No Gills — They Are Reptiles
Sea snakes do not have gills. They are reptiles — air-breathing vertebrates that evolved from land snakes and returned to the sea, just as whales evolved from land mammals. Every sea snake must surface to breathe, no exceptions. Yet sea snakes are extraordinarily well-adapted to aquatic life, with several remarkable physiological solutions to the challenge of breathing air while living underwater.
The Single Lung Adaptation
Most terrestrial snakes have two lungs — a small left lung and a larger right lung. Sea snakes have taken this further: in most species, the right lung extends almost the entire length of the body, acting as both a respiratory organ and a buoyancy control device. By compressing or expanding this lung, sea snakes can adjust their depth in the water column — a function that in fish is served by the swim bladder.
Cutaneous Respiration: Breathing Through the Skin
Here is where sea snakes become genuinely remarkable. Unlike terrestrial snakes, sea snakes can absorb a significant amount of oxygen directly through their skin — a process called cutaneous respiration. Studies have found that sea snakes absorb up to 33% of their oxygen needs and release up to 90% of their carbon dioxide through their skin while submerged.
This cutaneous gas exchange allows sea snakes to extend their dives dramatically. A sea snake that would need to surface every few minutes if relying solely on lung breathing can remain submerged for 30 minutes to over 2 hours by supplementing with skin breathing. Some species have been recorded at depths of over 100 meters.
Valve-Like Nostrils
Sea snake nostrils are equipped with muscular valves that seal tightly when submerged, preventing water from entering the respiratory tract. These valves open automatically when the snake surfaces to breathe. The nostrils are also positioned on the top of the snout — rather than the sides as in most terrestrial snakes — allowing the snake to breathe while barely breaking the surface.
How Long Can Sea Snakes Stay Underwater?
Most sea snakes surface every 30 minutes to 2 hours under normal conditions. During periods of low activity or sleep (sea snakes sleep underwater, held by currents or resting on the bottom), they may remain submerged for much longer — some species have been documented remaining underwater for over 8 hours, relying almost entirely on cutaneous respiration during these extended resting periods.
Frequently Asked Questions
Can sea snakes breathe underwater?
Not in the way fish do — sea snakes have no gills. But they can absorb oxygen through their skin (cutaneous respiration), which allows them to extend dives for hours without surfacing.
How long can a sea snake hold its breath?
Most species surface every 30 minutes to 2 hours. During rest, some species can remain submerged for over 8 hours using skin breathing.
Do sea snakes drown?
If prevented from surfacing for extended periods, yes — sea snakes can drown. This is a significant cause of mortality from fishing net entanglement.