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 makes up almost all of its diet.[3] Marine iguanas are the only extant lizard that spends time in a marine environment.[4] Large males are able to dive to find this food source, while females and smaller males feed during low tide in the intertidal zone.[5] 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.[6] 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.[7]
Marine iguanas vary in appearance between the different islands and several subspecies are recognized.[8] Although relatively large numbers remain and it is locally abundant,[9] this protected species is considered threatened, primarily from El Niño cycles, introduced predators and chance events like oil spills.[1]
Taxonomy and evolution[edit]
Species description and etymology[edit]
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,[10] a locality now known to be erroneous.[11]
Its generic name, Amblyrhynchus, is a combination of two Greek words, Ambly- from Amblus (ἀμβλυ) meaning "blunt" and rhynchus (ρυγχος) meaning "snout".[12] 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.[11]
Conservation[edit]
Status and threats[edit]
The marine iguana has a relatively small range and is currently considered vulnerable by the IUCN.[1] Most subpopulations have the same IUCN rating, but those of San Cristóbal, Santiago and Genovesa Islands are considered endangered.[70][71]
On some shorelines they can be very numerous, with densities as high as 8,000 per kilometer (almost 13,000 per mile),[7] and their biomass compared to the area they occupy may surpass that of any known reptile.[37] However, their distribution is patchy,[7] and colonies are generally found within 100 m (330 ft) of the ocean, naturally limiting their range.[72] The total population for the entire archipelago is estimated to be 200,000–300,000 individuals,[9] although this number is labelled with considerable uncertainty.[7] Most subpopulations have not been surveyed in detail because their lifestyle and habitat make it difficult to survey with a high level of accuracy.[50] By far the largest subpopulation—likely including around 2⁄5 of all marine iguanas—lives on Fernandina Island, but estimates vary greatly from 15,000 to 120,000 individuals. In contrast, the maximum size of the subpopulations on the islands of San Cristóbal, Darwin and Pinzon is estimated to be 400, 800 and 900, respectively.[7] It is estimated that Marchena Island has 4,000–10,000 marine iguanas, Rabida Island has 1,000–2,000 and Santa Fé Island has 15,000–30,000.[1] Although individuals may on occasion be transferred between islands by ocean currents, marine iguanas are unable to swim between all but the nearest islands in the archipelago because of their slow speed and limited stamina in the relatively cold water.[63]
The periodic El Niño events reduce the cold water needed for algae to grow and this can drastically reduce the marine iguana population, on some islands with as much as 90%.[73] Population recoveries after El Niños are fast; even when reduced by 30–50%, the population is generally able to recover within four years.[7] However, recoveries can be partially impeded by the invasive brown algae Giffordia mitchelliae. When their food algae (red and green algae) disappears during El Niños, the areas may be taken over by this brown algae, causing starvation among the marine iguanas.[3][7][39] With global warming, it is expected that El Niño events will be stronger and occur more frequently.[73]
Introduced predators, to which they have little or no protection, include animals such as pigs, dogs, cats and rats. Dogs may take adult marine iguanas, while the others may feed on their young or eggs.[6] This inhibits reproduction and the long-term survival of the species.[74] Introduced predators represent a major problem on the islands of Santa Cruz, Santiago, Isabela, Floreana and San Cristóbal where very few marine iguana hatchlings survive.[75] Marine iguanas are ecologically naïve and have not developed efficient anti-predator strategies against the introduced species. For example, the first dogs were introduced to the Galápagos Islands more than 100 years ago, but marine iguanas have not developed an anti-predator strategy against them.[72] In general, native predators represent a less serious threat to the marine iguana. Natural land predators include the Galápagos hawk, short-eared owl, lava gull, herons and Galápagos racer snakes that may take small marine iguanas.[9][14][49][76] When swimming, marine iguanas are occasionally attacked and eaten by sharks, although the two often behave indifferently to each other, even when close together.[62] Of the native predators, the Galápagos hawk is likely the most important,[49] and it may also take weakened adults (not just young),[7] but this hawk is quite rare with a total population numbering only a few hundred individuals.[77] Marine iguanas have anti-predator strategies that reduce the impact of the Galápagos hawk,[3][78] including an increased vigilance when hearing the alarm call of the Galápagos mockingbird, another species that is sometimes preyed upon by the hawk.[79]
Marine iguanas can easily be approached by humans as they are very tame and generally make little or no attempt to escape.[37][78] Individuals that have been caught before are only slightly more wary when again encountering humans.[78] Even when repeatedly caught and deliberately mishandled each time, they do not attempt to bite or lash with their tail in self-defense (although the sharp claws can cause painful scratches when the iguana attempts to gain a grip) and only move a few feet once released, allowing themselves to be caught again with ease.[66] Pathogens introduced to the archipelago by humans pose a serious threat to this species. The marine iguana has evolved over time in an isolated environment and lacks immunity to many pathogens. As a result, the iguanas are at higher risk of contracting infections, contributing to their threatened status.[80]
Occasional oil spills in the region also present a threat. For example, the Santa Fé population was reduced by almost 2⁄3 as a result of the MV Jessica oil spill in 2001,[7][70] and even low-level oiling may kill marine iguanas. It is suspected that the primary cause of death during these events is starvation due to the loss of their sensitive gut bacteria, which they rely on to digest algae.[7]
Marine iguanas may even enter the town of Puerto Ayora, like this individual demonstrating its climbing ability on a wall.
