The marine iguana, found exclusively in the Galápagos Islands, is the only lizard on Earth that forages in the ocean. This unusual habit of diving into saltwater to feed on marine algae often leads to questions about its biological classification. Many observers wonder if this reptile has developed specialized aquatic features, such as gills, to breathe underwater. Understanding how this unique creature survives requires examining its respiratory system and physiological adaptations.
The Direct Answer: Breathing with Lungs
Marine iguanas do not possess gills and cannot extract oxygen from water like fish. As a reptile, the marine iguana relies entirely on a pair of lungs to breathe air, similar to its terrestrial relatives. This means it must regularly surface from its dives to exchange carbon dioxide for oxygen, a process that limits its time spent underwater.
The iguana’s respiratory system is adapted for efficiency. They possess a large lung capacity relative to their body size, which helps them hold a significant volume of air before a dive. Unlike mammals, they do not use a diaphragm; instead, they employ the muscular movements of their ribs and body wall to draw air in and out. The necessity of returning to the surface for every breath is the primary constraint on their diving behavior.
The Saltwater Adaptation
The “marine” part of the iguana’s name refers to its diet of salt-laden marine algae, which requires a unique physiological adaptation. Consuming this food introduces high concentrations of sodium, chloride, and potassium into the bloodstream. Since reptile kidneys cannot efficiently excrete this excess salt without losing excessive water, the iguana developed a specialized solution.
To manage its ion balance, the marine iguana uses a lateral nasal gland, a type of salt gland located near its nostrils. This gland filters excess salt from the blood and excretes it as a concentrated saline solution. The iguana forcibly expels this brine from its nostrils in a process resembling a vigorous “sneeze,” covering its head with a white, crusty residue of dried salt. This mechanism allows the species to thrive in a saltwater environment that would be toxic to most other lizards.
Underwater Survival and Diving Endurance
Despite being obligate air-breathers, marine iguanas exhibit remarkable underwater endurance. While a typical foraging dive to graze on algae lasts between five and ten minutes, they can remain submerged for up to 30 minutes. In extreme cases, iguanas have survived forced submersion lasting up to an hour.
This extended breath-holding is achieved through the dive reflex, an adaptation common in marine vertebrates. When submerged, the iguana dramatically lowers its heart rate, a phenomenon called bradycardia. Heart rates can drop from a resting rate of around 40 beats per minute to as low as 10 beats per minute during a deep dive, conserving the limited oxygen supply. Simultaneously, the iguana redirects blood flow away from non-essential organs, prioritizing oxygen delivery to the brain and heart.