A Cold-Blooded Diver
The Galapagos marine iguana, Amblyrhynchus cristatus, is the world's only lizard adapted to a marine lifestyle. It forages at sea, an evolutionary path taken by no other modern lizard. On Fernandina Island, one of the most pristine and volcanically active islands in the Galapagos, large colonies of these reptiles cover the black lava shorelines. The largest subspecies of marine iguana are found on Fernandina and neighboring Isabela Island.
As ectotherms, marine iguanas depend on external heat to function. The waters around the Galapagos can be as cold as 11°C, a dangerous temperature for a reptile. Before entering the water, they must bask on the volcanic rocks to raise their body temperature to an optimal 35.5°C. A dive causes their body temperature to drop by as much as 10°C. To conserve heat and oxygen, their heart rate slows dramatically during a dive. Most foraging dives are shallow, between 1 and 5 meters, and last for 5 to 10 minutes, though they have been recorded diving as deep as 30 meters and staying submerged for up to an hour. After foraging, they must return to the rocks to bask, absorbing solar radiation through their dark skin to regain their optimal body temperature. While warming up, they are sluggish and vulnerable, often huddling together in large piles at night to conserve heat.
An Engine of Salt
A diet of marine algae means ingesting a huge amount of salt, which would be lethal for a terrestrial reptile. The marine iguana's kidneys cannot process this much salt from its bloodstream. Instead, it has evolved specialized salt glands connected to its nostrils. These glands filter excess salt from the blood. The iguana then expels this highly concentrated salt solution by "sneezing." This salt spray often coats the tops of their heads, giving them a distinctive white and crusty appearance.
The size of marine iguanas varies dramatically across the archipelago, a difference driven by food availability. The largest individuals, found on Fernandina and Isabela, can weigh up to 12 kg, while the smallest subspecies, on Genovesa Island, weighs only around 1 kg. This adaptation is a direct response to the differing ocean temperatures and algae growth around the islands. The species is listed as Vulnerable by the IUCN, facing threats from climate change, pollution, and introduced predators like cats and dogs. El Niño events are particularly devastating, as warmer waters kill the algae, leading to mass starvation.
A Famous Symbiosis
The marine iguana's ecosystem includes examples of mutualism. Darwin's finches and Galapagos mockingbirds often perch on the iguanas to pick off and eat ticks and dead skin. The relationship provides the birds with a meal and relieves the iguanas of parasites. Lava lizards also congregate near iguana colonies, feeding on the flies attracted to the large reptiles. While the iguanas have few natural predators, they are prey for Galapagos hawks and snakes. Their eggs and young are especially vulnerable to invasive species like rats, cats, and dogs.