The visible hole on the side of a garden slug, which often opens and closes, is a common source of curiosity. Slugs are terrestrial gastropods, mollusks that have adapted to life on land without a large external shell. This specialized biological feature allows the soft-bodied creature to manage breathing air while simultaneously preventing fatal dehydration. This adaptation is a direct result of their evolutionary path from water-dwelling ancestors to moist terrestrial environments.
Identifying the Breathing Hole
The scientific name for this opening is the pneumostome, which literally translates to “lung mouth.” This muscular orifice is typically located on the right side of the slug’s mantle, the saddle-like structure positioned just behind the head. When the slug is active, the pneumostome often appears as a distinct, pulsing opening.
The pneumostome serves as the external entrance to the slug’s respiratory system, which is housed within the mantle cavity. For terrestrial slugs, this cavity has evolved into a highly efficient, single lung. The opening is easier to see on a slug than on a snail because the slug’s reduced shell does not obstruct the view.
The Mechanics of Respiration
The slug belongs to the group of mollusks called Pulmonata, meaning they possess a lung for gas exchange. Air is drawn directly through the pneumostome and into the pallial cavity, which functions as the lung. The ceiling of this internal cavity is lined with an extensive network of tiny blood vessels.
Oxygen from the inhaled air diffuses across this moist, thin tissue surface and into the slug’s hemolymph (blood), while carbon dioxide moves out. This gas exchange is analogous to how human lungs function, but it occurs across the lining of the mantle cavity instead of within alveoli. This specialized lung represents an evolutionary shift from the gills used by the majority of marine mollusks.
The slug controls the size and rate of the pneumostome opening, a behavior known as the pneumostome rhythm. Breathing is accomplished by the rhythmic lowering and raising of the cavity’s floor, similar to how a diaphragm moves in mammals. This mechanism ensures a continuous supply of oxygen to the slug’s tissues as it moves and feeds.
Why Slugs Need to Close the Opening
The primary reason slugs actively open and close the pneumostome is to regulate water loss, a constant threat to soft-bodied terrestrial organisms. Slugs are composed mostly of water, and since the internal respiratory surface must remain moist for gas exchange, it is also a site where water evaporates easily. Closing the breathing hole helps prevent desiccation, especially when the slug is resting or during dry periods.
When a slug begins to dehydrate, the frequency of pneumostome closures increases dramatically. A fully hydrated slug might close the opening less than once every two minutes, but this rate speeds up as water is lost. The slug’s body monitors the osmolality, or concentration, of its hemolymph to trigger this water-saving behavior.
The ability to seal the opening also provides a defense against debris or parasites entering the mantle cavity. Furthermore, the closure mechanism prevents water from flooding the lung when the slug is submerged or caught in heavy rain. This active control is a survival strategy that allows the slug to balance the competing demands of respiration and water conservation.