The Galapagos Islands represent a distinct evolutionary laboratory where iguanas have diverged dramatically in feeding habits, driven by unique environmental pressures. Two distinct groups, the Marine Iguana (Amblyrhynchus cristatus) and the Land Iguanas (Conolophus species), demonstrate extraordinary specialization in their diets. The availability of resources in their respective habitats—the nutrient-rich ocean and the arid volcanic interior—has resulted in feeding behaviors that are entirely unalike, showcasing remarkable adaptation to this challenging ecosystem.
Marine Iguanas Specialized Seaweed Diet
The Marine Iguana is the only lizard in the world that forages in the ocean, subsisting almost entirely on marine algae. Their diet consists primarily of red and green algae, such as Centroceras and Gelidium, which grow on submerged volcanic rocks. Iguanas primarily feed during low tide when algae is exposed on the intertidal zone, or by diving into the cold Pacific waters for subtidal growth.
Larger, adult iguanas are capable of diving deeper, sometimes reaching 30 meters, to access abundant patches of algae. Dives can last up to an hour, though shorter dives are more common. As ectotherms, the cold water causes their body temperature to drop significantly, sometimes 10°C below their optimal range. To counteract this heat loss, iguanas engage in prolonged periods of basking on black lava rocks immediately after foraging, which raises their internal temperature quickly and aids digestion.
The marine diet forces the reptile to consume a high concentration of salt, requiring a specialized mechanism for its elimination. Iguanas possess specialized nasal glands located above their eyes that connect to the nostrils. These glands concentrate and excrete the excess salt in a highly concentrated solution, which the iguana expels by a forceful “sneeze.” Younger iguanas tend to avoid diving, restricting their feeding to the warmer, exposed algae of the intertidal zone to minimize thermal stress.
Land Iguanas Terrestrial Cactus Foraging
Land Iguanas are obligate terrestrial herbivores, relying on the sparse vegetation of the arid, lowland zones. Their diet is overwhelmingly dominated by the prickly-pear cactus (Opuntia species), which is a key resource in their volcanic habitat. This cactus can account for up to 80% of their total food intake, depending on the island and season.
The iguanas consume the entire plant, including the succulent pads, flowers, and fleshy fruit. They exhibit a tolerance for the cactus spines, often eating the pads whole without attempting to remove the defenses. The Opuntia cactus serves as both a source of nutrition and hydration in an environment where fresh surface water is scarce.
During the long dry season, the moisture content of the cactus pads is vital for the iguanas’ survival. While the cactus is the primary food, land iguanas also consume sparse native leaves and flowers. They occasionally supplement their diet with small insects, centipedes, or carrion, especially younger individuals.
Biological Adaptations for Extreme Diets
The divergence in feeding habits is supported by specific morphological and physiological adaptations. Marine Iguanas possess short, blunt snouts and sharp, tricuspid teeth, which function as tiny rakes to scrape algae from underwater rocks. Their tails are laterally flattened, providing propulsion for swimming, while long, sharp claws allow them to anchor securely against strong ocean currents while grazing.
Physiologically, marine iguanas manage cold-water foraging by constricting peripheral blood vessels, shunting warmer blood toward their core organs to conserve heat. Digestion of tough marine algae requires a specialized community of symbiotic bacteria within their hindgut. These microbes ferment the algae and break down its complex cellulose structure.
Land iguanas have evolved strong jaws and flatter, grinding teeth suited for crushing the tough, fibrous material of the cactus pads. They also have a thick, leather-like texture on their mouths and tongues, which offers protection from the cactus spines during feeding. Their larger, more robust digestive tracts are adapted to process a high-fiber, low-nutrient diet.