The main difference between snails and slugs is the shell: snails carry a coiled shell on their backs, while slugs have either a tiny internal shell remnant or no shell at all. Beyond that single visible distinction, these two creatures are closely related gastropods that diverge in surprising ways when it comes to survival strategies, movement, habitat preferences, and lifespan.
They’re Closer Relatives Than You’d Think
Snails and slugs both belong to the class Gastropoda, a massive group of soft-bodied mollusks. Slugs aren’t a separate evolutionary line that developed independently. They evolved from shelled ancestors, losing their shells over time in a process that has happened repeatedly across different gastropod lineages, both on land and in the sea. Marine slugs (nudibranchs) lost their shells separately from land slugs, and even among land species, shell loss occurred more than once. In evolutionary terms, “slug” is less a strict category and more a body plan that keeps re-emerging because, under the right conditions, ditching the shell works.
What the Shell Actually Does
A snail’s shell is 87 to 96 percent calcium carbonate by weight, reinforced with smaller amounts of magnesium, chitin, and other bioactive compounds. It serves as a portable fortress against predators, physical impacts, and dehydration. When conditions turn harsh, snails can seal themselves inside with a mucus plug and enter a dormant state. Some freshwater snails survive droughts lasting weeks by estivating (a summer version of hibernation), dropping their metabolism to near zero and ramping up internal antioxidants to protect their tissues until water returns. Certain species tolerate temperatures as low as 4°C by hibernating inside their shells the same way.
That kind of extreme shutdown is much harder for slugs. Without a sealed chamber to retreat into, they’re far more vulnerable to drying out and temperature swings, which is why slugs tend to be most active at night or after rain.
How Slugs Survive Without a Shell
Losing the shell sounds like a terrible idea, but slugs compensate with a toolkit of alternative defenses. The most important is mucus. Both snails and slugs produce slime that is 90 to 99.7 percent water, with the remaining fraction packed with glycoproteins, antimicrobial peptides, enzymes, and metal ions. This slime helps with locomotion, moisture retention, and protection against bacteria and fungi. For slugs, though, the mucus layer takes on extra defensive roles since there’s no shell to fall back on.
Some slug species go further. Certain terrestrial slugs produce toxic compounds, specifically terpenes, that make them unpalatable or outright dangerous to predators. This mirrors what nudibranchs do in the ocean: sequestering or manufacturing chemical toxins as a replacement for the armor they lost. Several snake species that specialize in eating slugs have evolved tolerance to these toxins, which is indirect evidence of just how effective the chemical defense is.
Slugs can also squeeze into tight spaces that a shelled snail could never fit, like narrow crevices under bark, inside rotting logs, or between rocks. This flexibility lets them access food sources and hiding spots that snails are locked out of.
Movement and Energy Costs
Both snails and slugs glide on a layer of pedal mucus, contracting muscles along the bottom of their foot in rhythmic waves. But this mode of travel is expensive. Research on terrestrial slugs found that crawling costs about 904 joules per kilogram per meter, which is considerably higher than most other forms of animal locomotion. The bulk of that energy goes toward producing the mucus trail itself, not the muscular effort of moving forward.
Snails pay a similar mucus cost plus the additional burden of hauling a heavy calcium carbonate shell. Slugs, freed from that weight, can move somewhat more efficiently over short distances and navigate vertical or uneven surfaces more easily. The tradeoff is that slugs need to produce even more mucus for moisture protection, since they lack the shell’s ability to seal in water.
Where Each One Thrives
Snails are more tolerant of dry, exposed environments because they can withdraw into their shells and wait out unfavorable conditions. You’ll find garden snails in sunny Mediterranean climates, arid grasslands, and open yards. Slugs, by contrast, strongly prefer moist, shaded habitats. Forest slug communities organize themselves along gradients of moisture and food availability, with many species gravitating toward decaying wood and leaf litter where fungi and microbial biofilms grow. Old-growth forests with closed canopies and plenty of deadwood are particularly slug-friendly environments.
This habitat difference is why gardeners in rainy climates tend to battle slugs, while those in drier regions deal more with snails.
Diet and Garden Damage
Snails and slugs are mostly omnivores. The majority feed on fungi, decaying plant material, and soil organisms. Some are predatory, eating other snails. Only a relatively small number of species actually damage living plants, though those few can be prolific pests. Both leave the same signature damage: irregular, oblong holes chewed through the margins and centers of leaves and flowers, sometimes eating only one side of thicker leaves.
Because their diets overlap so heavily, the practical difference in your garden is less about what they eat and more about when and where. Slugs do most of their feeding at night in damp conditions, often targeting seedlings and tender new growth close to the soil. Snails feed in similar patterns but may climb higher on plants and garden structures, using their shells as a safe retreat when the sun comes up.
Lifespan and Reproduction
Slugs generally live faster and die younger. Many slug species reach reproductive maturity in just 3 to 6 months, while the common brown garden snail takes about 2 years to mature. Slug lifespans typically range from 1 to 5 years depending on species, whereas some snail species live a decade or more in favorable conditions. The shell’s protection from predators and environmental stress likely contributes to this longevity gap.
Both snails and slugs are often hermaphrodites, carrying both male and female reproductive organs. They still typically mate with a partner to exchange genetic material, and both lay clusters of small, translucent eggs in moist soil or under debris. Slugs’ faster maturation means they can produce new generations more quickly, which partly explains why slug populations seem to explode overnight after a stretch of wet weather.