The marine iguana (Amblyrhynchus cristatus), also known as the sea iguana, saltwater iguana, or Galápagos marine iguana, is a species of iguana found only on the Galápagos Islands of Ecuador.

These unique reptiles are the only lizards fully adapted to marine life. They have the ability to forage in the sea for algae, which makes up almost all of their diet. They live mostly in colonies on rocky shores, where they bask after visiting the relatively cold water, but can also be found in marshes and on beaches. Like many other reptiles, after mating the female digs a nest hole in the soil where she lays her eggs, leaving them to hatch on their own.

Large males are able to swim out to sea and dive to find algae, while females and smaller males feed during low tide in the intertidal zone (the area along the coast which is above water level at low tide and underwater at high tide). Regardless of where they dine, the iguanas take in a lot of salt along with their algae. To rid themselves of the excess, they have large cranial glands which they empty with forceful sneezes. With each dramatic snort, the lizard ejects a thin fluid filled with sodium, potassium, and sodium chloride.

Marine iguanas vary in appearance between the different islands; several subspecies are recognized. Although relatively large numbers remain, as a species they are protected and considered threatened, primarily by El Niño cycles, introduced predators, and chance events like oil spills.

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