The only ocean-going lizard evolved from land iguanas stranded on the islands. Genetic analysis shows they developed the ability to dive 9 meters and slow their heart rate to survive.
RAF-YYC from Calgary, Canada, CC BY-SA 2.0, via Wikimedia Commons
An evolutionary divergence
The Galápagos marine iguana, Amblyrhynchus cristatus, is the only lizard on Earth that forages in the sea. Genetic data indicates its lineage split from the Galápagos land iguanas (Conolophus genus) approximately 4.5 million years ago. The ancestral iguanas likely arrived on the volcanic islands by rafting on debris from the South American mainland. Once there, the populations separated, with one group adapting to a terrestrial life and the other turning to the ocean.
Competition for scarce food on land is a leading hypothesis for this divergence. The marine iguana's ancestors began to forage in the intertidal zone, eating marine algae. This food source was abundant but required a new set of biological tools. Today, their diet consists almost exclusively of red and green algae found on submerged rocks. They use their short, blunt snouts and small, sharp teeth to scrape the algae from the surface. Different subspecies, which vary in size and color, exist across the archipelago. These range from the large, dark iguanas of Isabela and Fernandina islands to the smaller, more colorful "Christmas iguanas" on Española, which have red and green markings.
Physiology of a sea lizard
Surviving in the cold Pacific waters required significant physiological changes. To forage, a marine iguana can dive to depths of 9 meters (30 feet), and some large males have been recorded as deep as 30 meters (98 feet). To conserve oxygen during these dives, the iguana has extreme bradycardia, slowing its heart rate from around 40 beats per minute to as few as 10 beats per minute. A typical foraging dive lasts between 10 and 20 minutes, though they can remain submerged for up to an hour.
A flattened tail provides propulsion through the water, while long, sharp claws allow the iguana to grip volcanic rock in strong currents. To deal with the high salt intake from their diet, they have specialized exocrine glands connected to their nasal passages. These glands filter salt from their blood, which they then expel by sneezing a fine mist. This process often leaves white salt crystals encrusted on their snouts. During periods of food scarcity, such as an El Niño event, marine iguanas can shrink their body size by up to 20 percent. This reduction includes soft tissue and their bones, which they can regrow when food becomes plentiful again.
💡Fun Facts
Marine iguanas "sneeze" out excess salt through specialized nasal glands, a behavior that often leaves white, crusty patches on their heads.
During El Niño-induced famines, they can shrink their skeletons, losing up to 20% of their body length, and regrow the bone when food returns.
Their dark coloration is an adaptation that helps them rapidly absorb solar radiation and warm up after cold dives in the Pacific Ocean.
The largest subspecies, found on Isabela and Fernandina islands, can weigh up to 12 kilograms.
Access to Galápagos National Park sites is with a licensed guide, typically during daylight hours (approx. 6:00 AM to 6:00 PM).
Admission
The Galápagos National Park entrance fee for most foreign adults is $200 USD, paid in cash upon arrival. This fee is subject to change.
Accessibility
The terrain is volcanic, uneven, and rocky. Most iguana colonies are reached by authorized tour boats, which require either a wet or dry landing on shore.
