Slugs are a common sight in gardens and damp environments. While slugs appear to lack a shell, unlike snails, the answer is nuanced. A slug is a gastropod mollusk, belonging to the same broad group of animals as snails, but typically characterized by the absence of a visible, external shell.
The Hidden Remnant: Internal Shells
Although slugs do not carry an external shell, many species retain a small, internal, often vestigial shell or calcareous plate. This remnant is usually embedded within the mantle, a fleshy area behind the slug’s head. This internal structure stores calcium salts and provides structural support. This internal shell is a significant clue to the evolutionary history of slugs. All terrestrial slugs evolved directly from shelled terrestrial snails, with the reduction or loss of the large external shell occurring over millions of years. The shell-less condition has arisen independently multiple times across different slug lineages. However, this internal remnant does not offer the same external physical protection against predators or desiccation that a snail’s prominent shell provides.
Slugs and Snails: Understanding the Distinction
The primary difference between slugs and snails is the prominent, external, coiled shell found in snails, which is absent or greatly reduced in slugs. A snail’s shell functions as a mobile home, allowing it to fully retract its soft body for protection from predators and to conserve moisture. This ability makes snails more resilient to dry conditions. Conversely, without a large external shell, slugs gain advantages in mobility and access to confined spaces. They can squeeze into tight crevices under rocks, logs, or soil where shelled snails cannot. This difference impacts their appearance, habitat preferences, and overall lifestyle. Slugs rely on other methods to survive where snails depend on their hard, protective coverings.
Survival Strategies Without an Outer Shell
Without the physical defense of an external shell, slugs have developed several adaptive strategies to protect themselves and thrive. A primary defense involves copious mucus production, a slimy substance serving multiple purposes. This mucus lubricates movement, allowing slugs to glide across surfaces while protecting their delicate skin from abrasion and desiccation. When threatened, many slugs secrete a thicker, stickier, and sometimes foul-tasting defense mucus. This specialized slime makes them difficult for predators to grasp, unpalatable, and may even trap smaller insects. Beyond mucus, slugs employ avoidance behaviors, often being nocturnal to escape drying effects and avoid diurnal predators. During the day, slugs seek shelter under rocks, logs, leaf litter, or by burrowing into the soil to maintain moisture and avoid detection. Their coloration provides natural camouflage, allowing them to blend seamlessly with their environment. Some slug species also possess chemical defenses, producing distasteful or toxic secretions that deter potential predators, adding another layer of protection.