Marine iguana

The marine iguana (Amblyrhynchus cristatus) is a lizard species that lives only on the Galápagos Islands, which belong to Ecuador. It is also called the sea iguana, saltwater iguana, or Galápagos marine iguana. Among all living lizards, it is unique as a marine reptile that can feed in the ocean, where it gets almost all its food from algae.

Feeding

It is the only existing lizard that regularly spends time in a marine setting. Large males can dive to reach this food, while females and smaller males eat during low tide in the intertidal zone. These iguanas mostly live in colonies on rocky shores, where they bask after being in the cold water or intertidal area, but they also turn up in marshes, mangrove swamps, and beaches. Large males defend territories for a short time, but smaller males use other breeding methods.

Reproduction and life cycle

After mating, the female digs a nest hole in the soil, lays her eggs, and leaves them to hatch on their own a few months later. Their appearance varies across different islands, and several subspecies are recognized. Though still relatively numerous and locally abundant, this protected species is considered threatened, mainly because of El Niño cycles, introduced predators, and chance events like oil spills.

Species description and etymology

The marine iguana was first described in 1825 by Thomas Bell, who named it Amblyrhynchus cristatus. He noted several of its distinctive traits but mistakenly thought the specimen came from Mexico, a location now known to be wrong. Its genus name, Amblyrhynchus, comes from Greek words: "Ambly-" from "amblus," meaning "blunt," and "rhynchus," meaning "snout." The species name "cristatus" is Latin for "crested," referring to the low spine crest along its back. Amblyrhynchus is a monotypic genus, containing only this one species.

Evolution

Researchers think that Galápagos land iguanas (genus Conolophus) and marine iguanas evolved from a common ancestor after arriving on the islands from Central or South America, likely by rafting. Together, these land and marine iguanas form a clade, with their closest relatives being the Ctenosaura iguanas of Mexico and Central America. One study using mtDNA estimated that marine iguanas split from land iguanas between 5 and 15 million years ago, which is older than any current Galápagos island.

This led to the idea that the ancestral species lived on parts of the volcanic archipelago that are now underwater. However, a later study using both mtDNA and nDNA suggests the split happened about 4.5 million years ago, close to the age of the oldest existing islands, Española and San Cristóbal. Despite being separated for millions of years and placed in different genera, marine iguanas and Galápagos land iguanas can still breed with each other.

They have hybridized where their ranges overlap, producing a so-called hybrid iguana. This is very rare and has only been seen a few times on South Plaza, a tiny island where the usually separate breeding territories and seasons of the two species overlap. These hybrids have intermediate features, stay on land, and are infertile.

Different marine iguana populations fall into three main clades: western islands, northeastern islands, and southeastern islands. These can be split further, with each subclade generally matching iguanas from one or two main islands, except on San Cristóbal, which has two subclades (northeastern and southwestern).

Even the oldest split among marine iguana populations is quite recent—no more than 230,000 years and likely less than 50,000 years. Occasionally, an individual reaches another island, leading to hybridization between different marine iguana populations. There is one confirmed record of a marine iguana outside the Galápagos: in 2014, one appeared on Isla de la Plata near the Ecuadorian mainland.

Traditionally, seven or eight subspecies have been recognized, listed alphabetically: A. c. albemarlensis (Isabela Island), A. c. ater (Pinzón Island, not always recognized), A. c. cristatus (Fernandina Island), A. c. hassi (Santa Cruz Island), A. c. mertensi (San Cristóbal and Santiago Islands), A. c. nanus (Genovesa Island), A. c. sielmanni (Pinta Island), and A. c. venustissimus (Española Island and adjacent tiny Gardner Island). In 2017, the first comprehensive taxonomic review in over 50 years used morphological and genetic evidence to reach a different result, recognizing five new subspecies (three from small islands not previously assigned to any subspecies): A. c. cristatus (Isabela and Fernandina Islands, with albemarlensis and ater as junior synonyms), A. c. godzilla (northeastern San Cristóbal Island, named after the character Godzilla), A. c. hassi (Santa Cruz Island and smaller adjacent islands like Baltra), A. c. hayampi (Marchena Island), A. c. jeffreysi (Wolf Island, Darwin Island, and Roca Redonda), A. c. mertensi (southwestern San Cristóbal Island), A. c. nanus (Genovesa Island), A. c. sielmanni (Pinta Island), A. c. trillmichi (Santa Fé Island), A. c. venustissimus (Española Island, including adjacent tiny Gardner Island, and Floreana Island), and A. c. wikelskii (Santiago Island and smaller adjacent islands like Rábida).