The Marine Iguana: The Only Lizard That Went to Sea

There are roughly 7,000 species of lizard in the world, but only one has taken to foraging in the ocean. The marine iguana (Amblyrhynchus cristatus) is found only in the Galápagos, where it lives on every major island in the archipelago. Prolonged isolation has produced 11 traditionally recognized subspecies, each shaped by the island it inhabits.

Marine iguanas feed almost entirely on algae, including red and green species from at least 10 genera such as Centroceras, Gelidium, and Polysiphonia. To reach it, they dive into the sea. Most feeding dives stay below 5 metres and last around three minutes. Larger iguanas can go deeper, holding their breath for 15 to 30 minutes at depths of 7 metres or more. The verified record is 30 metres, roughly the height of a ten-storey building, on a single breath.

Marine iguanas have specialized cranial glands that remove excess salt from the bloodstream and expel it through the nostrils. That’s why they appear to sneeze after feeding. After a morning in the water, their heads can become coated in white salt. It may look dramatic, but it’s simply part of how they deal with life at sea.

During El Niño events, warmer water can reduce the algae marine iguanas depend on, sometimes leading to starvation. As an unusual response, they can reabsorb some of their own bone mass, reducing their total body length by up to 20%. This is a measurable and reversible change in skeletal length. The iguanas can then regain their size when algae becomes available again. No other known vertebrate is documented to do this. For more context, see our Galapagos wildlife guide

The marine iguana’s dependence on nearshore algae is also its greatest vulnerability. When an El Niño event warms the waters around the Galápagos, the cold upwellings that support algae growth are suppressed. The algae dies back, the food supply collapses, and marine iguanas, which eat very little besides algae, can begin to starve.

During strong El Niño events, population losses on some islands have reached 90 percent. The 1997–1998 El Niño is the main documented example, and it was particularly devastating for marine iguana populations. The population’s recovery afterward is almost as significant as what happened during the event. Within four years, even populations that had lost 30–50 percent of their individuals had rebounded, because the surviving iguanas reproduced rapidly once the algae returned.

The body-shrinking adaptation becomes especially important during these events. When food is scarce, marine iguanas can reabsorb some of their own bone mass, reducing their total body length by up to 20 percent. After all, a smaller body requires less food to maintain. When conditions improve and algae becomes available again, they can regain their size. This is a documented physiological adaptation and one of the most extreme responses to food scarcity known among vertebrates. No other reptile is known to respond to starvation by reversibly reducing its own skeleton.

The MV Jessica oil spill in 2001 showed a different kind of threat. The tanker ran aground near San Cristóbal and spilled fuel oil into the waters around the archipelago. The marine iguana population on Santa Fé, already a substantial subpopulation, fell by nearly two-thirds. A single localized event severely affected one island’s population even while the species as a whole remained viable. This illustrates why small island populations, particularly individual subspecies, can be especially vulnerable to local disasters.

The subspecies with the smallest populations face the greatest risk. San Cristóbal has approximately 400 individuals, Darwin around 800, and Pinzón around 900. A severe El Niño event, another oil spill, or the introduction of a disease could pose a serious threat to any of these small populations. Climate change is also expected to affect El Niño patterns in the eastern tropical Pacific, although the extent to which future events will become more frequent or severe remains an area of ongoing scientific research.