People such as Charles Darwin described the marine iguanas as black lizards thriving on the sea. But marine iguanas are not usually really black in color. The younger marine iguanas take the dorsal stripe color whereas the adults are grayish. Their color may be really dull but there is a reason behind this. Such color allows them to easily absorb heat right after they emerge out of the water. Their food is usually marine algae. They expel the remaining salts from their nasal glands as they bask under the heat of the sun. The salt contained in their bodies makes their faces appear whiter.

Mature male marine iguanas have varying colors — depending on the season. During the breeding season, the adult male marine iguanas become teal-green or reddish in color. Those thriving in Santa Cruz appear black and brick red, while those found in Fernandina appear dull green and brick red. Their sizes likewise differ depending on the island that they live on. Those found in Isabela and Fernandina are the largest marine iguanas on the entire island of Galapagos. Meanwhile, the smallest marine iguanas are to be found in Genovesa.

As poikilothermic animals, marine iguanas can spend very little time out in the cold sea whenever they dive for their algae food. Thus, they commonly swim within the very shallow water off the island. After swimming, they proceed to bask under the sun to have their bodies warmed up again. During cold days, marine iguana cannot effectively move thus making them very vulnerable to their predators. Since they cannot move fast, what they do is to strike their tails up in the air and bite their enemies. During the entire breeding season, male marine iguanas mate with the females and likewise guard them against the rest of the male reptiles.
Moreover, marine iguanas adjust their sizes to be able to adapt to the food available. There was a time during the onset of the El Niño when the algae decreased so the marine iguanas actually decreased in length too. When the food supply started to normalize, they returned to their normal size. Marine Iguanas are not as fast as the rest of their relatives. When taken as pets, their predators are dogs and cats. They can easily be attacked by these predators since they are slow and tame.

The government of Ecuador has set up laws that aim to protect the Marine Iguanas. Artificial nesting sites have been made on the smaller islands so that there will be fewer predators that can harm them.