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While snails are born with shells, they are often so small and translucent that spotting a newborn can be challenging. For instance, the species Angustopila psammion hatches with a shell less than 0.5 mm across and an egg the size of a grain of sand. Newborn shells typically lack the vibrant hues seen in adults, rendering them nearly invisible to the naked eye.
The diversity of gastropods—estimated at 40,000 to 60,000 species—means that shell characteristics vary widely. Newly described species such as Figuladra robertirwini, named after wildlife broadcaster Robert Irwin, and well‑known garden snails illustrate this range. In taxonomy, snails and slugs both belong to the class Gastropoda, with the defining difference being the presence of a protective shell.
However, the boundary between slugs and snails can be blurred. Many terrestrial slugs evolved from shelled ancestors and retain vestigial shells—sometimes too small for full retraction, as in semi‑slugs, or reduced to a tiny internal structure. For our discussion, we focus on snails capable of fully retracting into their shells.
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Unlike hermit crabs that adopt empty shells, snails manufacture their own. As the body expands, new shell material is added to the opening, creating additional whorls that grow forward rather than outward.
The journey begins with the protoconch—a miniature embryonic shell that serves as the foundation for subsequent growth. Some species’ protoconches already display multiple whorls, while others start with a single spiral. Although initially soft and lightly pigmented, the protoconch rapidly hardens after birth.
Immediately after hatching, young snails seek calcium‑rich food to fortify their shells. The first meal often consists of the calcium of their own eggshell. They also graze on bone fragments, rocks, soil, and even conspecific shells. The mantle processes the ingested calcium, depositing it at the protoconch aperture and building a durable shell within days.
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Snails and slugs form the class Gastropoda, one of seven classes within the Mollusca phylum, which also includes cephalopods and bivalves. The common ancestor of all mollusks likely possessed a mantle and a soft, moist body—features that persist in modern species. When gastropods transitioned to terrestrial life, the evolution of a watertight shell became essential for protection and, in some cases, locomotion.
Other mollusks also grow shells. The nautilus constructs its chambered shell while still in the egg, and this structure is vital for buoyancy control. Bivalves such as oysters and scallops develop their shells shortly after hatching; oysters’ shells harden within 12 hours, anchoring them as filter‑feeders, while scallops maintain mobility thanks to their hinge‑formed shell. In contrast, octopuses remain entirely soft-bodied, and squids possess only a reduced internal shell.
