An odd condition on the Galapagos is that reptiles are more common than mammals. This is not the case in most areas of the world. The ancestors of the reptiles found today on the Galapagos were hardy enough to make it across 600 miles of ocean from the South American mainland aboard rafts of vegetation. Mammals and amphibians for the most part did not survive such direct exposure to the sun, and could not survive as reptiles did, with out water for weeks on end.
Perhaps the ancestor of the land and marine iguanas was similar to this green iguana found in Ecuador.
Marine iguanas forage on red and green algae. Young iguanas and females don’t venture too far off the rocky coast, but males can forage to depths of 10 meters. Being ectotherms they walk a tight rope between feeding the in cold water and staying warm enough to actively feed and digest their algae salads. The body temperature of the male iguanas can drop as much as 10 degrees Celsius while they are foraging. They are much larger than the females, with some males weighing as much as 10kg.
The animals I saw spent a great deal of time basking in the sun. The young iguanas like those below clustered in areas away from the adults.
These iguanas are the only ones in the world who forage in the ocean, and as you can imagine they consume a great deal of salt in their diet. To rid their bodies of this salt they blow it out their nose from salt glands behind their eyes. They have the most efficient salt glad of any reptile, but I wouldn’t be surprised if the salt gland found in ocean going birds was more advanced. :) Take a look at the nostrils of the iguana in the photo below. The slight tube shape helps them expel the salt. The sneezing noise they make as they excrete salt is the only noise these lizards make.
Their dark color helps them absorb heat, and is great camouflage. There are numerous marine iguanas in the photo below. How many do you see?
El Nino events can raise the water temperature from a normal temperature of 18 to 32 degrees Celsius. This kills the algae upon which these iguanas feed, and can cause their populations to crash. During times of extreme starvation these animals can reabsorb bone and actually shrink in size to survive.