Insects & Invertebrates comparison

Snail vs Slug: Key Differences Explained

The short answer

A slug is essentially a snail without an external shell, and shell loss has happened repeatedly and independently in different gastropod lineages. The shell provides protection and reduces water loss but is heavy and requires calcium. Losing it allows a slug to squeeze into narrow spaces and survive in calcium-poor soils, at the cost of much greater vulnerability to drying out and to predators.

The main difference

A slug is a gastropod that has lost or greatly reduced the external shell, trading protection and water retention for flexibility and freedom from a calcium requirement.

The pair isolates a single evolutionary trade-off — keep the shell or lose it — and shows why both answers persist side by side in the same habitats.

At a glance

Comparison of Snail and Slug across 6 dimensions, with an interpretation of each difference.
What differs	Snail	Slug	What it means
Shell	External coiled shell the animal can withdraw into	Absent or reduced to a small internal remnant under the mantle	The presence or absence of an external shell is the whole distinction.
Water loss	Reduced — the shell limits evaporation and allows sealing during dry spells	High — must stay in damp conditions or shelter underground	Vulnerability to drying is the main cost of losing the shell.
Calcium requirement	Substantial, for building and maintaining the shell	Much lower	Slugs can thrive on acidic, calcium-poor soils where snails struggle.
Access to narrow spaces	Limited by shell width	Can compress the body into very narrow crevices and soil cracks	Squeezing into gaps provides shelter and access to food a snail cannot reach.
Defence	Withdraws into the shell and can seal the opening	Relies on mucus, distastefulness, camouflage and hiding	Some slugs produce notably thick or sticky mucus that deters predators.
Taxonomic status	Describes shelled gastropods generally, across many families	Describes a body form that has evolved independently in several lineages	Slug is not one natural group, so some slugs are closer to certain snails than to other slugs.

Shell: SnailExternal coiled shell the animal can withdraw into
SlugAbsent or reduced to a small internal remnant under the mantle
The presence or absence of an external shell is the whole distinction.
Water loss: SnailReduced — the shell limits evaporation and allows sealing during dry spells
SlugHigh — must stay in damp conditions or shelter underground
Vulnerability to drying is the main cost of losing the shell.
Calcium requirement: SnailSubstantial, for building and maintaining the shell
SlugMuch lower
Slugs can thrive on acidic, calcium-poor soils where snails struggle.
Access to narrow spaces: SnailLimited by shell width
SlugCan compress the body into very narrow crevices and soil cracks
Squeezing into gaps provides shelter and access to food a snail cannot reach.
Defence: SnailWithdraws into the shell and can seal the opening
SlugRelies on mucus, distastefulness, camouflage and hiding
Some slugs produce notably thick or sticky mucus that deters predators.
Taxonomic status: SnailDescribes shelled gastropods generally, across many families
SlugDescribes a body form that has evolved independently in several lineages
Slug is not one natural group, so some slugs are closer to certain snails than to other slugs.

How they are related

Both are gastropod molluscs. Slug is not a single natural group: shell reduction and loss have occurred independently in several gastropod lineages, so a given slug may be more closely related to a particular snail than to another slug. The same is true in the sea, where sea slugs arose separately from land slugs.

Telling them apart

The presence of an external shell settles it. Semi-slugs complicate matters slightly, carrying a shell too small to withdraw into, and represent an intermediate stage. A small remnant shell is often present under the mantle shield of a true slug but is not visible externally.

External coiled shell: snail. No visible shell, or a shell too small to withdraw into: slug. The relationship is not one of two separate groups but of a shell that some gastropod lineages kept and others reduced or abandoned, more than once and in different places.

Habitat and distribution

Both occupy damp habitats — gardens, woodland, hedgerows and leaf litter — and both are most active at night and in wet weather. Slugs are more dependent on continuous moisture and shelter in soil and under objects during dry periods. Snails can seal the shell opening with a mucus membrane and remain dormant through drought or cold.

Diet and ecological role

Both are chiefly herbivores and detritivores, rasping plant material, fungi and decaying matter with a radula, a ribbon of tiny teeth. Some species are omnivorous or predatory, taking earthworms and other slugs. Both are important decomposers, and both are garden pests where they feed on cultivated plants.

Behaviour

Both move on a muscular foot over a layer of mucus that reduces friction and allows travel over rough or sharp surfaces. Most land species are hermaphrodites, and courtship in some is elaborate and prolonged. Both navigate using chemical trails, and some species follow their own or others' mucus trails back to shelter.

Senses and adaptations

Land species carry eyes on the tips of the upper pair of retractable tentacles, with the lower pair used for smell and touch. Vision is limited to light and coarse shape, and chemical sensing does most of the work in locating food and mates. Mucus is a sophisticated material that changes properties under pressure, allowing both adhesion and gliding.

More on this topic: Animal senses and adaptations.

People and these animals

Both are significant agricultural and garden pests, and both are food for birds, mammals, amphibians and beetles. Some species are intermediate hosts for parasites affecting livestock and, in a few cases, people, which is why hand-to-mouth contact after handling should be avoided. Several introduced species have become invasive.

What they have in common

Both are gastropod molluscs that move on a muscular foot over a mucus layer.
Both feed using a radula, a ribbon of tiny teeth that rasps food.
Both are most active at night and in damp conditions and depend on moisture.
Most land species of both are hermaphrodites with elaborate courtship.

Key differences

Snails have an external shell they can withdraw into; slugs do not.
Slugs lose water far more readily and depend on continuously damp shelter.
Snails need substantial calcium for the shell, while slugs need much less.
Slugs can compress into narrow crevices that a shelled snail cannot enter.
Snails can seal the shell and remain dormant through drought or cold.

Where people go wrong

Assuming slugs form a single natural group, when the body form arose several times.
Believing slugs have no shell at all, when many retain an internal remnant.
Grouping sea slugs with land slugs as close relatives.

Frequently asked questions

Is a slug just a snail without a shell?

Functionally that is a good description, though the history is more interesting. Shell reduction and loss have occurred independently in several gastropod lineages, so slugs are not one natural group. Many slugs retain a small internal shell remnant beneath the mantle. It is a body form that different lineages have converged on rather than a single evolutionary event.

Why would a snail lose its shell?

Because the shell has costs as well as benefits. It is heavy, restricts access to narrow spaces, and requires substantial calcium to build and maintain — a real constraint on acidic or calcium-poor soils. Losing it allows the animal to compress into soil cracks and crevices for shelter and food. The price is much greater vulnerability to drying out and to predators.

Why do both still exist if one strategy is better?

Because neither is better overall — each suits different conditions. Snails do well where calcium is available and dry or cold periods must be survived, since sealing the shell allows dormancy. Slugs do well in consistently damp, calcium-poor habitats where squeezing into crevices matters more than armour. Both strategies persist because both work in the circumstances that favour them.

Are sea slugs related to garden slugs?

Both are gastropods that lost the external shell, but they did so independently in separate lineages, so they are not closely related to each other. Sea slugs include nudibranchs, many of which are brightly coloured and chemically defended, some by incorporating stinging cells from cnidarian prey. The shared name reflects a shared body form rather than shared ancestry.

Read the full profiles

Sources and methodology

How this comparison is built: FaunaHub comparisons are assembled from the sourced animal profiles behind them rather than written as standalone claims. Figures such as size, mass, and lifespan are typical ranges that vary by subspecies, population, region, sex, age, and — where relevant — whether an animal lives in the wild or under human care, so they are presented as ranges rather than fixed values. Where a common name covers a group rather than a single species, the page says so instead of implying a species-to-species match. Conservation status changes over time and should be confirmed against the current IUCN Red List. Dimensions that could not be supported from the underlying profiles are omitted rather than filled in.

What this page does not do: it does not rank the two animals, predict the outcome of a confrontation, or present either one as stronger, smarter, or better. It is not veterinary, medical, first-aid, handling, capture, hunting, or pest-control guidance. For an animal in your care, or any situation involving injury, contact a qualified veterinarian, your local wildlife authority, or emergency services.

Sources and further reading

Authoritative references used for general educational context. External links open in a new tab. These sources do not endorse FaunaHub.

UniversityAnimal Diversity Web — University of Michigan Museum of Zoology — Peer-edited reference accounts for animal species
ReferenceBritannica — Snail — Editor-reviewed encyclopedia entry
Wildlife referenceIUCN Red List of Threatened Species — Authoritative source for current conservation status
ReferenceBritannica — Slug — Editor-reviewed encyclopedia entry
ReferenceEncyclopaedia Britannica — Animals reference — Editor-reviewed encyclopedia overview entries
ReferenceWoRMS — World Register of Marine Species — Authoritative register of marine species names

Last updated: 2026-08-04