Snails breathe through remarkably different structures depending on whether they live on land, in water, or somewhere in between. Aquatic species typically rely on gills to extract dissolved oxygen, while land snails have evolved a modified mantle cavity that functions as a primitive lung. Some groups, like apple snails, carry both a gill and a lung simultaneously, letting them switch between water and air as conditions demand. The variety across snail species is striking, and the evolutionary story behind it connects ancient ocean-dwelling ancestors to the garden snails crawling across your patio.
How Aquatic Snails Use Gills
Most water-dwelling snails breathe with gills, thin tissue structures packed with blood vessels where oxygen from the surrounding water passes into the snail’s circulation. In simple terms, water flows over the gill surface, oxygen diffuses into the blood, and carbon dioxide diffuses out. The gill sits inside the mantle cavity, a space between the snail’s body and its shell that acts as a respiratory chamber. Blood flowing through the gill picks up oxygen and carries it to the rest of the body.
The arrangement works well in water that contains enough dissolved oxygen, but it has limits. Gills need to stay wet to function. If the water becomes too warm or too stagnant, dissolved oxygen drops, and the snail can struggle. Some freshwater snails in ponds and ditches cope with low-oxygen water by supplementing gill respiration with trips to the surface, which brings us to a more unusual approach.
The Pulmonate Lung and the Pneumostome
Land snails and many freshwater snails belong to a group called the pulmonates, and their solution to breathing is elegantly simple: they converted the mantle cavity into a lung. Instead of housing a gill, the cavity’s inner walls are lined with a dense network of blood vessels that absorb oxygen directly from air. The snail opens a small breathing pore called the pneumostome to let air in and out. You can actually watch this pore open and close if you observe a garden snail closely.
In the freshwater pond snail Lymnaea stagnalis, for example, this aerial breathing behavior has been studied in detail. The snail rises to the water’s surface, opens its pneumostome at the air-water interface, and exchanges gases before descending again. A small circuit of nerve cells controls this rhythmic surfacing behavior, timing how long the pneumostome stays open and when the snail returns to the bottom.1Canadian Journal of Zoology. Behavioural and network plasticity following conditioning of the aerial respiratory response of a pulmonate mollusc
The lung’s internal structure, studied in detail in the aquatic pulmonate Pomacea canaliculata, reveals layers of tissue arranged for maximum gas exchange. The lung roof consists of an outer skin layer, a muscle layer, blood sinuses surrounded by storage tissue, and finally a thin respiratory surface where gas exchange happens. The lung floor mirrors this layout with an even thicker vascular layer and a double muscle layer that lets the snail actively ventilate the chamber.2PubMed Central. Morphological grounds for the obligate aerial respiration of an aquatic snail: functional and evolutionary perspectives The result is a respiratory organ that, while far simpler than a mammalian lung, is well suited to pulling oxygen from air.
Apple Snails and the Dual-Breathing Strategy
Some snails refuse to commit to one method and carry both a gill and a lung. Apple snails of the genus Pomacea are the classic example. These large freshwater snails have a fully functional gill on one side of the mantle cavity and a fully functional lung on the other. The gill handles underwater respiration, while the lung lets the snail breathe air at the surface. The lung also doubles as a flotation device, trapping a bubble of air that helps the snail bob along the water’s surface when it wants to travel.3Journal of Shellfish Research. Microscopic Anatomy of the Gill and Lung of the Apple Snail Pomacea maculata, with Notes on the Volume of the Lung
What makes this dual system especially interesting is how differently the two modes perform. In Pomacea maculata, oxygen uptake in air is roughly three times higher than in water, with aerial respiration measured at about 63 µL of oxygen per gram per hour compared to around 19 µL/g/h underwater, even though the snail’s heart rate stays similar in both conditions.4American Malacological Bulletin. Respiratory Physiology and Heart Rate of Pomacea maculata in Water, Air, and during Aestivation Air is simply a richer source of oxygen than water, so the lung can extract far more with each breath. This helps explain why apple snails regularly surface to gulp air even when their gills are working fine underwater.
Mangrove Snails and Life Between Tides
Intertidal and mangrove snails face a particular challenge: the tide constantly changes whether they are submerged or exposed to air. Different species in the same mangrove forest have evolved very different solutions, and the pattern tracks neatly with how high on the shore they live.
A study of three mangrove snail species illustrates this well. Nerita articulata, found throughout the mangrove zone and exposed to air between half and most of the time, retains a gill but breathes air about 2.7 times more efficiently than it breathes water. The other two species, Cerithidea obtusa and Cassidula aurisfelis, live higher up where they spend over 95% of their time in air. Both have converted their mantle cavities into lungs and breathe air five to six times more efficiently than they breathe water.5ScienceDirect. Respiration in air and water of three mangrove snails The lower-dwelling species hedges its bets with a gill; the higher-dwelling species have fully committed to air.
Similarly, among littoral (shoreline) snails studied across a tidal gradient, species living in zones with more aerial exposure tend to have aerial respiration rates that approach or match their aquatic rates. One limpet-like snail, Acmaea testudinalis, manages nearly equivalent oxygen uptake in air and water across its normal temperature range, even though it still technically has a gill.6PubMed. Temperature relations of aerial and aquatic respiration in six littoral snails in relation to their vertical zonation These shoreline species represent a kind of living snapshot of the transition between water breathing and air breathing.
Sea Slugs and Other Unusual Approaches
Not all gastropods rely on gills or lungs in the conventional sense. Sea slugs in the cladobranch group have evolved a striking alternative: fleshy projections called cerata that cover their backs. These finger-like extensions are filled with branches of the digestive system and are surrounded by thin tissue where gas exchange takes place. Some species grow hundreds of cerata as they mature, with one study documenting up to 280 per individual.7PubMed Central. Development of Cerata in the Cladobranchian Sea Slug Pteraeolidia semperi The cerata serve multiple roles simultaneously, including defense and even housing symbiotic algae, but their large combined surface area makes them effective respiratory organs.
This is a reminder that “snail” and “slug” cover a huge range of body plans. The common thread is that gas exchange requires a thin, moist, well-vascularized surface. Whether that surface takes the form of a feathery gill, a vascularized mantle wall, or a forest of cerata, the physics are the same: oxygen moves from a high-concentration area into the blood, and carbon dioxide moves out.
Why Snail Blood Is Usually Blue
The oxygen-carrying molecule in most snail blood is not hemoglobin, the red iron-based pigment found in mammals. Instead, most snails use hemocyanin, a copper-based protein that floats freely in the blood (rather than being packed inside red blood cells). Hemocyanin turns blue when it binds oxygen, which is why a snail’s blood is often a pale blue color.8PubMed Central. Molluscan hemocyanin: structure, evolution, and physiology These hemocyanin molecules are enormous, among the largest proteins known, and they drift through the hemolymph (snail blood) in massive cylindrical complexes.
There are exceptions. Some pulmonate snails, particularly certain freshwater species, use hemoglobin instead. The pond snail Biomphalaria glabrata carries a high-molecular-weight hemoglobin in its blood that binds oxygen very tightly, with a half-saturation oxygen pressure of just 6.1 mmHg at normal conditions. This extremely high affinity for oxygen is likely an adaptation to murky, oxygen-poor pond water where every molecule of dissolved oxygen counts.9PubMed. Oxygen binding and its allosteric control in hemoglobin of the pulmonate snail, Biomphalaria glabrata The snail’s hemoglobin also shows a strong Bohr effect, meaning its oxygen-binding behavior shifts with blood pH, which helps it load oxygen at the respiratory surface and release it efficiently in tissues that are producing carbon dioxide.
Estivation and Shutting Down to Survive
When conditions become too hot or too dry, many land snails enter estivation, a state of deep dormancy that can last for months. During estivation, the snail withdraws into its shell and seals the opening with a dried mucus membrane called an epiphragm. Metabolic rate drops dramatically, and the snail shifts to burning stored fats while keeping water loss to a minimum.
The Mediterranean land snail Otala lactea has been studied as a model for this process. During estivation, it relies on aerobic metabolism at a profoundly reduced rate, conserving both energy and water. The epiphragm is not airtight: it allows slow gas exchange so the snail can keep breathing at a trickle without losing too much moisture.10PubMed. Life in the slow lane: molecular mechanisms of estivation Some land snails in arid regions can survive in this suspended state for remarkably long periods, essentially shutting respiration down to the bare minimum needed to keep cells alive.
Apple snails, too, can estivate when their ponds dry up. Pomacea maculata shows a measurable decline in both respiration rate and heart rate during prolonged air exposure that transitions into estivation. The snail gradually shifts from active aerial breathing to a torpid state where oxygen demand is minimal. This ability to throttle respiration up and down is part of what makes amphibious snails so successful in unpredictable freshwater habitats where water levels fluctuate seasonally.
How Air Breathing Evolved From the Sea
The transition from water breathing to air breathing did not happen in one leap. Phylogenomic research tracing the evolutionary tree of air-breathing snails suggests that the shift happened in stages, with marginal habitats like estuaries, mangrove forests, and intertidal zones acting as stepping stones. Snails that colonized these environments, where tidal exposure meant regular contact with air, gradually evolved the pneumostome and modified their mantle cavities for aerial gas exchange. The ancestral air-breathing snail likely had both a pneumostome and a reduced gill, a combination still seen in some living species.11PubMed Central. Phylogenomic resolution of the root of Panpulmonata, a hyperdiverse radiation of gastropods: new insight into the evolution of air breathing
This pattern echoes what we see today in mangrove and shoreline snails. Species that spend more time in air have better-developed lungs and less reliance on gills, while species in lower tidal zones retain functional gills and treat air breathing as a supplement. The evolutionary pressure seems clear: snails that could extract oxygen from air had access to habitats that purely aquatic species could not reach, giving them an edge in competition and predator avoidance. Over millions of years, some lineages completed the transition fully and became the terrestrial pulmonates we know as garden snails and land slugs.
Environmental Stress and Snail Respiration
Rising temperatures and ocean acidification are putting pressure on aquatic snail respiration. In the common periwinkle Littorina littorea, a marine intertidal snail, exposure to elevated carbon dioxide levels caused a roughly 31% drop in respiration rate. When higher COâ‚‚ was combined with warmer temperatures, respiration still fell by about 15% compared to normal conditions. The snails compensated by shifting toward anaerobic metabolism, producing waste products that suggest they were struggling to meet energy demands through normal oxygen-based pathways.12PubMed. Exposure to elevated temperature and Pco(2) reduces respiration rate and energy status in the periwinkle Littorina littorea
Freshwater snails face analogous problems. In warming lakes and streams where dissolved oxygen is declining, the invasive New Zealand mud snail Potamopyrgus antipodarum shows altered thermal performance under low-oxygen conditions. Its respiration becomes limited at temperature extremes, and at warmer temperatures the snail appears to rely more heavily on anaerobic energy production, a stopgap that cannot sustain the animal indefinitely.13PubMed. Hypoxia Decreases Thermal Sensitivity and Increases Thermal Breadth of Locomotion in the Invasive Freshwater Snail Potamopyrgus antipodarum For snails that breathe water, the combination of warming and deoxygenation is a double threat: warmer water holds less oxygen while simultaneously raising the animal’s metabolic demand.
Land snails are somewhat buffered from aquatic deoxygenation, but they are not immune to climate stress. Higher temperatures increase water loss from the moist respiratory surface, and prolonged drought forces extended estivation that depletes energy reserves. The lung may be a powerful adaptation for life on land, but it still requires a film of moisture to function, and that moisture evaporates faster in hotter, drier conditions.
Deep-Sea Vents and the Far Edges of Snail Respiration
At the other extreme from sun-baked gardens and tidal pools, snails also thrive around deep-sea hydrothermal vents, where superheated, chemical-laden water billows from cracks in the ocean floor. These environments have no sunlight and very little dissolved oxygen in places, yet dense communities of vent snails flourish. Their secret is symbiosis: the snails house chemoautotrophic bacteria inside their greatly enlarged gills, and these bacteria convert hydrogen sulfide or methane from the vent fluid into energy the snail can use.
The vent snail Alviniconcha hosts symbiotic bacteria in its expanded gill tissue, with the bacteria sitting in an intermediate state between inside and outside the gill cells.14PubMed. Multi-omics analyses of the Alviniconcha holobiont reveal multi-faceted adaptations to deep-sea hydrothermal vents Another vent species, Gigantopelta aegis, carries two different bacterial partners simultaneously, one that oxidizes sulfur and another that oxidizes methane, providing nutritional flexibility in an environment where chemical conditions can shift rapidly.15PubMed Central. Hologenome analysis reveals dual symbiosis in the deep-sea hydrothermal vent snail Gigantopelta aegis
In these snails, the gill has been repurposed far beyond simple gas exchange. It serves as a combined respiratory organ, a housing complex for symbiotic bacteria, and a chemical processing plant. The gill tissue is massively enlarged compared to related non-vent species, providing enough surface area to support both the snail’s oxygen needs and the bacterial colony that feeds it. It is one of the most dramatic examples of how a basic respiratory structure can be co-opted for entirely new functions when the environment demands it.