The endemic Marine Iguana (Amblyrhynchus cristatus, iguana marina) is the only sea-faring lizard in the world. Adult marine iguanas are mostly black or dark gray, but some races have a colorful red and green lichen-like covering on their backs, the result of dies in their algal diet. They also have an elongated tail to help them swim; a flat head; a pronounced crest running the length of their backs; and large, webbed feet.

Marine iguanas live on land, but they feed on red and green algae in the cool ocean waters. Smaller iguanas keep to the intertidal zone, but others venture to depths of up to 10 meters and stay submerged for up to ten minutes looking for food. After a swim, they will return to land to bask in the sun or huddle with others for warmth (they are cold-blooded) and perform an unforgettable snort in order to release an excess of salt from their nostrils.

Marine iguanas have the same general reproductive cycle as land iguanas, laying their eggs in the beginning of the year. Their mating behavior, however, is a bit different. Marine iguanas are polyganous, meaning female iguanas accept a number of male partners, often as many as fifteen. Females lay three eggs in their underground nests instead of one, which take up to three months to incubate. Young marine iguanas face the same predatory threats when they hatch, but they have the added challenge of thwarting marine predators. If they survive through the formative period, they are expected to live for 40 years.

Marine iguanas are interesting to watch, not only because they are unusual and entertaining, but because they tell an interesting evolutionary tale. When iguanas originally arrived to the islands, they were strictly land-based creatures; they evolved the ability to swim over time, so that they could make use of unexplored niches. Furthermore, every El Niño year, you can witness the complicated dynamics of natural selection at work upon the marine iguanas: sexual selection favors larger individuals, while climatic conditions work against them, resulting in more medium-sized individuals in subsequent generations.

Marine iguanas are found on all of the major islands.