Slug vs Snail: Shells, Bodies and Key Differences

Key differences at a glance

Slugs and snails are both gastropods, the largest group of mollusks. They share the same basic body plan: a muscular foot, a head with tentacles, a mantle that covers the internal organs, a lung for breathing air (in land species) and a radula, a tongue covered in tiny teeth. The differences come mainly from that one feature, the shell, and from how each animal copes without it or with it.

To compare real animals, this page uses two familiar species: the leopard slug (Limax maximus) and the garden snail (Cornu aspersum). Both are native to Europe and have been introduced to North America and many other regions.

Shell versus mantle

A snail’s shell is part of its body, not a house it can leave. It is made mostly of calcium carbonate and grows with the snail, which adds new material at the opening. The body is attached inside by a strong muscle, and many organs sit permanently in the coils. When threatened, the snail pulls its head and foot inside. How shells are built is explained in how do snails grow their shells?

A slug has a mantle on its back instead, a fleshy shield behind the head. In keeled slugs such as the leopard slug, the mantle hides a thin, flat shell plate about 13 mm (1/2 in) long. Roundback slugs (Arion) have only scattered chalky grains, and some slugs have no shell at all. In both slugs and snails, the breathing pore, or pneumostome, opens on the right side of the mantle.

Between the two lie the semi-slugs, which have a small external shell that is too small for them to retreat into.

Why did some snails become slugs?

Slugs are not one natural group. Shell loss has evolved many separate times in different families, on land and in the sea. That is why some slugs are more closely related to certain snails than to other slugs.

Losing the shell has clear advantages. A shell needs a lot of calcium, which is scarce in acidic soils, and carrying it costs energy. Without a shell, a slug can squeeze into narrow cracks in soil, wood and stone, and can burrow deep underground to escape frost and drought. The disadvantage is that a slug has no armor against predators and no portable shelter from drying out.

Coping with drought and cold

Both slugs and snails lose water through their skin and slime, so both are most active at night and after rain. Their strategies for bad weather differ.

A snail can retreat into its shell and seal the opening with a thin, papery lid of dried mucus called an epiphragm. Inside, it can remain inactive for weeks or months, a state called aestivation in summer and hibernation in winter. Garden snails often cluster in sheltered spots to do this.

A slug has no such shelter, so it must hide. Slugs move into soil, under logs, stones and leaf litter, into wall cracks and cellars, and coat themselves with thick mucus that slows water loss. Slugs therefore depend more on damp microhabitats, while snails can survive more exposed places, such as dry grassland and sand dunes.

Diet: what slugs and snails eat

Most slugs and snails are generalist plant-eaters and decomposers. They eat living and decaying leaves, fungi, algae, fallen fruit and seedlings, scraping food with the radula. You can read about the radula in do snails have teeth?

Each group also includes predators. The leopard slug eats fungi and rotting plants but also chases other slugs. Shelled slugs of the genus Testacella hunt earthworms underground. Among snails, the rosy wolfsnail of the southeastern United States eats other snails and slugs.

Mating and reproduction

Nearly all land slugs and snails are hermaphrodites, with male and female organs in each adult. They usually mate in pairs, exchanging sperm, and both partners later lay eggs in damp soil.

Their courtships can be remarkable. Garden snails stab each other with a chalky “love dart” before exchanging sperm. Leopard slugs mate while hanging in mid-air from a rope of slime, as described in how do leopard slugs mate? Neither group cares for its eggs after laying them. Young snails hatch with a tiny, delicate shell that grows as they do; young slugs hatch as miniature versions of the adults.

Speed, size and predators

Slugs and snails move the same way, on waves of muscle contraction along the foot over a layer of mucus. Both are slow. Garden snails in races reach only a few millimeters per second, and a hunting leopard slug manages about 15 cm (6 in) per minute.

Size depends on the species, not on the shell. The largest land slugs reach 25–30 cm (10–12 in) long when stretched out, while the largest land snails, such as the giant African land snail, have shells up to about 20 cm (8 in) long.

Both are eaten by hedgehogs, shrews, toads, slow worms, ground beetles and birds. Song thrushes are famous for smashing snail shells on a favorite stone, called an anvil. Without a shell, slugs rely on hiding, sticky or bad-tasting mucus and, in some species, on contracting into a tough, rounded shape.

Can slugs and snails interbreed?

No. Slugs and snails are separate species, often from different families, and cannot mate with each other. Even very similar slug species usually differ in courtship and in the shape of their reproductive organs, which helps keep species apart.

Which is better for the garden?

Neither is simply good or bad. Many slugs and snails damage soft plants and seedlings, but others feed mainly on fungi, rotting plants and algae, recycling nutrients into the soil. Both are also important food for birds, mammals, amphibians and beetles, so a garden with slugs and snails supports many other animals.