When Charles Darwin visited the Galapagos Islands in 1835, he was a young, probably still a bit immature man, and thus I can forgive him the unflattering description of one of the most amazing reptiles that has ever lived on our planet, which he included in his diary of the voyage of the Beagle. Marine iguanas (Amblyrhynchus cristatus) are the world’s only marine lizards. They feed on nothing else but a few species of marine algae that they graze underwater in the cold seas that surround the Galapagos, and never stray away from the forbidding, volcanic shores of the islands. Their appearance evokes the image of the long-gone reptilian giants that once ruled the world, and seeing them in person was to me one of the most thrilling experiences of my life.
Not so for young Charles, who found these remarkable reptiles rather underwhelming, and opined that “It is a hideous-looking creature, of a dirty black colour, stupid, and sluggish in its movements.” But by watching them, and torturing them a little (“One day I carried one to a deep pool left by the retiring tide, and threw it in several times as far as I was able.”), he ascertained that their “apparent stupidity may be accounted for by the circumstance, that this reptile has no enemy whatever on shore, whereas at sea it must often fall prey to the numerous sharks.” He was mostly right about this, although marine iguanas have an enemy on land, the Galapagos hawk, and young animals often fall prey to other birds and snakes. In fact, marine iguanas have learned to recognize the alarm calls produced by mockingbirds when these birds detect the presence of a hunting hawk, and react to the sound by immediately scattering and hiding among boulders.
Marine iguanas, despite their primordial appearance, are a young species, derived from its South American ancestor who arrived to the newly formed archipelago no more than a few million years ago. But within these few million years the ancestral iguana, which is speculated to have been similar to the modern green iguanas (Iguana), completely changed its appearance and physiology. Its face turned puggish and perfectly adapted for grazing short algae growing on underwater rocks, the tail became laterally flattened and fin-like and, most importantly, marine iguanas evolved the ability to drink sea water and expel the excess of salt from their bodies. Their skin turned black to speed up the process of warming up before getting into the chilly waters and digesting the algae afterwards, and the animals developed high tolerance for the crowded, gregarious conditions forced upon them by the shortage of available space on the shores of the Galapagos.
Currently, seven subspecies of marine iguanas are recognized, varying in their coloration and body size. The smallest animals are found on the island of Genovesa, where they reach the maximum weight of 500 g (about 1 lb), and when I saw them there I was convinced that they were very young juveniles. In fact, they are small because of the limited availability of marine algae, which in turn drove the selective pressure towards the smallest reproductive individuals. On the island of Isabela, however, live truly monstrous animals, with many males reaching the weight of 12 kg (about 26 lb). This, of course, is the result of rich algal pastures in the waters that surround this island.
Nobody really knows how many marine iguanas live in the Galapagos, and estimates of their numbers vary wildly between 50 and 250 thousand individuals. But all in all, they are doing quite well, albeit introduced dogs and rats have had a negative effect on their populations. While in the Galapagos I was mildly frustrated by all the restrictions and regulations put in place by the Ecuadorian national park authorities, but at the same time I am incredibly impressed with the effectiveness of their conservation actions and strategies. Young iguanas were basking on the busy piers of Puerto Ayora, the largest town in the Galapagos, a place where they once were virtually extirpated, and on almost every island that I visited iguanas ruled the rocky shores. One day I hope to go back, and be able to dive alongside these unique, magnificent animals.