“The marine iguana is the only living lizard that forages in the sea.”
The marine iguana is a lizard found only in the Galápagos Islands of Ecuador. It is the only living lizard that forages in the sea, grazing on marine algae and expelling excess salt through specialized nasal glands.
By Wildlife Encyclopedia · Updated
Amblyrhynchus cristatusSquamataIguanidae
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The marine iguana is the only lizard alive today that feeds in the sea. Found nowhere except the Galápagos Islands of Ecuador, it spends much of its life on bare lava rock at the water's edge, entering the cold Pacific to graze on algae and hauling back out to warm in the sun. Charles Darwin, who examined the species during the 1835 voyage of HMS Beagle, described the dark, blunt-faced lizards as "imps of darkness." Behind that unflattering first impression sits a set of adaptations that let a reptile make a living from the ocean.
Marine iguanas are stout, dark-colored lizards with a spiny crest running along the back and a long, laterally flattened tail. That tail is the main swimming organ: pressed side to side, it drives the animal through water while the limbs are held close to the body. Adults range from roughly 0.6 to 1.5 metres in total length including the tail, with body mass varying widely between islands and between the sexes; large males in the biggest subspecies can weigh well over 10 kilograms, while animals on other islands stay much smaller.
Several features suit the species to its unusual diet. Long, sharp claws grip wave-washed rock so the iguana is not swept off while feeding, and a short, blunt snout lets it press its jaws flat against the substrate to scrape algae free. Dark skin absorbs solar heat quickly, which matters because cold water rapidly cools the body during dives. Perhaps the most distinctive adaptation sits inside the skull: glands connected to the nostrils remove salt taken in with seawater and food, concentrating it and ejecting it in a sharp sneeze. The dried salt often leaves a pale crust on the head.
The marine iguana is endemic to the Galápagos archipelago, meaning it occurs naturally nowhere else on Earth. Its range covers the rocky shorelines of the islands, a coastal habitat of black lava, tide pools, and surf-battered ledges. Populations differ from island to island, and taxonomists recognize a number of subspecies tied to particular islands such as Fernandina, Española, and Genovesa.
These lizards stay close to the sea. They bask, rest, and nest on shore, and feed in the intertidal zone or in shallow water just offshore. Because the surrounding currents are cold, the marine iguana depends on sun-warmed rock to raise its body temperature high enough to be active, then loses that heat again each time it enters the water. This tie between warm rock and cold sea shapes almost everything the animal does during the day.
Marine iguanas are herbivores, and their diet is narrow: they eat marine algae, chiefly red and green species, along with other seaweed growing on rock. Smaller and younger animals tend to graze in the intertidal zone at low tide, scraping films of algae from exposed rock. Larger adults, especially big males, can swim out and dive to feed on algae growing below the surface, reaching depths of around 30 metres and remaining submerged for several minutes at a time.
Feeding underwater in cold water is costly. The iguana can only stay in the sea while its body stays warm enough to function, so foraging bouts are limited and are followed by long periods of basking. The salt taken in with this marine diet is dealt with by the nasal glands rather than the kidneys, which is why a resting marine iguana is often seen sneezing out fine sprays of salty fluid.
Day-to-day life alternates between two states: warming on rock and cooling in water. After a dive, an iguana's body temperature can drop sharply, so it returns to dark lava and basks, sometimes pressing flat against the stone or facing the sun to control how much heat it takes in. Large numbers often pile together on favored rocks, and this clustering can help retain warmth overnight.
Behavior shifts during the breeding season. Males become territorial, defending patches of rock and displaying to rivals with head-bobbing and, if pushed, wrestling matches in which they push head to head. The rest of the year the animals are far more tolerant of one another and gather in dense groups without much conflict. One of the species' most remarkable responses is physiological rather than social: during El Niño years, when warm water kills off the algae they depend on, marine iguanas can actually shrink in body length, reducing their food needs, and then regrow once cooler, productive conditions return.
Breeding follows the territorial season. Females move to sandy or ash-covered ground, sometimes well back from the shore, and dig a burrow in which they lay a small clutch, typically one to six eggs. The eggs are buried and left to incubate underground for roughly three months, warmed by the substrate rather than by a parent. A female may guard her nest site briefly against other females competing for digging space, but there is no extended parental care.
Hatchlings dig their way to the surface and are immediately independent. This early stage is the most dangerous of their lives: newly emerged young are small, slow on land, and exposed to a range of predators before they reach the relative safety of the adult population.
The marine iguana (Amblyrhynchus cristatus) is a reptile in the order Squamata and the family Iguanidae, the same family as the familiar green iguana. It is the only species in the genus Amblyrhynchus and its closest relatives are the Galápagos land iguanas, from which it is thought to have diverged after the ancestors of both colonized the islands. The split between a plant-eating land lizard and a sea-grazing one is a classic example of how isolation on the Galápagos produced unusual forms.
Comparisons with other large lizards highlight how specialized the marine iguana is. The Komodo dragon is a giant carnivorous monitor of Indonesian islands, while the frilled lizard is an arboreal, insect-eating agamid of Australia and New Guinea. Among Galápagos animals, the marine iguana shares its islands and its Vulnerable-to-threatened conservation context with the giant Galápagos tortoise, another endemic reptile shaped by life on the archipelago.
The IUCN Red List classifies the marine iguana as Vulnerable, with an overall population trend listed as decreasing. Estimating numbers is difficult because the total population swings enormously with ocean conditions: strong El Niño events can kill a large share of animals in a single famine, while productive years allow recovery. Several of the island subspecies are considered more threatened than the species as a whole.
The clearest human-linked threat comes from introduced animals. Feral cats, dogs, rats, and pigs prey on eggs and hatchlings on several of the islands, and populations near these predators are especially at risk. Oil spills, marine pollution, and disturbance add further pressure, and climate change is expected to make El Niño famines more frequent or severe. Because the entire species exists only in the Galápagos, its future depends heavily on controlling invasive predators and protecting the archipelago's shorelines.
Kingdom
Animalia
Phylum
Chordata
Class
Reptilia
Order
Squamata
Family
Iguanidae
Genus
Amblyrhynchus
Species
A. cristatus
The marine iguana is the only living lizard that forages in the sea.
It expels excess salt through nasal glands, producing a distinctive sneeze that leaves a white crust on its head.
During lean El Niño years, marine iguanas can shrink their body length by up to about 20 percent, then regrow when food returns.
Charles Darwin, who saw them during the 1835 voyage of HMS Beagle, called them 'imps of darkness'.
Large males can dive to depths of around 30 metres and stay submerged for many minutes while grazing.
Their dark skin helps them reabsorb heat quickly after cold-water dives lower their body temperature.
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The marine iguana (*Amblyrhynchus cristatus*) is a lizard endemic to the Galápagos Islands of Ecuador. It is the only living lizard that feeds in the sea, grazing on marine algae along rocky shores.
They live only in the Galápagos Islands, an Ecuadorian archipelago in the eastern Pacific. They stay close to rocky coastlines, where they bask on lava rock and enter the water to feed.
They are herbivores that feed almost entirely on marine algae, including red and green species. They scrape algae from rocks in the intertidal zone or while diving in shallow water.
Feeding in seawater loads their bodies with salt. Specialized glands connected to the nostrils concentrate that salt, and the iguana ejects it in a forceful sneeze that often leaves a white crust on the head.
Cold Galápagos currents lower their body temperature during dives, so they cannot stay in the water long. After feeding they haul out onto dark rocks and bask, using their dark skin to reabsorb heat before their next dive.
The IUCN Red List classifies the species as Vulnerable, with a decreasing trend. Numbers swing sharply with El Niño famines, and introduced cats, dogs, rats, and pigs prey on eggs and young on several islands.
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