Marine iguana: how a lizard feeds in the sea and sneezes out salt

On a black lava shore in the Galapagos, a dark lizard hauls itself out of the Pacific, settles on a warm rock and sprays a squirt of liquid out of its nose. Visitors call it a sneeze. It is the lizard’s way of getting rid of salt, and it has just finished lunch, which was seaweed and was eaten underwater.

The marine iguana (Amblyrhynchus cristatus) lives only in the Galapagos Islands, according to the Animal Diversity Web, and it eats almost nothing but marine algae. That is an unusual way to make a living for a lizard: its dinner table is the sea. None of the sources we read names another lizard that forages in the ocean.

This article covers how it feeds (not every iguana dives), how it deals with all that salt, what happens when El Niño wipes out the algae, and why the species is rated Vulnerable. We also point out where popular pages and scientific papers disagree, because on this animal they often do.

Meet the marine iguana

The Animal Diversity Web describes a gray-to-black lizard with pyramid-shaped scales on its back and a shorter, blunter snout than its land cousins. The genus name comes from Greek words for “blunt” and “snout,” says the Reptile Database. Young iguanas have a lighter stripe down the back. The animal lives on steep rock cliffs, low ledges and intertidal flats, and it needs sandy or ashy ground for nesting.

Two oddities from the Reptile Database. First, the first formal description, by Bell in 1825, gave Mexico as the home of the species. That is recorded as an error, and the location was later corrected to Fernandina Island. Second, the eleven subspecies include one named godzilla. Nobody said the lizards had to look the part.

The Animal Diversity Web adds that land and marine iguanas may have split at least 10 million years ago, on islands now below sea level. The Reptile Database reports that hybrids with Galapagos land iguanas have been described, with viable offspring. That page is a database summary, so we treat the hybrid reports as reports, not as a measured rate.

Two ways to eat algae

Not every marine iguana dives. The Animal Diversity Web says larger individuals dive at high tide, while smaller ones feed in the intertidal zone, the strip of shore that the tide uncovers, at low tide. The blood-chemistry study adds that intertidal and subtidal algae are different mixes, and the Galapagos Conservancy lists three ways to eat: grazing exposed rocks, working the subtidal zone, or diving in cold seawater.

How deep is deep? A 2025 study says in its introduction that large adults can dive to below 30 meters, where algae are most abundant. It cites a 2004 paper by Wikelski and Nelson for this, so it is a reported figure, not a new measurement. The introduction adds that smaller, younger iguanas mostly stay in shallow water and on rocky shores. We could not find a measured dive time in the peer-reviewed sources we opened. Popular pages give times anywhere from a few minutes to an hour, which is too wide to trust, so we leave it out.

A cold sea is a problem for an animal that relies on outside heat. The Animal Diversity Web says the iguana basks on rocks to warm up, that its dark color helps it absorb heat, and that males can switch to breeding colors to rewarm faster. That is the whole day in one sketch:

Treat the table as a sketch. Real timing follows the tides and varies by island.

The nose does the dishes

Seawater is salty, and algae are salty too. The blood-chemistry study says an algae diet brings a lot of sodium and chloride into the body, and that salt glands excrete most of the salt. The authors suggest that tolerance of high sodium may have helped the ancestors of marine iguanas start eating algae at all. The Galapagos Conservancy says the gland connects to the nostrils.

What comes out looks like a sneeze. The Galapagos Conservation Trust, quoted by Discover Wildlife, says the lizard filters its blood at the nose and sneezes out the excess, and that the salt often forms crystals on the snout. A white crust on an iguana’s snout is therefore not dandruff. It is dishes done with the nose.

Hard times: El Niño, shrinking and slower living

El Niño is a warm-water event. It warms the Pacific, reduces the upwelling of cold, nutrient-rich water, and can cause large die-offs of the algae the iguanas eat, according to the 2025 study. The Galapagos Conservancy says El Niño is the biggest cause of death, with up to 70 percent dying in some populations in the 1982 to 1983 event. The 2025 paper says mortality can reach 90 percent in the most susceptible populations. The two figures come from different sources and are not the same measurement, so neither is the last word.

Survivors do something odd. In 2000, Martin Wikelski and Corinna Thom reported in Nature, as covered by Science News, that some iguanas shrank by as much as 20 percent (up to 6.8 centimeters of snout-to-vent length) over two years of famine. More than 100 of about 6,000 iguanas tracked across four El Niño events showed the shortening, and it reversed afterward. Wikelski calculated that shrinking connective tissue could explain only about a tenth of it, so bones themselves must shrink. In 1992 to 1993, large adults that shrank more lived longer.

Not everyone is convinced it helps. One researcher quoted by Science News suggested the shrinking may simply be starvation. Another proposed that big males shrink most because defending territory leaves them little time to feed. A third said only X-rays could confirm the bone change. So the shrinking is real in the data, and the explanation is still debated.

A 2025 study on San Cristóbal Island found another response. After an El Niño year, the resting metabolic rates of juveniles and subadults (19 measured in each year) were about 20 percent lower than after a neutral year, at the same body mass. The authors suggest an energy-saving response, but warn the sample is small and that they could not separate metabolic depression from other effects.

Common mistakes and myths about marine iguanas

Mix-up: all marine iguanas dive. Most feed on exposed rocks at low tide. Diving is mainly for the largest individuals.

Wrong reading: the sneeze is a cold. It is a salt gland at work, not an illness.

Myth: size and dive numbers are well settled. Popular pages disagree on dive depth and time, and even on size. Measured numbers come from small studies, like the 35 iguanas above.

Overstated: the shrinking is a settled survival trick. One researcher suggested it may simply be starvation, so whether it helps is still debated.

Mix-up: they are the same lizard as the land iguanas of the same islands. They are related and may hybridize, but the Animal Diversity Web says the two groups may have split at least 10 million years ago.

Mix-up: it is the only reptile in the ocean. Other reptiles live in the sea, such as the Green sea turtle and the Yellow-bellied sea snake. The marine iguana is the lizard in that company. For another island lizard, see the Komodo dragon.

Conservation status

The marine iguana is listed as Vulnerable on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Vulnerable means the species was evaluated and judged to face a high risk of extinction in the wild. It is the lowest of the three threatened categories.

The pressures in our sources are natural and human. El Niño die-offs are the biggest, per the Galapagos Conservancy. Introduced cats have had a major impact there, the Animal Diversity Web names feral rats, dogs and cats as threats to eggs and adults, and birds take young iguanas. The 2001 Jessica oil spill is another. About 62 percent of the marine iguanas on Santa Fe Island died within a year of it, according to ScienceDaily’s report of a Nature study, which suggested the oil killed gut microbes that help digest algae. ScienceDaily puts the spill at nearly 800,000 gallons, a figure we did not cross-check. The blood-chemistry study also lists pollution and disease, and says one subspecies, from San Cristóbal, has been described as endangered.

There is some good news. The Galapagos Conservancy estimates 200,000 to 300,000 iguanas in all, with up to 4,500 per mile of coast in some places. We have no confirmed population trend: none of the sources we opened states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.