How Do Snails Develop Their Shells?

Many pet owners enjoy pampering their hermit crabs. A simple online search reveals a variety of decorative shells designed specifically for these tiny crustaceans. These ornate shells are in demand because hermit crabs typically inhabit abandoned shells from other creatures. If removed from their borrowed homes, these crabs will quickly seek out a replacement.

Snails, however, lack this luxury. These slow-moving mollusks are born with and grow their own protective shells, to which they are permanently attached. Separation from their shell is fatal, as snails cannot survive without their natural, calcium-based shelter. But how do these shells form, and what makes them unique compared to other forms of animal protection?

Before diving deeper, let’s cover some snail basics: There are potentially up to 43,000 snail species. While many of us recognize land or pond-dwelling snails, marine varieties also thrive. Reproduction methods vary, with some species relying on sexual reproduction and others being self-fertilizing hermaphrodites. Whatever works, right?

One universal trait among snails is that they all emerge from eggs. Typically, these are laid in soft soil or attached to rocks, though some species are ovoviviparous, meaning eggs hatch inside the mother before the young are born. Once out, the newborns must navigate their new environment.

Returning to the shell question, a snail’s shell starts developing during gestation. The mantle, a vital organ in mollusks like snails, is responsible for shell creation. Composed mainly of calcium carbonate with traces of protein, the shell forms as the mantle generates an electric current to position calcium ions effectively.

Prior to hatching, a baby snail forms a protoconch, the initial layer of its shell. After emerging from the egg, the snail’s diet becomes critical. The mantle needs more calcium to grow and fortify the shell. Newborns instinctively consume the calcium-rich remnants of their egg, establishing a lifelong dietary pattern.

Throughout their lives, snails must consume calcium-rich foods. This dietary need often labels them as pests, as they frequently target calcium-loaded crops like spinach, broccoli, and turnips. They also obtain calcium by ingesting soil or chewing on limestone.

The protoconch features a tiny opening, or "mouth." From beneath, the mantle continuously deposits fresh layers of calcium carbonate and proteins around this opening. As these materials harden at the mouth, the shell expands. Spiraling coils develop around the protoconch, rotating progressively to form the apex, or the highest point, of the snail's ever-growing shell. Depending on the species, the protoconch may either stay intact indefinitely or break off eventually.

Unlike nautilus shells, snail shells consistently spiral either to the left or right, a trait that varies by species. While most snails have shells that coil to the right, some species exhibit leftward spirals, and in rare cases, the direction can vary within a species.

Examining a cross-section of a snail shell reveals multiple distinct layers. The outermost layer, called the periostracum, is a thin outer layer typically composed of organic material. Beneath it lies a sturdy layer of calcium carbonate, supported by a foundation of nacre, a durable material commonly referred to as "mother of pearl."

However, unlike turtle shells, snail shells lack nerves and blood vessels. Turtle shells are essentially fused bones, such as ribs and vertebrae, covered by hard plates. When damaged, turtles can repair their shells biologically. Snails, on the other hand, rely on calcium and protein secretions from their mantles to mend cracks and reinforce weakened areas.

Despite their self-healing capabilities, snail shells don’t ensure complete safety. Ironically, many birds prey on snails because their calcium-rich shells provide essential nutrients. To evade predators, some snails have evolved remarkable defenses. For instance, the deep-sea species Chrysomallon squamiferum is covered in iron sulfide, giving it a metallic appearance. Scientists are exploring how this unique adaptation could inspire better armor for human military use, continuing a long tradition of drawing inspiration from nature.