Snail Is an Adult
When a snail becomes an adult and growth stops, a flare or thickening often forms at the opening of the shell [1]. This feature is thought to be created when the snail reaches its adult size and stops growing. It is used as an indicator of the shell’s developmental stage [3].
The structure of the shell opening is usually formed only after the snail becomes an adult. It is explained that this final finishing step requires a large amount of energy [4]. While the snail is still young, the edge of the opening remains thin [3].
Note: If the edge becomes thick or flared, it often means the snail has reached adult size and stopped growing. These features are indicators of the shell’s development stage. The shell grows by adding material from the mantle to the edge. In many species, when adult size is reached, the snail stops adding material and instead thickens or flares the edge. But can we always say that a flared edge means "adult"? [3]
3. Can a Snail Be Adult Without a Flared Shell Edge?
Not all snails follow the same rule. In some groups, such as the *Helicella* genus, the edge of the shell opening remains thin even when the snail is an adult [3]. Therefore, while a flared edge is one clue that a snail is an adult, you cannot decide that a snail is a child just because it lacks a flare.
There are also examples where growth does not stop completely but just slows down. A study of a small land snail (*Notodiscus hookeri*) living on sub-Antarctic islands found that shell growth did not stop after sexual maturity; it only became very slow. The largest shells were about 7.5 to 7.7 mm [7]. However, this is a case of one specific species. We cannot say this applies to all common snails.
Some species stop growing, while others just grow slowly. In all cases, the shell cannot grow or become thicker without material. Where does this material come from?
4. Where Do Snails Get the Calcium for Their Shells?
Between 95% and 99% of a shell’s weight is mineral, mainly calcium carbonate. The remaining 1% to 5% is organic matter, such as proteins [6]. For a shell weighing 100 g, this means 95 to 99 g is inorganic material and 1 to 5 g is organic. The organic matter is said to give the shell strength, flexibility, and lightness.
Calcium is a resource that is difficult for snails to replenish. It can be a limiting factor for their population numbers. The fact that there are more species and individuals in limestone areas is thought to be a result of this [4].
Land snails usually get calcium from the soil, plant debris like fallen leaves, and exposed limestone rock. There are reports that they may also absorb it directly through the soles of their feet [5]. However, the amount or proportion of absorption is not stated in that source.
When the usual sources are not enough, snails turn to unexpected items.
5. Do Snails Really Eat Concrete and Empty Shells for Calcium?
Snails have a radula in their mouth, which works like a file. Besides eating leaves, vegetables, and moss, they are said to scrape and eat concrete to get calcium for their shells [1]. Calcium carbonate dissolves from concrete that is wet with rain, so it can also be a source of material. The connection to why snails increase during the rainy season is only a speculative explanation [6].
However, concrete is not a staple food. You should understand this as "they eat it sometimes." Snails also sometimes eat the shells of dead snails or unused shells to replenish calcium [4].
If the source of materials makes a difference, does it show up in the shape of the shell?
6. How Does Low Soil Calcium Affect Snail Shells?
In a survey of the land snail *Notodiscus hookeri* in sub-Antarctic regions, shells tended to be thicker in places with more exchangeable calcium in the soil. For shells measuring 4.0 to 6.5 mm in size, the thickness was about 35 to 72 µm (0.035 to 0.072 mm) in calcium-rich soil. In soil with less calcium, the thickness was about 35 to 41 µm [7]. A thickness of 72 µm is thinner than ordinary copy paper. Comparing the higher values, there is a difference of about 1.8 times.
On the other hand, in places where calcium was scarce, shells tended to be thinner but larger [7].
Note: This means that under conditions with less material, the same shell might distribute its resources differently between thickness and size. [7]
How far can we generalize this? This is an example of one species. We cannot say for certain that it applies to all common snails. Still, the view that "since calcium is the material, the amount in the soil may relate to the shell’s shape" is supported by one survey.
7. How to Compare Shell Openings of Snails You Find
If you find a snail, do not grab it with your hands. Instead, observe the edge of the shell opening. Is it flared, or is it thin? If you compare several snails in the same place, you will notice individual differences in the shape of the edge. If you touch the snail, wash your hands.
A flared edge is one clue that a snail is an adult, but if it is not flared, you cannot decide it is a child. You cannot fully interpret the shape of the edge unless you know what kind of mark the specific species creates.
You can draw the shape of the shell opening on paper at home, noting if it is flared, thick, or thin. This allows you to compare them later. As the number of observations increases, you will realize that "you cannot decide based on the edge alone."
For the Japanese materials that formed the basis of this article, you can read HondaWoods’ "Snail Special Skills: Why? What?" [1].