The marine iguana is an iguana found only on the Galápagos Islands that has the ability, unique among modern lizards, to forage in the sea, making it a marine reptile. The iguana can dive over 9 m (30 ft) into the water.
It has spread to all the islands in the archipelago, and is sometimes called the Galápagos marine iguana. It mainly lives on the rocky Galápagos shore to warm from the comparatively cold water, but can also be spotted in marshes and mangrove beaches.
Marine iguanas are medium-sized lizards (200–340 mm (7.9–13.4 in), adult snout–vent length) and are unique as they are marine reptiles due to their foraging on inter-
and subtidal algae only. These iguanas forage exclusively in the cold sea, which leads them to behavioral adaptations for thermoregulation.
On his visit to the islands, despite making extensive observations on the creatures, Charles Darwin was revolted by the animals’ appearance, writing:
The black Lava rocks on the beach are frequented by large (2–3 ft [0.6–0.9 m]), disgusting clumsy Lizards. They are as black as the porous rocks over which they crawl
& seek their prey from the Sea. I call them ‘imps of darkness’. They assuredly well-become the land they inhabit.
Researchers theorize that land iguanas and marine iguanas evolved from a common ancestor since arriving on the islands from South America, presumably by rafting.
The marine iguana diverged from the land iguana some 8 million years ago, which is older than any of the extant Galapagos islands. It is therefore thought that the ancestral species inhabited parts of the volcanic archipelago that are now submerged.
The two species remain mutually fertile in spite of being assigned to distinct
genera, and they occasionally hybridize where their ranges overlap.
Although the marine iguana resembles a lizard, it has developed several adaptations that set it apart. These include blunt noses for efficiently grazing seaweed, powerful limbs and claws for climbing and holding onto rocks, and laterally flattened tails for improved swimming.
Compared to the land iguana its limb bones, especially those from the front limbs, have become more heavy and compact (osteosclerosis), providing ballast to help with diving.
The marine iguana has no evolved defences against introduced predators. These include rats, which tend to feed on the eggs, cats, which can feed on juveniles, and dogs
which may threaten adults.
Amblyrhynchus cristatus is not always black; the young have a lighter coloured dorsal stripe, and some adult specimens are grey, and adult males vary in colour with
the season. Dark tones allow the lizards to rapidly absorb heat to minimize the period of lethargy after emerging from the water. The marine iguana lacks agility on
land but is a graceful swimmer. Its laterally flattened tail and spiky dorsal fin aid in propulsion, while its long, sharp claws allow it to hold onto rocks in strong