Land slugs are soft-bodied, shell-less gastropod mollusks common in gardens and other damp terrestrial environments. They are relatives of snails, differing primarily in their reduced or absent external shell. These creatures have elongated bodies, typically brownish or grayish, and range in size from tiny to several inches long. Land slugs thrive in moist habitats, often found under rocks, logs, and leaf litter, as their bodies are prone to drying out. They secrete a layer of mucus, leaving a characteristic slime trail that aids their movement and helps prevent desiccation.
The Simple Answer for Land Slugs
Land slugs cannot live underwater. While they require moisture to survive and are frequently observed in damp conditions, prolonged submersion is fatal. Their physiological design is not adapted for an aquatic existence, meaning they will eventually drown if unable to access air. This inability stems from their method of respiration and their permeable body structure.
Why Land Slugs Don’t Thrive Underwater
Land slugs breathe air through a specialized respiratory opening called a pneumostome, located on the right side of their mantle. This opening leads to a lung-like cavity that facilitates gas exchange. When a land slug is submerged, the pneumostome closes to prevent water from entering the cavity. This closure prevents the slug from extracting oxygen from the water, leading to suffocation as its oxygen reserves deplete.
Furthermore, their permeable skin, while enabling them to absorb moisture from their environment, is not suited for aquatic life. When fully submerged, water can pass through their skin into their bodies, disrupting their internal fluid balance through osmosis. This uncontrolled water absorption can dilute their internal fluids, leading to an imbalance that compromises their survival underwater. Additionally, the typical food sources for terrestrial slugs, such as decaying plant matter, fungi, and living plants, are not readily available underwater.
Introducing Aquatic Slugs
It is important to distinguish land slugs from “aquatic slugs,” which are organisms adapted to live underwater. Most creatures called sea slugs, such as nudibranchs, are marine gastropod mollusks that have lost their shells entirely or have greatly reduced internal shells. Unlike land slugs, these aquatic forms possess gills or other specialized structures for extracting oxygen from water.
Nudibranchs, for instance, often have exposed, feather-like gills or finger-like appendages called cerata that increase their surface area for respiration. Some aquatic snails, also often called aquatic slugs due to their lack of a prominent external shell, similarly use gills to breathe underwater. These aquatic species are evolved for marine or freshwater environments, with physiological adaptations that allow them to thrive where land slugs cannot.
What Happens When Land Slugs Get Wet?
Land slugs require a moist environment to survive and move, producing mucus to prevent their soft bodies from drying out. They are most active after rainfall or during humid nights when moisture is abundant. However, temporary submersion in standing water can still be detrimental. While a slug might survive for a short period, typically ranging from minutes to a few hours, if it falls into water, its lack of aquatic adaptations means it will eventually drown.
Some sources suggest slugs can survive underwater for up to 24 hours, possibly by holding their breath or slowing their metabolism. However, this is a temporary state. Prolonged immersion, such as in a puddle or during heavy rain that causes flooding, will lead to death if the slug cannot escape to air. The ability to withstand brief periods of wetness, like a passing shower, is different from being able to live indefinitely submerged, as their respiratory system is designed for air, not water.