While often confused, slugs and snails are not the same animal, though they are closely related. The primary distinction lies in the presence or absence of a prominent external shell, which profoundly influences their biology and behavior.
The Gastropod Connection
Both slugs and snails are members of the class Gastropoda, which is the largest and most diverse class within the phylum Mollusca. Gastropods are characterized by a distinct head, a ventral muscular foot used for movement, and a visceral hump containing their internal organs. A defining developmental process in gastropods is torsion, a 180-degree twisting of their internal organs during development. All gastropods possess a radula, a ribbon-like structure with microscopic teeth used for feeding.
Understanding Snails
Snails are gastropods primarily identified by their prominent, typically coiled external shell. This shell, composed mainly of calcium carbonate, is an integral part of their anatomy, providing physical protection from predators and a retreat during unfavorable environmental conditions like dry weather. The snail can retract its soft body completely into this shell for safety and to prevent desiccation.
The presence of this rigid shell influences a snail’s movement, which involves rhythmic muscular contractions of its foot while secreting a layer of mucus to reduce friction. Their shell also affects their habitat preferences, as they often seek calcium-rich environments to maintain and grow their shells. Most snails are herbivorous, grazing on plants, algae, or decaying organic matter using their radula.
Understanding Slugs
Slugs, while also gastropods, are distinguished by the reduction or complete absence of a prominent external shell. Many slug species possess only a small, internal shell remnant embedded within their mantle, which serves primarily for calcium storage. This lack of a large external shell means slugs are more susceptible to desiccation, requiring them to inhabit moist environments and often being most active at night or after rain.
The absence of a bulky shell offers slugs increased flexibility and maneuverability, allowing them to access tighter spaces that shelled snails cannot. To compensate for the lack of physical protection, slugs often rely on other defensive mechanisms, such as producing large amounts of slime, which can be unpalatable or toxic to predators. Their diet is varied, with many species being generalists that consume decaying plant material, fungi, or living plants.