Why Do Slugs Have Holes on Their Sides?

That small hole on the side of a slug is called a pneumostome, and it serves as the animal’s sole airway. Slugs are pulmonates, meaning they breathe air through a simple lung rather than gills, and the pneumostome is the opening that lets air in and out of that lung. It sits on the right side of the fleshy shield near the slug’s head, called the mantle, and if you watch closely you can see it opening and closing in a slow, rhythmic cycle. Far from being a wound or a defect, the pneumostome is one of the most important structures on a slug’s body, and it does more than just let the animal breathe.

What the Pneumostome Actually Is

The pneumostome is a muscular pore that the slug can actively open and close. It connects the outside air to the mantle cavity, a chamber inside the slug’s body that functions as a lung. The walls of this cavity are densely packed with blood vessels, creating a surface where gas exchange happens: oxygen diffuses in, carbon dioxide diffuses out. In the tropical slug Trichotoxon copleyi, researchers found that the respiratory surface inside the lung is “profusely vascularized by anastomosing ‘blood’ capillaries,” essentially a web of tiny vessels woven together to maximize the area available for absorbing oxygen.¹

The setup is surprisingly similar to what happens in your own lungs, just radically simplified. You have millions of tiny air sacs surrounded by capillaries; a slug has a single chamber surrounded by capillaries. You breathe through your nose and mouth; a slug breathes through one adjustable pore. The pneumostome is muscular enough that the slug can control its diameter, opening it wide when it needs more air and narrowing it when conditions demand water conservation. This adjustability turns out to be critical for an animal that lives with its entire body exposed to the elements.

How Oxygen Moves Through a Slug’s Body

Once air enters the mantle cavity and oxygen diffuses across the vascularized lung wall, it enters the slug’s circulatory fluid, called hemolymph. Hemolymph is not blood in the mammalian sense. It is a pale, sometimes bluish fluid that bathes the slug’s internal organs directly rather than flowing through a closed network of arteries and veins. Oxygen travels through this fluid in two ways: dissolved directly in the watery plasma, or bound to a protein called hemocyanin.

Hemocyanin is the slug’s equivalent of hemoglobin, the oxygen-carrying molecule in human blood. Where hemoglobin uses iron and turns red when oxygenated, hemocyanin uses copper and turns blue. Oxygen inhaled into the lung first dissolves in the hemolymph after crossing the lung membrane, and only then binds to hemocyanin for transport to the tissues. When the hemolymph reaches oxygen-hungry cells, the process reverses: oxygen detaches from hemocyanin, re-enters its dissolved state, and crosses into the tissue.² It is an elegant relay system, though a relatively slow one compared to mammalian circulation. This is part of why slugs are not exactly built for speed: their oxygen delivery system works, but it was never designed for high metabolic demands.

How the Pneumostome Balances Breathing and Drying Out

If you watch a slug for a few minutes, you might notice the pneumostome opening and closing at a lazy, steady pace. In a well-hydrated slug, this rhythm runs at fewer than about one closure every two minutes. The slug opens the pore, lets air circulate into the mantle cavity, then closes it again. It is unhurried because the slug has water to spare and can afford to keep the pore open for longer stretches.

The problem is that every time the pneumostome opens, the slug loses moisture. The lung lining is wet, and exposing it to air means water evaporates. For an animal that has no waterproof skin and is essentially a tube of mucus sliding across the ground, water loss is a constant existential threat. The pneumostome’s ability to close is what makes terrestrial life possible for slugs: it lets them ration their exposure to dry air.

When a slug starts to dehydrate, the rhythm changes. Research on slugs found that once the animal has lost roughly 15 to 20 percent of its body weight through evaporation, the frequency of pneumostome closures begins climbing. The slug closes the pore more often, and the opening itself gets physically smaller. Each individual closure lasts about the same length of time, but because the closures happen more frequently, the total proportion of time the pneumostome spends sealed shut increases significantly.³ The slug is essentially choosing to breathe less in order to lose less water. It is a trade-off between suffocating and drying out, and the slug manages it by tightening access to its only airway.

This dehydration response is controlled internally by the concentration of the hemolymph. As the slug loses water, the hemolymph becomes more concentrated, and peptide hormones signal the pneumostome muscles to increase their closing rate.³ It is a feedback loop: the body senses water stress and responds by limiting the main route of water loss, even though that route also happens to be the only way to breathe. This is why slugs are overwhelmingly nocturnal and prefer damp environments. Daytime and dry conditions force them into a state where they are constantly throttling their own breathing to survive.

Why the Pneumostome Is Always on the Right

If you flip over enough slugs (or, more humanely, observe them crawling up a glass surface from below), you will notice the pneumostome is consistently on the right side of the mantle. This is not random. The vast majority of land slugs and snails belong to a group called Stylommatophora, and in these animals the body plan is asymmetric: the lung, the heart, and certain other organs are concentrated on one side. The pneumostome sits on the right because the mantle cavity it opens into is on the right side of the body.

This asymmetry traces back to the coiled body plan of snails. Snails carry a spiraling shell, and most species spiral in a right-handed direction. The internal organs are arranged to fit inside that coil, which places the lung opening on the right. Slugs evolved from snails, losing the shell over evolutionary time but retaining the same internal layout. The pneumostome stayed on the right because the underlying body asymmetry did not change just because the shell shrank or disappeared.

There are rare exceptions. A handful of slug and snail species have left-handed body plans, and in those animals the pneumostome appears on the left. But for the slugs you are most likely to encounter in a garden, the hole will be on the right side of the mantle, roughly in the rear half of that fleshy shield.

How Slugs Evolved to Breathe Air

The ancestors of modern slugs and snails were aquatic, and the transition to air-breathing did not happen all at once. Phylogenomic research into the deep history of pulmonate gastropods suggests that the evolution of the pneumostome was preceded by shifts into marginal habitats, areas like estuaries, mangrove swamps, and intertidal zones where animals experienced fluctuating exposure to both water and air. These transitional environments appear to have accelerated the development of air-breathing structures. The ancestor of the group that includes modern air-breathing slugs and snails already possessed a pneumostome alongside a one-sided gill, a kind of dual system that could handle both aquatic and aerial respiration.⁴

Over time, fully terrestrial lineages lost the gill entirely and relied exclusively on the pneumostome and lung. This is the system modern garden slugs use. But in some semi-aquatic relatives that still live in wet coastal habitats, vestiges of the older gill-based system persist. The pneumostome was not a brand-new invention so much as a repurposed and refined version of an opening that already existed for managing airflow in creatures that lived at the boundary between water and land.

This evolutionary history also helps explain why slugs remain so tied to moisture. Their lung is not a highly efficient, deeply internalized organ like a mammalian lung. It is a wet cavity open to the outside through a single pore. The slugs that moved onto land were never fully liberated from water; they just developed enough of an air-breathing apparatus to survive on damp ground, and the pneumostome’s ability to close became the key adaptation that made it work.

The Pneumostome Does More Than Breathe

In some slug species, the pneumostome area is a surprisingly busy intersection. In the Australian red triangle slug Triboniophorus graeffei, the pneumostome, the anus, and the excretory pore are all closely associated in the same region of the mantle.⁵ This arrangement means the breathing pore sits right next to the openings for waste disposal. It is an efficient bit of anatomy for a soft-bodied animal with limited real estate, though it strikes most people as a rather unappealing design choice.

This clustering of openings is not universal across all slugs, but it illustrates a broader point about slug anatomy: the mantle is the control center for several major body functions, not just respiration. The mantle secretes mucus, houses the remnant shell (in species that still have an internal shell plate), manages water balance, and in some groups provides the exit route for reproductive and excretory products. The pneumostome is the most visible feature on the mantle’s surface, but it sits within a zone that is doing a lot of physiological work behind the scenes.

Spotting the Pneumostome in Your Garden

If you want to see a pneumostome in action, the best time to look is on a mild, damp evening when slugs are most active. Find a slug moving across a flat surface and look at the mantle, the smooth, oval-shaped shield behind the slug’s head. On the right side, toward the back edge, you should see a small opening, sometimes round, sometimes more slit-like. Wait and watch. You will eventually see it close and reopen. The rhythm is slow enough that you might need a minute or two of patience.

One common source of confusion is mistaking the pneumostome for damage. People sometimes see the hole and assume the slug has been injured, bitten, or parasitized. In reality, the pneumostome is perfectly normal anatomy. If the hole you are looking at is on the right side of the mantle, opens and closes, and has smooth edges, it is almost certainly the pneumostome. Actual wounds tend to look ragged and may leak hemolymph, which appears clear or slightly bluish.

Another thing people wonder about is whether slugs can drown. They can. Because the pneumostome opens into an air-filled lung, submerging a slug in water blocks its ability to breathe. Slugs caught in heavy flooding or trapped in a water-filled container will eventually suffocate. Some species can survive brief submersion by closing the pneumostome and relying on oxygen already in the mantle cavity, but this buys them minutes, not hours. The pneumostome is an air-breathing structure through and through.

Slugs Versus Snails and the Question of Vulnerability

The pneumostome works the same way in land snails as it does in slugs. Both groups are pulmonates, and both breathe through a muscular pore on the right side of the mantle leading to a vascularized lung cavity. The difference is that a snail can retract into its shell and seal the opening with a mucus plug or a hard plate called an operculum, dramatically reducing water loss and protecting the pneumostome from predators and desiccation. A slug has no shell to retreat into. Its pneumostome sits exposed on the surface of the body at all times.

This vulnerability is a significant part of why slugs are so much more dependent on humid microhabitats than their shelled relatives. A snail in dry conditions can withdraw, seal up, and wait out a drought for days or even months in a state of dormancy called estivation. A slug’s only options are to find shade, burrow into moist soil, or curl up under debris and hope conditions improve before it dries out. The pneumostome rhythm ramps up its closing rate, but without the physical barrier a shell provides, the slug is always fighting a losing battle against evaporation in dry weather.

This trade-off between shell and no-shell has played out repeatedly across gastropod evolution. Slugs have evolved independently from shelled ancestors at least a dozen times across different lineages, suggesting that shedding the shell offers real advantages in certain environments, likely reduced energy costs and the ability to squeeze into tight spaces for shelter. But the cost is always the same: greater exposure and a more precarious relationship with water. The pneumostome, and its ability to modulate how much air gets in and how much water gets out, is the adaptation that makes that trade-off survivable.