How Do Snails Grow Their Shells? Calcium, Coils and Repairs

What is a snail shell made of?

A snail shell is mostly calcium carbonate, the mineral found in chalk, limestone and eggshells. The mineral is laid down as tiny crystals within a framework of protein, which makes the shell tougher and less brittle than pure mineral would be.

The shell has several layers. On the outside is the periostracum, a thin skin of protein that protects the mineral from wear and from being dissolved by acidic water, and that carries much of the color and pattern. Beneath it are layers of calcium carbonate, with the crystals arranged in different directions in each layer, a bit like plywood. The innermost layer, in contact with the body, is smooth.

How does a baby snail get its shell?

A land snail begins making its shell while it is still inside the egg. A garden snail hatches with a tiny, thin, almost transparent shell. This embryonic shell, called the protoconch, is the tip, or apex, of the shell on every adult. It is the oldest part of the shell, and it often looks smoother than the rest.

Newly hatched snails need calcium straight away. Many begin by eating their own eggshells, which are themselves rich in calcium carbonate, before they start feeding on plants.

How does the shell grow?

All new growth happens at the rim of the opening, called the aperture. The edge of the mantle, which lies against the inside of the shell, adds a thin band of new material there, first the outer protein skin and then the mineral layers. Each new band is a little wider than the one before, so the shell expands outward. Fine lines across the shell surface, called growth lines, mark successive additions, a bit like the rings of a tree.

The spiral shape comes from uneven growth. The mantle adds material faster on one side of the rim than the other, so the growing tube curves and winds around a central pillar called the columella. Each complete turn is a whorl, and a garden snail’s shell has four to five. The mantle also thickens older parts of the shell from inside.

Some snails keep growing all their lives. Others, including the garden snail, stop when they become adults. At that point a garden snail builds a thickened, white lip around the opening and adds no more length to the shell. A lipped shell is therefore a sign of an adult, as explained in how long do snails live?

Where do snails get calcium?

Snails need a lot of calcium for their shells and their eggs. They get it from plants, from licking and eating chalky soil, stones and old mortar, and sometimes from empty shells of other snails. Where the soil is rich in lime, land snails tend to be more common and to have thicker shells; on acidic soils, they are often scarcer and thinner-shelled.

This affects other animals too. In the 1990s, Dutch scientists found that great tits and other small birds in forests on acidified soils were laying eggs with thin, weak shells. Acid rain had leached calcium from the soil, snails had become scarce, and the birds were missing the snail shells they normally eat as a source of calcium in the breeding season.

Why do most snail shells coil to the right?

Hold a snail shell with the tip pointing up and the opening facing you. In most species the opening is on the right, so the shell is called right-handed, or dextral. More than 90% of snail species are right-handed, although some species are always left-handed.

The direction of coiling is set very early, in the first few cell divisions of the embryo. In the great pond snail, it is controlled by a single gene, and unusually, by the mother’s version of that gene rather than the baby’s own. Left-handed individuals of normally right-handed species are rare. In garden snails, scientists estimate about one in 40,000. In 2016, a left-coiled garden snail found in London and nicknamed Jeremy became world news when scientists appealed for another “lefty” to mate with it, because a left- and a right-coiled snail have great difficulty mating. Jeremy’s 56 offspring all had right-coiled shells.

Spirals in shells are often linked to mathematical patterns. Whether the famous nautilus shell follows the golden ratio is checked in is the nautilus shell a golden spiral?

Can a snail repair its shell?

Yes, within limits. If a shell is cracked or holed, the mantle underneath can lay down new shell material from the inside to seal the damage. The repair usually looks different from the original shell, often paler, rougher or lumpy, and many wild snails carry such scars from falls, stones, birds or small mammals. A snail needs calcium to repair its shell, and it cannot rebuild missing parts of the original surface pattern.

Damage that breaks into the upper coils, where the internal organs lie, or that injures the soft body, is far more serious, and snails often die from it.

Why can’t a snail leave its shell?

A snail’s shell is not a house it can move out of, as cartoons sometimes suggest. The body is attached inside by the columellar muscle, which runs along the central pillar and pulls the snail back into its shell when it is threatened. The heart, kidney, digestive gland and reproductive organs sit permanently in the upper coils, in a part of the body called the visceral mass. A snail pulled from its shell would die.

Hermit crabs, which do move from shell to shell, use the empty shells of dead sea snails. Slugs, meanwhile, evolved from snails by reducing their shells over millions of years, as explained in slug vs snail.