Marine iguanas are unique reptiles found exclusively in the Galápagos Islands, a remote archipelago off the coast of Ecuador. They are the only lizards globally that regularly forage for food in the ocean. This unusual lifestyle has led to highly specialized dietary habits and distinct physical features that enable their survival in a marine environment.
Algae: Their Primary Food
Marine iguanas are herbivores, feeding almost entirely on marine algae. They primarily consume red and green algae, abundant on the submerged rocks surrounding the Galápagos islands. Specific genera like Grateloupia, Gelidium, Hypnea, Polysiphonia, and Ulva are common components of their diet, providing essential nutrients.
These iguanas graze on algae found in both the intertidal zone, exposed during low tide, and the subtidal zone, which remains submerged. The exact types of algae they consume can vary depending on the specific island, season, and availability of different species. This specialized diet forms the foundation of their existence in an environment where terrestrial plant matter is often scarce.
Specialized Feeding Techniques
To access their algal food source, marine iguanas display unique feeding behaviors, often involving aquatic excursions. Large males frequently dive into the ocean, sometimes reaching depths of up to 30 meters (98 feet), though typical dives are shallower. While underwater, they can remain submerged for several minutes, with some individuals capable of staying down for 30 minutes or even an hour.
When foraging, they use strong claws to cling firmly to submerged rocks, resisting ocean currents. Their flattened snouts and sharp, three-pointed (tricuspid) teeth scrape algae directly off rocky surfaces. Smaller individuals, including females and juveniles, often feed in the intertidal zone during low tide, where algae is exposed and less diving is required.
Physical Adaptations for Their Diet
Marine iguanas possess several distinct physical adaptations that facilitate their specialized marine diet. Their short, blunt snouts allow them to press their mouths closely against rocks, while their razor-sharp, tricuspid teeth efficiently scrape algae from uneven surfaces. These teeth are replaced at a higher rate compared to other iguana species, compensating for wear from their abrasive diet.
Their powerful limbs are equipped with long, sharp claws, which are indispensable for maintaining a secure grip on slippery, algae-covered rocks, especially in strong currents. A laterally flattened tail acts as a paddle, providing efficient propulsion through the water during dives. After ingesting salty food and seawater, specialized salt glands near their nostrils filter excess salt from their blood. This concentrated salt solution is then forcefully expelled through their nostrils in a “sneeze-like” action, often leaving white crusts on their heads.
After foraging in cool ocean waters, a marine iguana’s body temperature can drop significantly. Their dark skin pigmentation allows them to rapidly absorb heat from the sun by basking on dark volcanic rocks, raising their internal temperature for digestion.
Dietary Flexibility and Constraints
While marine algae forms the overwhelming majority of their diet, marine iguanas exhibit some dietary flexibility, particularly during periods of food scarcity. During events such as El Niño, warmer ocean temperatures can reduce the availability of their preferred red and green algae. In such circumstances, they may resort to consuming less digestible brown algae, which can be detrimental to their health, or occasionally terrestrial plants, grasshoppers, or crustaceans.
Their digestive systems are specialized for marine algae, making alternative food sources less efficient and potentially leading to starvation. A remarkable adaptation to severe food shortages is their ability to temporarily shrink in body length, potentially by as much as 20%, which reduces their metabolic needs. Younger iguanas often forage in tidal pools or feed on different types of algae accessible without deep diving, and some hatchlings may even consume the feces of larger iguanas to acquire necessary gut bacteria for digestion.