The Galapagos Marine Iguana: At a Glance
- Scientific name: Amblyrhynchus cristatus
- Taxonomy: Reptilia > Squamata > Iguanidae > Amblyrhynchus
- Conservation Status: Vulnerable (IUCN)
- Endemic to the Galapagos Islands: Yes – the world’s only sea-going lizard.
- Feeds mainly on red and green algae in the intertidal zone, usually within about 50 m of the shore and to depths of around 10 m.
- Bask on black lava rocks before and after feeding to warm its body; avoids overheating by changing its posture so body temperature stays below ~35 °C.
- Can remain submerged for 10 minutes or more; some individuals have been recorded diving to around 30 m, with heart rate slowing from roughly 40 to about 10 beats per minute.
- Expels excess salt through a specialised gland above the eyes, “snorting” out salty droplets that leave a whitish crust on the face.
- Seven recognised subspecies vary greatly in size and colour:
- Largest animals occur on Isabela (up to 1.3 m long and around 12 kg).
- Smallest on Genovesa (sometimes under 1 kg).
- Española iguanas are among the most brightly coloured, especially in the breeding season.
- Males are larger and more strongly coloured than females; during the mating season they can take on vivid red and green tints.
- Breeding season usually begins around December–January, with egg-laying in March–April. Females dig burrows up to 1 m long and lay up to four leathery eggs that incubate for three to four months.
- Hatchlings (about 10 cm long and almost entirely black) face heavy predation from frigatebirds, herons, hawks, snakes, moray eels and introduced predators such as feral cats and dogs.
- Adults have few land predators beyond hawks and feral animals; at sea their main natural enemies are sharks.
- Severe El Niño events can cause major population crashes; in extreme years around two-thirds of animals may die as their algal food disappears.
- Adult marine iguanas can shrink their body—including the skeleton—by more than 5 cm in years of food shortage, then regrow when conditions improve; individuals that shrink the most have the best survival chances.
Marine iguanas in the Galapagos
The marine iguana is one of the Galapagos’ most distinctive residents and an animal found nowhere else on Earth. Endemic to the archipelago, it is the only lizard that forages in the sea. What looks at first like a line of motionless black rocks along the shoreline often turns out to be a colony of iguanas warming themselves in the sun, conserving energy for their next feeding dive.
On our Safaris, guests encounter marine iguanas frequently: draped over warm lava at low tide, easing themselves into the surf, or grazing quietly underwater. They are a reminder that life here has adapted not just to survive, but to make full use of a coastal environment that appears, at first glance, almost bare.
Built for the ocean
Marine iguanas are reptiles that have committed to the sea without ever leaving the land. Their bodies tell the story. Dark, heat-absorbing skin helps them warm quickly after a cool-water dive. A long, flattened tail acts as a powerful rudder, pushing them through the water with a side-to-side motion. Strong limbs and sharp claws give them the grip they need to cling to slippery lava while waves break around them.
Once underwater, they graze on short swards of red and green algae growing on submerged rocks. Because they inevitably swallow seawater as they feed, they rely on a specialised salt gland above and between the eyes. From time to time they expel the excess with a sharp snort, sending a spray of brine from their nostrils. The whitish crust that often coats their faces is simply salt.
Like all reptiles, marine iguanas depend on external heat. They usually dive only when the sun has warmed both their bodies and the sea surface. In the hot season they may spend close to an hour a day in the water; in the cool season that window can shrink to twenty minutes. After each foraging trip they haul themselves back onto the rocks to warm up again, adjusting their posture to absorb more or less heat as needed.
Life across the islands
Although all marine iguanas belong to a single species, the landscape of each island leaves a different mark on them. Seven subspecies are recognised. On Isabela, where nutrient-rich upwellings support dense algae growth, iguanas grow largest, with record animals reaching around 1.3 metres and 12 kilos. On Genovesa, where resources are more limited, the largest individuals may weigh less than a kilo.
Colour varies too. Most marine iguanas are black or dark grey, blending almost perfectly with the lava. During the breeding season, however, males develop striking red and green hues, particularly on Española, where some animals look almost painted. Males set up small territories on the shoreline and defend them vigorously, while smaller males adopt more discreet tactics to secure mating opportunities of their own.
Living with extremes
Marine iguanas live in a place where the rules change regularly. In normal years, cold currents deliver the algae they depend on. During strong El Niño events, those currents weaken and warm water spreads through the islands. Red and green algae die back, replaced by tougher brown species that are far less nutritious.
When this happens, the impact can be severe. During the extreme El Niño of 1982–83, researchers recorded the loss of roughly two-thirds of the population on some islands, with the youngest and the very old suffering the most. In the decade that followed, scientists noticed something even more unusual: measurements of marked individuals showed that adult iguanas were literally becoming shorter.
Marine iguanas possess a flexible skeleton that allows them to shrink when food is scarce, sometimes by more than five centimetres in length, and to regrow when conditions improve. Animals that shrink the most appear to have the best chance of surviving hard years. It is one of the clearest examples of how finely tuned these reptiles are to their environment.
Darwin’s encounters
Early visitors did not always warm to marine iguanas as quickly as we do today. Charles Darwin described them as “disgusting, clumsy lizards,” and referred to them as “imps of darkness,” a phrase that had also been used a decade earlier by Captain George Byron of HMS Blonde when he encountered crowds of iguanas on Fernandina.
What began as revulsion quickly shifted to curiosity. Dissections revealed stomachs “largely distended with minced seaweed,” showing Darwin that their diet came entirely from the sea, and he was struck by how well they swam, using their tails and claws to move with surprising grace underwater.
One of his most memorable observations came from an iguana he repeatedly tossed into a tidal pool, only for it to climb back to him each time rather than remain in the water. Today, this behaviour is understood as a sensible instinct: on land, adult marine iguanas have few natural predators, whereas at sea they face sharks. For Darwin, it was another clue to how animals adapt to the particular challenges of island life.
Where to see marine iguanas
Marine iguanas are widespread along rocky shores throughout the archipelago, and are often among the first wildlife guests notice on arrival. On our land-based Safaris you are likely to encounter them on the coastlines of Santa Cruz and Isabela,
Their behaviour changes with the tide and the sun. In the cool of the morning, they bask in tight groups on lava ledges, eyes half-closed. As the day warms, some slip into the water to feed, while others remain ashore, adjusting their position to the angle of the sun. For many guests, the most memorable encounters happen while snorkelling, watching them graze calmly on the rocks as reef fish swirl around them.
Plan your Safari
To learn more about how you can encounter marine iguanas and other Galapagos wildlife on a land-based itinerary, see our Safaris. Our team will help you choose the right combination of islands, activities and pace, with plenty of time to watch these remarkable reptiles going about their daily lives.
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