Marine Iguanas are large and dark with variable coloration. They are distinguished from land iguanas by the flattened, square nose which is an adaptation for feeding on marine algae. Marine iguanas also have a flattened tail which is an adaptation for swimming.
The adult male has a row of long spines on the head and along the back and tail. They have a variable pattern on the body which ranges from red and black to green, yellow, and black. Marine iguanas on Fernandina Island in the Galapagos have adapted to the lava rocks by having a dark sooty grey coloration.
During the mating season, the adult male assumes a a considerably brighter coloration. The adult female is considerably smaller than the adult male and is wholly dark, with a ridge of short spines along the back and tail. Marine Iguanas are highly territorial, particularly during mating season. Mating season is from November to December. Males take up to eight years to reach sexual maturity.
Marine iguanas are found predominantly along the rocky shores, with the females and younger iguanas basking together in large groups. The marine iguana is the only marine lizard in the world, and can spend up to an hour under water.