Reptiles
Marine Iguana
Amblyrhynchus cristatus · also called Galápagos marine iguana, sea iguana
The marine iguana is the world’s only sea-going lizard — a Galápagos reptile that dives into the cold Pacific to graze algae off the rocks, sneezes the salt back out, and can literally shrink its own body to survive when the ocean turns against it.
Overview
Marine iguanas are found only in the Galápagos, where they live along the rocky coasts of nearly every island, basking in heaps on the black lava and swimming out to feed. Each island’s population has diverged in size, colour, and behaviour — from the small dark iguanas of Genovesa to the large, breeding-season-red-and-green “Christmas iguanas” of Española. They descend from a green iguana-like ancestor that rafted to the islands from South America millions of years ago and adapted to a marine diet in the absence of much land vegetation.
Built for grazing the sea
Life as a cold-blooded animal feeding in cold water demands compromises:
- Thermoregulation. A marine iguana warms itself to around 35 °C on sun-baked lava, then dives into water often below 25 °C, which rapidly chills it. It can slow its heart and shunt blood away from the skin to hold heat, but it must return to land to bask before it can digest its meal.
- Feeding gear. A blunt snout and short, tricusped teeth for scraping algae flush to the rock; long, sharp claws to cling on in the surge; a laterally flattened tail for propulsion.
- Salt handling. A powerful nasal salt gland removes the huge salt load from a seaweed diet, expelled in the marine iguana’s characteristic sneeze.
Shrinking to survive
Marine iguanas are tightly bound to the productivity of the ocean. In El Niño years, when warm water wipes out their preferred algae, whole populations starve and die. Individuals that survive do so partly by getting smaller — losing not just fat and muscle but bone length, up to about 20% — which lowers their food needs. When cool, productive conditions return, they regrow. This reversible shrinking is almost unknown among vertebrates.
Reproduction
Males defend small territories on communal basking grounds and display with head-bobs and shoving matches. Females travel to sandy areas, sometimes some distance inland, to dig a burrow and lay one to six eggs, then guard it briefly before leaving. The eggs incubate for three to four months.
Threats and conservation
The marine iguana is listed as Vulnerable. It has no defence against introduced predators — cats, dogs, rats, and pigs take eggs, hatchlings, and adults — and small island populations are especially fragile. Oil spills, plastic pollution, and disturbance by tourism and development add pressure, and climate change is expected to make severe El Niño famines more frequent. Protection focuses on eradicating invasive mammals from key islands and limiting coastal disturbance.
Marine Iguana: common questions
How does the marine iguana feed in the ocean?
It grazes algae. Small and young iguanas, and most females, feed in the intertidal zone at low tide, cropping algae off exposed rock. Large males dive, swimming with a side-to-side sculling of the flattened tail down to the seabed, gripping the rock with long claws against the surge, and scraping algae with blunt, tricusped teeth for several minutes before returning to the surface to breathe. After feeding in cold water they must haul out and bask to warm back up before they can digest.
Why do marine iguanas sneeze salt?
Eating seaweed and swallowing seawater loads the body with far more salt than a reptile's kidneys can remove. Marine iguanas have a specialised gland connected to the nostrils that pulls excess salt from the blood and concentrates it into a fluid, which the iguana expels in a forceful sneeze. The dried salt often lands on the animal's own head, giving many marine iguanas a whitish "wig."
Can a marine iguana really shrink?
Yes, and it is one of the few vertebrates known to do so. During El Niño years, warm water kills off the green and red algae the iguanas eat and replaces it with a brown alga they digest poorly, causing mass starvation. Researchers tracking individuals found that starving marine iguanas not only lose fat and muscle but actually get shorter — by as much as a fifth of their length — apparently by reabsorbing bone. When food returns, they grow back. Shrinking iguanas survive famines better than those that stay large.
Are marine iguanas dangerous or venomous?
No. They are harmless herbivores, famously unbothered by the humans who walk among them on Galápagos beaches. Charles Darwin, who disliked them, called them "imps of darkness" and "hideous-looking," and threw one into the sea repeatedly to see what it would do (it kept swimming back to shore, apparently because the water is where its predators are). They will puff up, bob their heads, and bite in a territorial dispute with another iguana, but not at people.