There is only one true living sea lizard: the marine iguana (Amblyrhynchus cristatus), a native of the Galapagos Islands. Despite their terrific look, with short snout, black eyes, large mouth and the crest of spikes (which are in fact soft at touch), long, flattened and powerful tail, long, sharp claws that give them an aspect of Mesozoic monsters, these lizards are peaceful vegetarians. They dive and eat algae and weeds from the sea bottom, but also those plants exposed by the low tide. From time to time, they eject, through their nostrils, a brine liquid, excreted by special salt glands, an adaptation that makes the iguanas cope with the salted sea water.
Adult marine iguanas do not overpass 1.3 m (4.3 ft) in length, but each island of the archipelago has its type of marine iguana, with different sizes and colors. Males can weigh up to 3.5 kg (8 pounds).
In Fernandina Island, the marine iguanas are black, resembling the color of the lava, while those of the Espanola Island display a combination of red and green and never exceed 1 m (3.3 ft) in length. The marine iguanas of the Genovesa Island are not longer than 45 cm (1.5 ft), while those of Cristobal Island are dark-gray. Those of Santa Cruz Island are yellowish-green. Zoologists consider there are about 50 subspecies of this iguana species.
The Galapagos archipelago is not older than 5 million years and none could clearly say how these lizards reached the islands. They surely came from South America, located 968 km (605 mi) away, and ground South American iguanas are known to be able to cross rivers.
The marine iguana uses its flattened tail to swim and dive, even in agitated waters, as the violent waves seem not to pose a problem for them. Once they return to the shores, they get grips on the rocks using their powerful claws. Still, their ancestors did not come from South America swimming; they rather traveled on tree trunk, transported by the Humboldt current to make the distance from the mainland to the islands.
No fossil of marine iguana was found in South America, and it seems that this iguana entered the sea after reaching the Galapagos, probably forced by the scarcity of suitable food on land. A curious issue is that the iguanas seem not to pass from one island to another. In fact, the islands can be separated by narrow canals crossed by strong currents. Moreover, the marine iguanas do not go off the shore more than 400 m (1,330 ft).
On land, these iguanas have no enemies, while in the water they can be attacked by sharks. Sometimes, seals play with them, grabbing the lizards of their tails in the water.
When diving, the iguanas empty their lungs of air to annul their floatability. A dive is usually made between depths of 3-15 m (10-50 ft) and lasts 15-20 minutes. But the marine iguanas can stand more than one hour underwater. The limit of the dives is 28 m (93 ft); at this depth the base of the thorax and the abdomen are almost flat. The heartbeat of a diving male marine iguana descends from 45-50 beats per minute in resting conditions to 8-9 at a depth of 15 m after 15 minutes. After that, the rhythm drops to 4-5 beats per minute, the animal stopping blood circulation, which is restricted just to the brain.
The main problem for the iguanas is not the oxygen, but the water temperature. Even if located at Equator, the cold waters of the Humboldt current plummets the temperature of the ocean waters around Galapagos to 12-22o C. The waters around Galapagos are with 8-10o C colder than other equatorial sea waters, while the sand of the beaches can frequently reach 60o C.
Lizards are cold-blooded animals; the iguanas depend on the sun heat. That's why they spend a lot of time taking sun baths before entering the water for feeding, then, at the return, they bask again so that their body temperature returns to 35-37o C.
Marine iguanas start their breeding season to the end of the February. The males delimit their territories which they will defend and where females are attracted. The males attempt to retain as many females as possible. Courtship is like in any iguana: after a few head bobbings, the male grabs the female's nape with his mouth and the copulation takes place. After mating, females gather in nesting places, which are not very numerous. They are represented by sandy embankments, 50-60 m (166-200 ft) tall. Females fight against each other for the possession of a place where to dig a 30-40 cm (1-1.3 ft) long tunnel, in which they lay their eggs. The nests can be so close that a female found in her tunnel can be covered by the sand thrown by another female digging her lair.
The eggs have a soft shell and incubate in 2 months. Meanwhile, the females remain in the area, checking the eggs with their tails.
The 16 January 2001 oil spill of "Jessica", just 100 tonnes of petroleum, in the Galapagos, produced huge damages to the marine iguanas, fur seals, pelicans, sea turtles, boobies, and albatrosses. The petroleum did not attack directly too many large animals,but it destroyed algae, food of the iguanas, and fish populations.
Fluctuations of the El Niño cause huge natural mortalities in the species depending on the sea resources, as their 'food' (fish, crustaceans, corals and algae) dies or gets scarce. In 1982-1983, this phenomenon cut the population of sea lions and sea birds to half and the marine iguanas were menaced with extinction, as the green algae on which they feed were replaced, due to the higher temperatures, by brown algae which they cannot digest.
Some monitor lizards dwell the sea beaches looking for corpses brought by the sea, but they are not genuine sea lizards; some may cross short distances on the sea (like the Komodo dragon), but they do not depend on the sea. In fact, real sea lizards were the mosasaurs, marine monsters from the dinosaur era, relatives of the living monitor lizards. Some reached up to 15 m (50 ft) in length, and were the most fearsome reptiles ever to enter the seas.