Snakes are air-breathing vertebrates that do not possess gills. Their respiratory system relies entirely on lungs for gas exchange, unlike aquatic organisms like fish. This means snakes, regardless of their habitat, must always access atmospheric oxygen to survive.
How Snakes Breathe
Snakes breathe using a lung system that has adapted to their elongated body shape. Most snakes have one functional lung, which is stretched to extend along much of their body cavity. A smaller, often non-functional or vestigial, left lung may also be present. The functional lung features a respiratory portion near the head, where oxygen and carbon dioxide are exchanged. The posterior part of this lung often acts as an air sac, serving as a reservoir for air but not actively participating in gas exchange.
Unlike mammals, snakes do not have a diaphragm, the muscular sheet that aids in breathing. Instead, they rely on the muscles located between their ribs to facilitate respiration. These intercostal muscles contract to expand the rib cage, creating negative pressure that draws air into the lungs. Exhalation occurs when these muscles relax, causing the rib cage to contract and push air out. When a snake is consuming large prey, it can extend its glottis, the opening to its windpipe, out the side of its mouth to ensure continuous breathing.
Aquatic Adaptations for Breathing
Even aquatic snakes, such as sea snakes, are air-breathing reptiles. These species must regularly surface to take in atmospheric oxygen, similar to other marine air-breathers like whales and dolphins. Their adaptations to an aquatic lifestyle involve physiological and behavioral mechanisms to maximize their time underwater.
Aquatic snakes can hold their breath for extended periods by slowing their metabolic rate, which reduces their oxygen demand. Their lung also functions as a large air reservoir, storing oxygen for longer dives. Some species, like the annulated sea snake, possess specialized vascular networks, particularly on their heads, that can absorb some oxygen directly from the water through their skin, acting as supplementary respiratory surfaces. This cutaneous respiration, while not a gill system, allows them to absorb a percentage of their oxygen needs through their skin, enabling prolonged submersion. Additionally, many aquatic snakes have valved nostrils that can close tightly to prevent water entry when submerged.