Marine iguana

The marine iguana (Amblyrhynchus cristatus) is a lizard species that lives only on the Galápagos Islands, part of Ecuador. It is also called the sea iguana, saltwater iguana, or Galápagos marine iguana. Among all living lizards, it is the only one that spends time in a marine environment, foraging in the ocean for algae, which makes up nearly its entire diet. Large males can dive for this food, while females and smaller males feed in the intertidal zone during low tide.

Reproduction and life cycle

These iguanas mainly live in colonies on rocky shores, where they bask after swimming in cold water or visiting the intertidal zone, but they can also be found in marshes, mangrove swamps, and beaches. Large males defend territories for a short period, while smaller males use other breeding strategies. 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 the different islands, and several subspecies are recognized. Although the species is locally abundant and has relatively large numbers, it is considered threatened. The main dangers are 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 distinctive features but mistakenly thought the specimen came from Mexico. The genus name Amblyrhynchus combines the Greek words ambly (blunt) and rhynchus (snout), while 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 believe that Galápagos land iguanas (genus Conolophus) and marine iguanas evolved from a common ancestor that arrived on the islands from Central or South America, likely by rafting. Together, they form a clade whose closest relatives are the Ctenosaura iguanas of Mexico and Central America. One study using mtDNA estimated that marine iguanas split from land iguanas 5–15 million years ago, older than any existing Galápagos island, suggesting the ancestor lived on parts of the archipelago now submerged. A more recent study using both mtDNA and nDNA places the split at about 4.5 million years ago, close to the age of the oldest current islands, Española and San Cristóbal.

Despite millions of years of separation and being in different genera, marine and land iguanas remain mutually fertile. They have hybridized where their ranges overlap, producing a so-called hybrid iguana. This is very rare and has only been documented a few times on South Plaza, a tiny island where their usually separate breeding territories and seasons overlap. The hybrids have intermediate features, stay on land, and are infertile.

Marine iguana populations fall into three main clades: western, northeastern, and southeastern islands. These can be further divided, 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 divergence between 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.

Subspecies

Traditionally, seven or eight subspecies have been recognized: 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 Gardner Island). In 2017, the first comprehensive taxonomic review in over 50 years used morphological and genetic evidence to recognize five new subspecies, resulting in this list: A. c. cristatus (Isabela and Fernandina Islands, with albemarlensis and ater as junior synonyms), A. c. godzilla (northeastern San Cristóbal, named after Godzilla), A. c. hassi (Santa Cruz and smaller adjacent islands like Baltra), A. c. hayampi (Marchena Island), A. c. jeffreysi (Wolf, Darwin, and Roca Redonda Islands), A. c. mertensi (southwestern San Cristóbal), A. c. nanus (Genovesa Island), A. c. sielmanni (Pinta Island), A. c. trillmichi (Santa Fé Island), A. c. venustissimus (Española, including adjacent Gardner Island, and Floreana Island), and A. c. wikelskii.