Slugs and snails are familiar garden and landscape occupants, often seen gliding across surfaces. Their similar appearance often leads to questions about whether they are the same. While they share many characteristics, a fundamental distinction defines each group.
The Defining Distinction
The primary difference between a slug and a snail lies in the presence and nature of their shells. Snails possess a prominent, external, coiled shell that serves as a protective home. Composed primarily of calcium carbonate, this calcified structure allows the snail to fully retract its soft body when threatened or facing adverse conditions like heat and dryness. It also provides defense against mechanical damage.
In contrast, slugs lack an external shell. Most slug species retain a remnant of a shell, known as a vestigial or reduced internal shell, usually hidden beneath the mantle. This internal structure often functions more for calcium storage than protection. Some slug species have no shell whatsoever.
Despite this clear difference, slugs are not simply snails that lost their shells; they represent a distinct evolutionary path. Slugs evolved from shelled ancestors, with the reduction or internalization of the shell occurring independently multiple times across different gastropod lineages over millions of years. This evolutionary change reflects adaptations to different lifestyles and environments.
Shared Biological Traits
Both slugs and snails belong to the same large biological group, Gastropoda, a class within the phylum Mollusca. The name “gastropod” translates from Greek as “stomach-foot,” referencing their anatomy where the body and internal organs are positioned above a large, muscular foot. This muscular foot is used for locomotion, propelling the animal forward through rhythmic muscular contraction.
As they move, both slugs and snails secrete mucus, commonly known as slime. This mucus serves multiple purposes: as a lubricant to reduce friction, an adhesive for climbing, and a protective barrier against sharp objects and moisture loss. The slime is primarily water, typically 97% to 99% of its composition.
These creatures also share similar sensory organs, featuring two pairs of retractable tentacles on their heads. The longer, upper pair typically contains light-sensing eyespots, while the shorter, lower pair is used for detecting smells and tactile exploration. This allows them to navigate and locate food sources.
Their diets are largely similar; most species are herbivorous or detritivorous, feeding on living plants, decaying organic matter, and fungi. They often prefer moist environments, found under logs, rocks, or in leaf litter, where humidity is consistently high.
Subtle Differences and Adaptations
The presence or absence of a shell leads to further subtle differences in the adaptations and lifestyles of slugs and snails. Lacking a bulky external shell, slugs gain flexibility. This allows them to squeeze into narrower crevices and tighter hiding spaces, such as under loose bark or stones, providing an advantage in avoiding predators and finding shelter.
However, this lack of an external shell also makes slugs more vulnerable to desiccation. They must actively seek moist conditions and often hide during dry or sunny periods to conserve moisture. Their soft bodies are also more exposed to predators.
Conversely, snails, with their protective shells, are less susceptible to dehydration and offer a more robust defense against predators. A snail can completely withdraw into its shell, sealing the opening with a mucous plug (epiphragm) to endure dry spells. This protection comes at the cost of agility and access to confined spaces. Additionally, the shell requires significant calcium investment to build and maintain.