7–11 ssp.; see text
The marine iguana (Amblyrhynchus cristatus), also known as the Galápagos marine iguana, is a species of iguana found only on the Galápagos Islands (Ecuador) that has the ability, unique among modern lizards, to forage in the sea, making it a marine reptile. This iguana feeds almost exclusively on algae and large males 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 warm after visiting the relatively cold water or intertidal zone, but can also be seen in marshes, mangrove and beaches.
Marine iguanas vary in appearance between the different islands and several subspecies are recognized. Although relatively large numbers remain and it is locally common, this protected species is considered threatened.
Taxonomy and evolution
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.
Researchers theorize that 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, and the nearest relative of this Galápagos clade are the Ctenosaura iguanas of Mexico and Central America. The marine iguana diverged from the land iguanas some 8–10 million years ago, which is older than any of the extant Galápagos islands. It is therefore thought that the ancestral species inhabited parts of the volcanic archipelago that are now submerged. The two species remain mutually fertile in spite of being assigned to distinct genera, and they occasionally hybridize where their ranges overlap, resulting in the so-called hybrid iguana of South Plaza Island.
- 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 Gardener 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 not previously assigned to any subspecies):
- A. c. cristatus Bell, 1825 (albermarlensis and ater are a junior synonyms) – Isabela and Fernandina Islands
- A. c. godzilla Miralles et al., 2017 – northeastern part of San Cristóbal Island
- A. c. hassi Eibl-Eibesfeldt, 1962 – Santa Cruz Island and smaller adjacent islands such as Baltra
- A. c. hayampi Miralles et al., 2017 – Marchena Island
- A. c. jeffreysi Miralles et al., 2017 – Wolf, Darwin and Roca Redonda islands
- 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 (including adjacent tiny Gardener Island) and Floreana Islands
- A. c. wikelskii Miralles et al., 2017 – Santiago Island and smaller nearby islands such as Rábida
A. c. albemarlensis on Isabela Island (a synonym of A. c. cristatus based on a 2017 study)
A. c. wikelskii on Santiago Island (formerly included in A. c. mertensi)
- 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 [0.6–0.9 m]), disgusting clumsy Lizards. They are as black as the porous rocks over which they crawl & seek their prey from the Sea. I call them 'imps of darkness'. They assuredly well-become the land they inhabit.
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 and spiky dorsal fin aid in propulsion, 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 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). Females show much less variation between the islands and are typically dark with less contrasting colours than the males.
Marine iguanas range from 12 to 49 cm (4.7–19.3 in) in snout-to-vent length and have a tail length from 17 to 84 cm (6.7–33.1 in). There are major differences between the islands. Average snout-to-vent length on Wolf and Darwin Islands (subspecies jeffreysi) is about 19 cm (7.5 in), and those on Genovesa Island (subspecies nanus) are only slightly larger. In comparison, Santa Cruz marine iguanas (hassi) have an average snout-to-vent length of about 35 cm (14 in), and those of Isabela and Fernandina (cristatus) are almost as large. Other subspecies are of intermediate size, in between the small Wolf, Darwin and Genovesa iguanas and the large Santa Cruz, Isabela and Fernandina iguanas.
The maximum weight of adult males ranges from 12 kg (26 lb) on southern Isabela to 1 kg (2.2 lb) 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.
Marine iguanas are sexually dimorphic with adult males weighing about 70% more than adult females. There is a correlation between longevity and body size, particularly for adult males. Large body size in males is selected sexually, but can be detrimental during El Niño events when resources are scarce. This results in large males suffering higher mortality than females and smaller adult males. The mortality rates of marine iguanas are explained through the size difference between the sexes.
Reproduction in the marine iguana begins during the cold and dry season, with breeding from December to March and nesting from January to April. The exact timing depends on the island. Female marine iguanas reach sexual maturity at the age of 3–5 years, while males reach sexual maturity at the age of 6–8 years. Sexual maturity is marked by the first steep and abrupt decline in bone growth cycle thickness.
Males are selected by females on the basis of their body size. Females display a stronger preference for mating with bigger males. It is precisely because of body size that reproductive performance increases and "is mediated by higher survival of larger hatchlings from larger females and increased mating success of larger males." During courtship, the male nods at the female and if she accepts the male mounts her while holding her by the neck. Roughly one month after copulation, females lay between one and six eggs. The eggs are laid in a nest that is 30–80 cm (12–31 in) deep and dug by the female. The female guards the nest site for several days after the eggs are laid, ensuring that they are not dug up by other nesting females. The leathery white eggs hatch after about three to four months, and the hatchlings are similar in size to lava lizards. The nest sites can be as much as 2 km (1.2 mi) inland.
In general, marine iguanas live in colonies, but adult males defend territories for up to 3 months during the breeding season. Fights between males sometime occur during the breeding season and may last for hours. Nevertheless, the fights are generally harmless; males will bob their heads as a threat and if the other suitor responds, both will thrust their heads together until one backs away.
The marine iguana forages almost exclusively on algae in the inter- and subtidal zones, and 4–5 red algal species are their food of choice. During neap low tides, however, the usually avoided Ulva lobata, a species of green algae, is eaten more often since the preferred red algae are not easily available. Among the red algae genera often consumed are Centroceras, Gelidium, Grateloupia, Hypnea, Polysiphonia and Pterocladiella, and in some populations Ulva dominates the diet. Several other red and green algae species have been recorded, and at least 10 genera are regularly consumed. Brown algae have also been recorded in their diet, but marine iguanas are unable to easily digest these and starve if this is the only type present. The algal diet varies in accordance to the algal abundance, preferences and foraging behaviour, and this also depends on the season and exact island. Some species with chemical deterrents, such as Bifurcaria, Laurencia and Ochtodes, are actively avoided, but otherwise algal food choice mainly depends on energy content and digestibility. At Punta Espinoza on northeastern Fernandina Island it has been estimated that the approximately 2,000 marine iguanas eat about 28 tonnes per year, but this is counterbalanced by the very high growth rate of the algae. Rarely they may feed on crustaceans, insects, and sea lion feces and afterbirth. The population on North Seymour Island also feeds on the land plant Batis maritima and these iguanas have a higher survival rate during periods where their normal algal food is reduced. However, the hindgut of marine iguanas is specially adapted to algae feeding, likely restricting the possibility of efficiently switching to other plant types. In the first months after hatching, the juveniles mainly feed on feces from larger marine iguanas, gaining the endosymbiotic bacteria needed for digesting algae.
Marine iguanas can dive as deep as 20 m (66 ft), and can spend up to one hour underwater. Most dives are shallower than 5 m (16 ft), and much shorter in duration with near-shore foraging individuals typical only spending about 3 minutes underwater. Only 5% of marine iguanas dive for algae offshore, and these individuals are the large males. The minimum size of these divers vary with island and subspecies, ranging from 0.6 kg (1.3 lb) on Genovesa Island (A. c. nanus) to 3 kg (6.6 lb) on Fernandina Island (A. c. cristatus). They are slow swimmers, averaging just 0.45 metres per second (1.5 ft/s) and the highest recorded speed is about twice that figure. Most females and smaller males feed on exposed algae in the intertidal zone during low tide, retreating once the water returns and starts washing over them. This separation in feeding behavior is advantageous because the large males experience less competition for food from smaller males and females. A few individuals of intermediate size may use both feeding strategies.
Foraging behavior changes in accordance to the seasons and foraging efficiency increases with temperature. These environmental changes and the ensuing occasional food unavailability have caused marine iguanas to evolve by acquiring efficient methods of foraging in order to maximize their energy intake and body size. During an El Niño cycle in which food diminished for two years, some were found to decrease their length by as much as 20%. When food supply returned to normal, iguana size followed suit. It is speculated that the bones of the iguana actually shorten as shrinkage of connective tissue could only account for a 10% change in length.
Marine iguanas have several adaptions that aid their feeding. Among these are flattened tails for efficient swimming, blunt heads ("flat noses") and sharp teeth allowing them to easier graze algae off rocks, powerful limbs and claws for climbing and holding onto rocks, and compact (osteosclerosis) limb bones compared to the land iguana, especially those from the front limbs, providing ballast to help with diving. As a sea reptile, much salt is ingested. The salt is filtered from their blood and then excreted by specialised cranial exocrine glands at the nostrils, expelled from the body in a process much like sneezing. The head may appear white from encrusted salt.
Lava lizards may scurry over marine iguanas when hunting flies, and Darwin's finches and mockingbirds sometimes feed on mites and ticks that they pick off their skin. Marine iguanas typically ignore these visits.
Marine iguanas are unique as they are marine reptiles that forages on inter- and subtidal algae almost exclusively. They forage in the relatively cold waters around the Galápagos Islands, which typically are between 11 and 23 °C (52–73 °F) at the sea surface. As their preferred body temperature is from 35 to 39 °C (95–102 °F) and the temperature declines throughout a foraging trip to the sea, sometimes by as much as 10 °C (18 °F), the marine iguana has several behavioral adaptations for thermoregulation.
As an ectothermic animal, the marine iguana can spend only a limited time in cold water diving for algae. Afterwards it basks in the sun to warm up. Until it can do so it is unable to move effectively, making it vulnerable to predation. However, this is counteracted by their highly aggressive nature consisting of biting and expansive bluffs when in this disadvantageous state. Their dark shade aids in heat reabsorption. To conserve heat during the night, they often sleep closely together in groups that may number up to 50 individuals, while others sleep alone below plants or in crevices.
In general, the time of each foraging trip is directly related to the water temperature; the colder the water the shorter the foraging trip. Additionally, marine iguanas that forage in or near the intertidal zone prefer to do so during low tides, allowing them to remain on land (on rocks exposed by the tide) or return to land faster. Individuals that forage further from the shore by diving are large males, which mainly feed during the hot midday (although it may occur from late morning to early evening), are less affected by the cool water because of their body size and are more efficient swimmers.
Under the tropical sun, overheating can also be a problem. To avoid this, they pant, and adopt a posture where they face the sun and lift their body up, thereby exposing as little as possible of their body to direct sun and allowing cooling air to pass underneath.
Status and threats
The marine iguana has a relatively small range and is currently considered vulnerable by the IUCN. On some shorelines they can be very numerous with as many as 2,800 per kilometer (4,500 per mile), and the total population is estimated to be 200,000–300,000 individuals. Most subpopulations have not been fully surveyed, but it is estimated that Marchena Island has 4,000–10,000 marine iguanas, Rabida Island has 1,000–2,000 and Santa Fe Island has 15,000–30,000.
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%. When their food algae disappears during El Niños, the areas may be taken over by the invasive brown algae Giffordia mitchelliae. With global warming, it is expected that El Niño events will be stronger and occur more frequently.
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. This inhibits reproduction and the long-term survival of the species. Among the few natural predators are Galápagos hawks, short-eared owls, lava gulls, herons and Galápagos racer snakes that may take small marine iguanas. Of the native predators, the hawk is likely the most important, but marine iguanas have anti-predator strategies that reduce its impact.
Although unintentional, human beings pose one of the most serious threats 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.
The marine iguana is completely protected under the laws of Ecuador, and is listed under CITES Appendix II. Almost all its land range is in the Galápagos National Park (only the 3% human-inhabited sections in the archipelago are excluded) and all its sea range is in the Galápagos Marine Reserve.
Studies and research have been done on Galápagos marine iguanas that can help and promote conservation efforts to preserve the endemic species. Monitoring levels of marine algae, both dimensionally and hormonally, is an effective way to predict the fitness of the marine iguana species. Exposure to tourism affects marine iguanas, and corticosterone levels can predict their survival during El Niño events. Corticosterone levels in species measure the stress that they face in their populations. Marine iguanas show higher stress-induced corticosterone concentrations during famine (El Niño) than feast conditions (La Niña). The levels differ between the islands, and show that survival varies throughout them during an El Niño event. The variable response of corticosterone is one indicator of the general public health of the populations of marine iguanas across the Galápagos Islands, which is a useful factor in the conservation of the species.
Another indicator of fitness is the levels of glucocorticoid. Glucocorticoid release is considered beneficial in helping animals survive stressful conditions, while low glucocorticoid levels are an indicator of poor body condition. Species undergoing a large measure of stress, resulting in elevated glucocorticoid levels can cause complications such as reproduction failure. Human activity has been considered a cause of elevated levels of glucocorticoid in species. Results of a study show that marine iguanas in areas central to tourism are not chronically stressed, but do show lower stress response compared to groups undisturbed by tourism. Tourism, thus, does affect the physiology of marine iguanas. Information of glucocorticoid levels are good monitors in predicting long term consequences of human impact.
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