The Marine Iguana: The Only Lizard That Went to Sea

The world has roughly 7,000 lizard species. Exactly one of them decided the ocean was worth the trouble. The marine iguana (Amblyrhynchus cristatus) is that lizard — the only one on Earth that forages at sea. It is found only in the Galápagos, on every major island in the archipelago, and it has been evolving here in isolation long enough to produce 11 traditionally recognized subspecies, each shaped by the island it inhabits.

The diet is almost exclusively algae — red and green species, at least 10 genera, including Centroceras, Gelidium, and Polysiphonia. To reach it, marine iguanas dive. Most foraging dives stay under 5 meters and last about 3 minutes. Larger individuals push deeper, holding their breath for 15 to 30 minutes at 7 meters or more. The verified record depth is 30 meters — about the height of a ten-story building, achieved on a single breath, by a lizard.

The salt problem is solved by a piece of anatomy found nowhere else in lizards: specialized cranial glands that filter excess salt from the bloodstream and expel it through the nostrils. The result is a lizard that appears to sneeze after every dive session. After a busy morning in the water, the head turns white from encrusted salt. It looks theatrical. It is entirely functional. Back to the Galapagos wildlife guide Galapagos wildlife guide for context on the ecosystem these animals inhabit.

During El Niño events, when warmer waters kill off algae, marine iguanas face starvation. The adaptation that follows is one of the strangest documented in any vertebrate: individuals can reabsorb their own bone mass, reducing their total body length by up to 20%. This is not metaphorical shrinkage. It is a measurable, reversible reduction in skeletal length. When algae returns, they grow back. No other vertebrate is known to do this.

The marine iguana’s dependence on nearshore algae is also its greatest vulnerability. When an El Niño event warms the water surrounding the Galápagos, the cold upwellings that sustain algae growth are suppressed. The algae dies back. The food supply collapses. Marine iguanas, which eat almost nothing else, begin to starve.

During strong El Niño events, population losses on some islands have reached 90 percent. The 1997–1998 El Niño is the documented reference event — the most severe on record and the most devastating for marine iguana populations. What happened after it is almost as significant as what happened during it: populations recovered. Within 4 years, even populations that had lost 30 to 50 percent of their individuals had rebounded — because when the algae returned, the iguanas that survived reproduced rapidly.

The body-shrinking adaptation becomes critical during these events. Individuals can reabsorb their own bone mass — reducing their total body length by up to 20 percent — as a response to starvation. A smaller body requires less food to maintain. When conditions improve and food becomes available again, they grow back. This is documented, physiologically verified, and unique among all vertebrates. There is no other animal known to science that responds to food scarcity by reversibly reducing its own skeleton.

The MV Jessica oil spill in 2001 demonstrated 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 — was reduced by nearly two-thirds. A single localized event devastated one island’s population even while the species as a whole remained viable. This is the pattern that conservation biologists watch most carefully: small island populations, particularly subspecies, are effectively irreplaceable if lost.

The subspecies facing the highest risk are those with the smallest populations. San Cristóbal carries approximately 400 individuals, Darwin approximately 800, Pinzón approximately 900. A single severe El Niño event — or another oil spill, or a disease introduction — could eliminate any of these subspecies permanently. Global warming is expected to intensify El Niño events in frequency and severity. This is not a theoretical projection; it is the current scientific consensus on climate trajectory in the eastern tropical Pacific.