The terrestrial slug, a soft-bodied mollusk, navigates the world without the protective shell of its snail relatives. This shell-less existence makes the slug vulnerable to environmental challenges, particularly desiccation and physical harm. Survival relies on the constant production of a specialized, complex fluid: slime. This glistening substance is an engineered hydrogel necessary for the slug’s survival, facilitating movement, defense, and communication.
The Chemistry of Slime
Slug slime is classified as a hydrogel because it is composed primarily of water, generally ranging from 91% to 99% of its total mass. The remaining small percentage of solid material provides the unique physical characteristics of the mucus. The most significant components are complex polymers known as mucins, which are heavily glycosylated glycoproteins. These large molecules contain long, O-linked carbohydrate chains responsible for the substance’s gel-forming properties.
The mucins interact to create a double-network structure, forming a three-dimensional skeleton that binds water and other components, including peptides, lipids, and metal ions. This molecular architecture gives the slime its viscoelastic nature, meaning it exhibits properties of both a viscous fluid and an elastic solid. It can flow like a liquid when under stress but maintains its shape and acts like a solid adhesive when at rest.
Slime’s Function in Locomotion
The slug’s movement is known as adhesive locomotion, relying entirely on the unique rheology of its pedal mucus. The slime acts as a layer between the muscular foot and the substrate, allowing the animal to glide across surfaces. Without this secreted trail, the slug’s soft tissue would be damaged by friction.
Movement is generated by rhythmic muscular contractions that travel along the length of the foot, known as “pedal waves.” As a wave of muscle contraction passes, it transmits a shear stress through the mucus layer onto the ground. The viscoelastic nature of the slime allows the mucus to temporarily flow like a liquid in the area undergoing the wave, reducing friction and allowing that section of the foot to advance.
Immediately after the wave passes, the mucus quickly reverts to its solid-like state, acting as an adhesive to provide grip and propel the body forward. This alternating state of flow and adhesion ensures the slug generates a stable propulsive force. This mechanism enables the slug to crawl vertically or upside down without falling.
Slime’s Role in Protection and Communication
Beyond enabling movement, slug slime is a powerful survival tool, primarily preventing desiccation. Slugs have a soft, permeable body that is largely water, making them highly susceptible to drying out. The mucus acts as a protective, hygroscopic barrier, sealing the skin and absorbing moisture from the environment to maintain internal hydration. This need for moisture explains why slugs are most active at night or during damp conditions.
The slime also serves as a chemical defense against predators. The mucus on the slug’s body is typically thicker and stickier than the trail used for locomotion, making the animal difficult to grasp. For some species, the slime can be distasteful, toxic, or thick enough to physically gum up an attacker. This defensive mucus can also contain antimicrobial proteins and peptides that offer protection against bacteria and other pathogens.
The glistening trail left behind by a moving slug is a critical communication system, not just a byproduct of locomotion. These mucous trails contain chemical signals, or pheromones, that other slugs detect and follow. This trail-following behavior is used for navigation, helping the slug return to a resting place, and finding a mate.