Marine iguanas are unique reptiles found exclusively in the Galápagos Islands. They are the only modern lizards that regularly forage in the sea, establishing a truly distinctive lifestyle.
The Primary Diet: Marine Algae
Marine iguanas are specialized herbivores, relying almost exclusively on marine algae, commonly known as seaweed. Their diet primarily consists of red and green algae found in the intertidal and subtidal zones around the Galápagos Islands. They selectively consume specific varieties that are digestible and provide necessary nutrients. At least 10 genera of algae are regularly part of their diet, including red algae like Centroceras, Gelidium, and Hypnea, and sometimes green algae such as Ulva dominates in certain populations.
This specialized diet presents certain challenges, as marine algae can have lower nutritional value compared to land plants and contain high salt concentrations. Marine iguanas must process large amounts of this plant material to meet their energy needs.
Foraging and Feeding Strategies
Marine iguanas exhibit distinct foraging behaviors to obtain their algal diet. Smaller females and juveniles typically feed in the intertidal zone during low tide, grazing on exposed algae from rocky shores. They often scurry back and forth, quickly taking bites before retreating from incoming waves. Larger males, however, are capable of diving into deeper waters to access submerged algae, sometimes descending to depths of 30 meters and remaining underwater for up to an hour.
To harvest algae, marine iguanas possess specialized physical features. Their blunt snouts and sharp, tricuspid teeth are well-suited for scraping algae off rock surfaces. They use their strong claws to cling firmly to rocks, even in strong currents, allowing them to graze effectively underwater.
Physiological Adaptations for Seaweed Consumption
Marine iguanas possess internal mechanisms that allow them to thrive on a high-salt, marine algal diet. Their specialized salt glands, located above their nostrils, filter excess salt from their bloodstream. This concentrated salt is then expelled through their nasal passages, often appearing as a salty sneeze. This process maintains their osmotic balance in a saltwater environment.
Their digestive tract is also adapted to process tough algal cellulose. It is longer and more complex than that of many other lizards, housing specialized microorganisms like bacteria and protozoa that aid in breaking down algal cell walls. During dives, marine iguanas can slow their heart rate from 40 to 10 beats per minute, conserving oxygen. They can also constrict blood vessels to redirect blood flow to vital organs, extending their underwater endurance.
Dietary Variations and Opportunistic Feeding
While marine algae forms the overwhelming majority of their diet, marine iguanas can exhibit limited dietary flexibility under specific circumstances. Newly hatched juveniles sometimes consume feces from larger iguanas, a behavior believed to help them acquire necessary gut bacteria for digesting algae. On rare occasions, juveniles may also consume small invertebrates.
Adult marine iguanas primarily remain herbivorous, but during severe environmental events, such as El Niño, when preferred marine algae become scarce, they may opportunistically consume land plants. For instance, the population on North Seymour Island has been observed supplementing its diet with coastal succulents like Batis maritima. These instances highlight a degree of adaptability in extreme conditions, though these alternative food sources are not their typical fare.