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."Marine Iguana." Journal of Herpetology, vol. 55, no. 3, Society for the Study of Amphibians and Reptiles, 2021, pp. i–i, https://doi.org/10.1670/0022-1511-55.3.i 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.
Taxonomy and evolution Species description and etymology 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.
Evolution
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.
Subspecies Seven or eight subspecies of the marine iguana, listed alphabetically, have traditionally been recognized:
A. c. albemarlensis Eibl-Eibesfeldt, 1962 – Isabela Island A. c. ater Gray, 1831 (not always recognized) – Pinzón Island A. c. cristatus Bell, 1825 – Fernandina Island A. c. hassi Eibl-Eibesfeldt, 1962 – Santa Cruz Island A. c. mertensi Eibl-Eibesfeldt, 1962 – San Cristóbal and Santiago Islands A. c. nanus Garman, 1892 – Genovesa Island A. c. sielmanni Eibl-Eibesfeldt, 1962 – Pinta Island A. c. venustissimus Eibl-Eibesfeldt, 1956 – Española Island and adjacent tiny Gardner Island
In 2017, the first comprehensive taxonomic review of the species in more than 50 years came to another result based on morphological and genetic evidence, including recognizing five new subspecies (three of these are small-island populations that were not previously assigned to any subspecies):
A. c. cristatus Bell, 1825 (albermarlensis and ater are junior synonyms) – Isabela Island and Fernandina Island A. c. godzilla Miralles et al., 2017 – northeastern part of San Cristóbal Island, named after the character of Godzilla A. c. hassi Eibl-Eibesfeldt, 1962 – Santa Cruz Island and smaller adjacent islands, such as Baltra Island A. c. hayampi Miralles et al., 2017 – Marchena Island A. c. jeffreysi Miralles et al., 2017 – Wolf Island, Darwin Island and Roca Redonda A. c. mertensi Eibl-Eibesfeldt, 1962 – southwestern part of San Cristóbal Island A. c. nanus Garman, 1892 – Genovesa Island A. c. sielmanni Eibl-Eibesfeldt, 1962 – Pinta Island A. c. trillmichi Miralles et al., 2017 – Santa Fé Island A. c. venustissimus Eibl-Eibesfeldt, 1956 – Española Island (including adjacent tiny Gardener Island) and Floreana Island A. c. wikelskii Miralles et al., 2017 – Santiago Island and smaller adjacent islands, such as Rábida Island
Appearance
Early visitors to the Galápagos Islands considered the marine iguanas ugly and disgusting. In 1798, captain James Colnett of the British Royal Navy wrote: The [i]guanas are small, and of a sooty black, which, if possible, heightens their native ugliness. Indeed, so disgusting is their appearance, that no one on board could be prevailed on, to take them as food.
On his visit to the islands in 1835, 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 []), most disgusting, clumsy Lizards. They are as black as the porous rocks over which they crawl & seek their prey from the Sea. Somebody calls them 'imps of darkness'. They assuredly well-become the land they inhabit.
Marine iguanas have a thickset body and relatively short, robust limbs. Adults have a row of spines extending from the nape, along the back to the tail. Males have longer spines, and larger bony plates on the top of their head than females. There are some differences in the spines depending on the island and they are most elaborate on Fernandina (subspecies cristatus). The scales on the top of the head of the marine iguana are quite conical and pointed. Although less extreme, the same can be seen in the Galápagos land iguanas (genus Conolophus), while these scales at most are slightly domed in other iguanas. Overall the skeleton of the marine iguana shows many similarities with the chuckwallas (genus Sauromalus), but this is an example of homoplasy, as the two are not closely related.
Marine iguanas are not always black; the young have a lighter coloured dorsal stripe, and some adult specimens are grey. 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 provides propulsion and the row of spines along the back provide stability, while its long, sharp claws allow it to hold onto rocks in strong currents. Adult males vary in colour with the season, becoming brighter when breeding. There are also major differences in the colour of the adult males depending on the subspecies. Males of the relatively small southern islands of Española, Floreana and Santa Fé (subspecies venustissimus and trillmichi) are the most colourful, with bright pinkish-red and turquoise markings. In comparison, those of the relatively small northern islands of Genovesa, Marchena, Pinta, Wolf and Darwin (jeffreysi, hayampi, sielmanni and nanus) are almost all blackish without contrasting markings. Other subspecies tend to resemble duller versions of venustissimus and trillmichi, or are blackish with markings in pale yellowish, ochre, greenish or grey (sooty to near white). It is suspected that the exact algal diet of each population plays a role in their colour. Females show much less variation between the islands and are typically dark with less contrasting colours than the males. In most places, females do not change colour conspicuously between the breeding and non-breeding season, but at least on Española (subspecies venustissimus) they do assume relatively bright male-like colours when nesting, which is possibly related to their need of defending the nest from other females on an island with few suitable sites.
Size
Marine iguanas typically range from in snout-to-vent length and have a tail length from . There are major geographical differences, as iguanas from large islands tend to grow relatively large as adults, while those from small islands are smaller in size. In one study, the average snout-to-vent length on Wolf and Darwin Islands (subspecies jeffreysi) was about , and those on Genovesa Island (subspecies nanus) were only slightly larger. In comparison, Santa Cruz marine iguanas (hassi) had an average snout-to-vent length of about , and those of Isabela and Fernandina (cristatus) were only slightly smaller. Other subspecies were of intermediate size, in between the small Wolf, Darwin and Genovesa iguanas and the large Santa Cruz, Isabela and Fernandina iguanas. In another study, the largest were from western San Cristóbal Island (mertensi), followed by those from Isabela (cristatus, in part), Floreana (venustissimus, in part), eastern San Cristóbal (godzilla), Fernandina (cristatus, in part) and Santa Cruz (hassi). The smallest by far were from Genovesa (nanus), but this study did not include any marine iguanas from Wolf and Darwin Islands. The remaining island populations were of intermediate size.
The maximum weight of adult males ranges from on southern Isabela to on Genovesa. This difference in body size of marine iguanas between islands is due to the amount of food available, which depends on sea temperature and algae growth.