Slugs are terrestrial mollusks highly dependent on water for survival. Unlike many land animals, their soft bodies and lack of a substantial outer shell mean their existence is intrinsically linked to maintaining high environmental humidity. This constant need for moisture dictates nearly every aspect of a slug’s physiology and behavior. Since their bodies are largely composed of water, they must continually seek damp conditions to prevent rapid desiccation. Managing body water is their most significant biological challenge, fundamental to their ability to function, move, and breathe.
Essential Role of Moisture
The physiological necessity for water in slugs centers on two functions: respiration and locomotion. Slugs primarily breathe using a single respiratory opening called the pneumostome, which leads to a lung-like structure inside the mantle cavity. However, they also rely on cutaneous respiration, which is the absorption of some oxygen directly through their skin. For this gas exchange to occur effectively, the skin surface must remain wet, as oxygen can only pass through a moist membrane.
The constant production of mucus demands a continuous supply of water. This mucus, which is approximately 98% water, is essential for locomotion, allowing them to glide across surfaces using muscular contractions of the foot. The slime trail facilitates movement and acts as a protective layer, shielding soft tissue from abrasion and injury. Because this watery secretion helps reduce evaporative water loss, slugs must frequently replenish the water lost in producing this hydrogel-like substance.
Desiccation The Constant Threat
The biological features that allow slugs to breathe and move on land also make them vulnerable to drying out. Lacking the protective shell of a snail or the keratinized skin of a vertebrate, their soft bodies are highly susceptible to evaporation. The moist skin required for oxygen absorption facilitates rapid water loss to the surrounding environment, especially in dry, warm conditions.
Active slugs can lose between 30 and 40% of their initial body weight in just two hours. Even when inactive, the rate of evaporation is significant, ranging from 3% to 5% of their body weight per hour in air with 45% relative humidity. As dehydration progresses, tissues lose turgidity, leading to an inability to move and eventual death if the water deficit is not corrected. To mitigate water loss, the slug can reduce the open diameter of the pneumostome, decreasing the surface area exposed to the air.
Survival Strategies and Habitat Selection
To counteract desiccation, slugs employ behavioral adaptations focused on water conservation and replenishment. Habitat selection is biased toward microclimates that offer sustained moisture, such as damp soil, under decaying logs, or beneath dense foliage. These locations provide shelter from direct sunlight and circulating air, which accelerates evaporation.
Activity patterns are predominantly nocturnal, as the nighttime environment features higher relative humidity and lower temperatures. This strategy allows them to forage and move while minimizing water loss. Slugs do not actively drink water; instead, they absorb moisture directly through their body surface, particularly the foot, from wet substrates like dew or damp soil. When severely dehydrated, a slug assumes a flattened posture on a moist surface, enabling rapid water uptake until hydration is restored. They also exhibit huddling behavior in dry conditions, forming tightly packed aggregations to collectively reduce the exposed surface area.