Marine iguana

The marine iguana (Amblyrhynchus cristatus), also known as the sea iguana, saltwater iguana, or Galápagos marine iguana, is a species of iguana found only on the Galápagos Islands (Ecuador). Unique among modern lizards, it is a marine reptile that has the ability to forage in the sea for algae, which make up almost all of its diet. Marine iguanas are the only extant lizard that spends time in a marine environment. Large males are able to dive to find this food source, while females and smaller males feed during low tide in the intertidal zone. They mainly live in colonies on rocky shores where they bask after visiting the relatively cold water or intertidal zone, but can also be seen in marshes, mangrove swamps and beaches. Large males defend territories for a short period, but smaller males have other breeding strategies. After mating, the female digs a nest hole in the soil where she lays her eggs, leaving them to hatch on their own a few months later.

Marine iguanas vary in appearance between the different islands and several subspecies are recognized. Although relatively large numbers remain and it is locally abundant, this protected species is considered threatened, primarily from El Niño cycles, introduced predators and chance events like oil spills.

The marine iguana was first described in 1825 as Amblyrhynchus cristatus by Thomas Bell. He recognized several of its distinctive features, but believed that the specimen he had received was from Mexico, a locality now known to be erroneous.

Its generic name, Amblyrhynchus, is a combination of two Greek words, Ambly- from Amblus (ἀμβλυ) meaning "blunt" and rhynchus (ρυγχος) meaning "snout". Its specific name is the Latin word cristatus meaning "crested," and refers to the low crest of spines along the animal's back.

Amblyrhynchus is a monotypic genus, having only one species, Amblyrhynchus cristatus.

Researchers theorize that Galapagos land iguanas (genus Conolophus) and marine iguanas evolved from a common ancestor since arriving on the islands from Central or South America, presumably by rafting. The land and marine iguanas of the Galápagos form a clade, the nearest relatives of which are the Ctenosaura iguanas of Mexico and Central America. Based on a study that relied on mtDNA, the marine iguana was estimated to have diverged from land iguanas some 5-15 million years ago, which is older than any of the extant Galápagos islands. It has therefore traditionally been thought that the ancestral species inhabited parts of the volcanic archipelago that are now submerged. However, a more recent study that included both mtDNA and nDNA indicates that the two split about 4.5 million years ago, which is near the age of the oldest extant Galápagos islands (Española and San Cristóbal).

The marine iguana and Galápagos land iguana remain mutually fertile in spite of being separated by millions of years and assigned to distinct genera. They have been known to hybridize where their ranges overlap, resulting in the so-called hybrid iguana. This is very rare and has only been documented a few times on South Plaza, a tiny island where the usually separated breeding territories and seasons of the two species overlap. The hybrids have intermediate features, stay on land and are infertile.

The different marine iguana populations fall into three main clades: western islands, northeastern islands and southeastern islands. These can be further divided, each subclade generally matching marine iguanas from one or two primary island, except on San Cristóbal where there are two subclades (a northeastern and a southwestern). However, even the oldest divergence between marine iguana populations is quite recent; no more than 230,000 years and likely less than 50,000 years. On occasion one makes it to another island than its home island, resulting in hybridization between different marine iguana populations. There is one confirmed record of a marine iguana outside the Galápagos Islands; in 2014 an individual appeared on Isla de la Plata near the Ecuadorian mainland.