The marine iguana, Amblyrhynchus cristatus, is a unique reptile found exclusively across the volcanic archipelago of the Galápagos Islands. It is the only modern lizard species in the world that actively forages in the ocean, adapting its biology and behavior to thrive in a harsh coastal and marine environment. This dramatic shift from the terrestrial lifestyles of its closest relatives required profound adaptations, ranging from specialized physical features to complex physiological controls.
The World’s Only Marine Foraging Lizard
The marine iguana is defined by its ability to exploit the sea for food, setting it apart from all other lizards globally. This required the evolution of specific physical traits for underwater movement. The most noticeable adaptation is the iguana’s tail, which is laterally flattened like a paddle, providing the primary source of propulsion as the animal swims with a powerful, serpentine motion.
Its powerful limbs are equipped with long, sharp claws, used to grip slippery, algae-covered volcanic rocks underwater. This anchoring ability is necessary while the iguana grazes against strong ocean currents. While many iguanas forage in the intertidal zone, the largest males dive up to 30 meters (98 feet) to reach subtidal algae beds. Though they can hold their breath for up to 30 minutes, most foraging dives last only a few minutes.
Specialized Salt Excretion Physiology
The constant consumption of marine algae and saltwater presents a significant physiological challenge. To prevent a buildup of salt in the bloodstream, the lizard evolved a specialized mechanism for excretion. Located above the eyes are nasal salt glands that filter excess sodium chloride from the blood.
This concentrated salt solution is then expelled through the nostrils in a process often described as “sneezing.” This action clears the glands and often leaves a white, crystalline crust on the animal’s face and snout. This mechanism provides a necessary countermeasure to the salty environment required by its marine diet.
Extreme Thermoregulation and Survival Tactics
Survival in the cold waters of the Galápagos, cooled by the Humboldt Current, poses a thermal problem for these ectothermic reptiles. Marine iguanas rely on external heat sources to maintain a body temperature between 35°C and 39°C for efficient digestion. Before entering the water, iguanas bask on black lava rocks, which absorb solar radiation and quickly raise their body temperature. Their dark coloration aids in this rapid heat absorption.
While diving, the iguanas employ bradycardia, a significant slowing of the heart rate, sometimes dropping from 40 beats per minute to about 10 bpm. This reflex conserves oxygen by reducing metabolic demand and redistributing blood flow to the brain and heart.
A unique survival tactic occurs during El Niño events, which warm the ocean and kill the iguanas’ preferred algae, leading to food scarcity. During these periods, iguanas can shrink their body length by up to 20%, a process thought to involve the reabsorption of bone tissue. This size reduction lowers their energetic needs and improves foraging efficiency, allowing them to survive until food resources return.
Diet and Endemic Habitat
The marine iguana’s diet is specialized, consisting almost exclusively of red and green marine algae or seaweed scraped from submerged rocks. They possess a short, blunt snout and small, sharp, tricuspid teeth suited for this grazing behavior.
The specific types of algae consumed determine the iguanas’ coloration, which varies dramatically across the islands and becomes brightest during the mating season. For example, iguanas on Española Island display patches of bright red and greenish-blue during this time due to their particular algal diet.
The species is entirely endemic to the Galápagos Islands, meaning it is found nowhere else on Earth. It is theorized that the ancestor of the marine iguana was a South American land iguana that drifted to the islands millions of years ago. This geographical isolation, combined with the lack of terrestrial food sources, drove the evolutionary pressure for the iguanas to seek sustenance in the sea.