Head to head
Slug vs Garden Snail
A slug is, quite literally, a snail without a shell. The two share the same body, the same muscular foot, the same slime, the same rasping tongue, and the same tentacles with eyes on the tips. The one real difference, the presence or absence of a coiled external shell, drives almost everything else about how they live.
| Slug | Garden Snail | |
|---|---|---|
| The one difference | No usable external shell | A coiled shell it can withdraw into |
| Shell remnant | Often a small internal plate under the shield | The full external shell |
| Surviving drought and winter | Burrows down into the soil | Seals the shell with a mucus lid and waits |
| Where it hides by day | Narrow damp cracks in soil, walls, and wood | Under leaves, in crevices, glued to walls |
| Water loss | Fast; more strictly nocturnal | Slower; can be out in milder conditions |
| Evolved how | A body form reached many times from snails | The ancestral gastropod condition |
| Diet | Mostly decaying matter; a few pest species | Mostly decaying matter; a few pest species |
| Everything else | Foot, slime, radula, tentacles, hermaphrodite | The same |
Slug vs Snail: common questions
Is a slug just a snail without a shell?
Essentially yes. A slug and a snail are the same type of animal, a land gastropod mollusc, and share the same body plan in every respect except the shell. Slug is not the name of one group of animals but a description of a shape, and it has evolved independently in many different snail families, each time by the shell shrinking and finally disappearing or becoming a small internal remnant. Some in-between animals, the semi-slugs, still have a visible shell that is too small to hide in. So a slug is best understood as one of several possible outcomes of the same family, produced whenever the advantages of not carrying a shell outweigh the protection it gives.
Why would an animal lose a protective shell?
Because a shell has real costs as well as benefits. Building and carrying a large calcium-carbonate shell takes energy and a steady supply of calcium, which is scarce in acidic soils and many leaf-litter habitats, and the shell adds bulk that stops the animal from squeezing into the narrowest, dampest, safest cracks. For a gastropod living in a consistently moist environment with plenty of tight hiding places, such as deep leaf litter, rotting wood, or soil, the shell may protect against fewer threats than it hinders, and natural selection has repeatedly favoured individuals with smaller shells until the shell is effectively gone. The trade-off is a much greater risk of drying out, which is why slugs are so tied to damp conditions and darkness.
Do slugs and snails do equal damage in a garden?
Roughly, and in both cases the damage is done by a small minority of species. Of the dozens of slugs and snails in a typical garden, only a few, such as the grey field slug and, among snails, the garden snail feeding on soft young growth, cause meaningful harm to living plants, and they concentrate on seedlings, salad leaves, and tender shoots. The majority feed mainly on dead and rotting material and are useful recyclers, and some large slugs even hunt the pest species. Control that targets the whole group, such as scattered pellets, therefore kills a lot of harmless and beneficial animals, so barriers around vulnerable plants, encouraging predators, and hand-collecting after dark are better approaches.