Marine iguana

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Marine iguana
Iguana marina (Amblyrhynchus cristatus), Las Bachas, isla Santa Cruz, islas Galápagos, Ecuador, 2015-07-23, DD 23.jpg
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Suborder: Iguania
Family: Iguanidae
Genus: Amblyrhynchus
Bell, 1825
Species: A. cristatus
Binomial name
Amblyrhynchus cristatus
Bell, 1825
Subspecies

7–11 ssp.; see text

Amblyrhynchus cristatus distribution map.svg

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) that has the ability, unique among modern lizards, to forage in the sea, making it a marine reptile.[2] 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.[3] 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.[4]

Marine iguanas vary in appearance between the different islands and several subspecies are recognized.[5] Although relatively large numbers remain and it is locally common,[6] this protected species is considered threatened.[1]

Marine iguana on Baltra Island, Galápagos

Taxonomy and evolution[edit]

Etymology[edit]

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.[7]

Evolution[edit]

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.[8][9] 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.[5] The marine iguana diverged from the land iguanas some 8–10 million years ago, which is older than any of the extant Galápagos islands.[10][11] 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.[12]

Subspecies[edit]

Seven or eight subspecies of the marine iguana, listed alphabetically, have traditionally been recognized:[12]

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):[5][7]

  • A. c. cristatus Bell, 1825 (albermarlensis and ater are 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., 2017Marchena Island
  • A. c. jeffreysi Miralles et al., 2017Wolf, 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., 2017Santa 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

Appearance[edit]

Marine iguanas from Española Island are the most colourful and have sometimes been called "Christmas iguanas"[6]

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.[13]

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.[14]

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.[3] Adult males vary in colour with the season, becoming brighter when breeding.[12] 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.[5][12] 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).[5][12] Females show much less variation between the islands and are typically dark with less contrasting colours than the males.[5] In most places females do not changed 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 possibly is related to their need of defending nest sites from other females.[15]

Size[edit]

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).[5] 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.[5]

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.[3]

Marine iguanas are sexually dimorphic with adult males weighing about 70% more than adult females.[16] 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.[17]

Behavior[edit]

Reproduction and life cycle[edit]

A marine iguana nest hole (center of image)
Juveniles on Isabela 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.[17] Marine iguanas can reach an age of up to 60 years,[6] but average is 12 years or less.[18]

Reproduction in the marine iguana begins during the cold and dry season,[17] with breeding from December to March and nesting from January to April.[9] The exact timing depends on the island,[19] occurring earliest in the western part of the archipelago where the growth of algae is most productive.[15]

Marine iguanas live in colonies that typically range from 20 to 500 animals,[20] but sometimes more than 1,000.[21] However, large males defend territories for up to three months during the breeding season.[22] A territory can be up to almost 40 m2 (430 sq ft),[23] but usually is less than 10 m2 (110 sq ft) and occasionally only 1 m2 (11 sq ft).[20] The territories tend to occur in clusters with several located adjacent to each other. Medium-sized males roam areas near the territories of large males or walk along beaches looking for females and small "sneaky" males may enter the territories of large males.[23] Fights between males sometime occur during the breeding season and may last for hours.[24] 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.[4]

Males are selected by females on the basis of their body size. Females display a stronger preference for mating with bigger males.[25] 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."[26] Other factors involved in the female's choice of partner are the display frequency by a male,[23] and the quality of a male's territory.[27] When inside a territory, females also receive less harassment from the opportunistic non-territorial males.[23] Both medium-sized males and "sneaky" small males will attempt to court and mate with females, but have a low succesrate compared to the large males that maintain a territory.[23] During courtship, the male nods at the female and if she accepts the male mounts her while holding her by the neck.[19] A mating typically lasts 3 to 4 minutes,[28] but it is comparably rapid in "sneaky" males, which easily are overlooked because their size is similar to that of a female. This rapid mating is necessary because large males will chase them out of their territory as soon as they are discovered.[23] During each breeding season, a male will mate with many females if given the chance, but the female only mates once.[27]

Roughly one month after copulation, the female lays between one and six eggs.[16] The leathery white eggs measure about 9.5 cm × 4.5 cm (3.7 in × 1.8 in) and weigh 80–120 g (2.8–4.2 oz).[15] This is large for an iguana,[15] and altogether the eggs may weigh up to one-quarter the weight of the female.[28] The nest sites can be as much as 2 km (1.2 mi) inland,[1] but typically are much closer to the coast.[29] They are occasionally as little as 20 m (66 ft) inland,[30] although they have to be above the high tide water mark.[15] The nest is 30–80 cm (12–31 in) deep and dug in sand or volcanic ash by the female.[16] On islands where there are few suitable sites and digging is difficult due to a relatively hard soil, the female guards the nest for several days after the eggs have been buried,[15] ensuring that they are not dug up by other nesting females.[19] Where there are more suitable sites and the soil is loose, females are less likely to fight for a location and do not guard their nest after the eggs have been buried.[15] They hatch after about three to four months.[19] The hatchlings are 9.5–13 cm (3.7–5.1 in) in snout-to-vent length,[15] and weigh about 50–60 g (1.8–2.1 oz).[28] As soon as they emerge from the nest they run for cover.[28]

Feeding[edit]

Large male marine iguanas forage for algae underwater at sea
While feeding underwater, marine iguanas are sometimes cleaned by fish, like this Cortez rainbow wrasse[31]
Lava lizards often scurry over marine iguanas when hunting flies; the iguanas generally ignore these visits[19]

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.[32] Among the red algae genera often consumed are Centroceras, Gelidium, Grateloupia, Hypnea, Polysiphonia and Pterocladiella, and in some populations Ulva dominates the diet.[3][32] Several other red and green algae species have been recorded,[32] and at least 10 genera are regularly consumed.[3] Brown algae have also been recorded in their diet,[32] but marine iguanas are unable to easily digest these and starve if this is the only type present.[33] The algal diet varies in accordance to the algal abundance, preferences and foraging behaviour, and this also depends on the season and exact island.[3] 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.[32] A 1 kg (2.2 lb) marine iguana typically eats about 8.6 g (0.30 oz) dry weight or 37.4 g (1.32 oz) fresh weight of algae per day.[34] At Punta Espinoza on northeastern Fernandina Island it has been estimated that the almost 1,900 marine iguanas eat about 27–29 tonnes (fresh weight) per year,[34] but this is counterbalanced by the very high growth rate of the algae.[19] Rarely they may feed on crustaceans, insects,[6] and sea lion feces and afterbirth.[35] 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.[35] In the first months after hatching, the juveniles mainly feed on feces from larger marine iguanas, gaining the endosymbiotic bacteria needed for digesting algae.[3]

Marine iguanas can dive as deep as 20 m (66 ft),[1] and can spend up to one hour underwater.[12] When diving to 7 m (23 ft) or deeper, they regularly remain submerged for 15 minutes to more than 30 minutes.[31] Most dives are much shorter in duration and shallower than 5 m (16 ft).[24] Individuals foraging near-shore, often at less than 1 m (3.3 ft) deep, typically only spend about 3 minutes underwater.[3][31] Only 5% of marine iguanas dive for algae offshore and these individuals are the large males.[3] 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).[3] Marine iguanas are buoyant and float to the ocean surface if they are not actively swimming or hold on to rocks underwater.[31] 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.[36] Most females and smaller males feed on exposed algae in the intertidal zone during low tide,[3] retreating once the water returns and starts washing over them.[19] They often scurry back-and-forth repeatedly, running to a patch of algae to take a few bites and then return fast to higher ground to avoid incoming waves.[37] The separation in feeding behavior is advantageous because the large offshore feeding males experience less competition for food from smaller males and females.[3] A few individuals of intermediate size may use both feeding strategies.[3] In general, each marine iguana has a specific feeding site it returns to day after day.[37] Most feed daily, but large offshore feeding males often only every second or third day. During bad weather with high waves marine iguanas do not feed, sometimes for more than a week, and large males often do not feed for several weeks when maintaining a breeding territory, resulting in a significant weight loss.[20]

Foraging behavior changes in accordance to the seasons and foraging efficiency increases with temperature.[32] 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.[3] 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.[38]

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,[2][32] and compact (osteosclerosis) limb bones compared to the land iguana, especially those from the front limbs, providing ballast to help with diving.[17] 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.[4][6] The head may appear white from encrusted salt.[19][39]

Lava lizards may scurry over marine iguanas when hunting flies, and Darwin's finches, mockingbirds and Sally lightfoot crabs sometimes feed on mites and ticks that they pick off their skin.[19][31] Marine iguanas typically ignore these visits.[19] When underwater, they are often cleaned by fish, like Pacific sergeant major that pick off moulting skin.[31]

Thermoregulation[edit]

Marine iguanas basking on Fernandina Island

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.[3] 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,[3] sometimes by as much as 10 °C (18 °F),[24] the marine iguana has several behavioral adaptations for thermoregulation.[17]

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.[29] When in the water and their temperature is falling, they have a low heart rate, allowing them to better conserve their warmth. When on land and heating up, the higher heart rate aids in spreading the heat throughout the body.[40] 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.[19]

In general, the time of each foraging trip is directly related to the water temperature; the colder the water the shorter the foraging trip.[3] 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.[3] 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.[3][19][24]

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.[19][24]

Conservation[edit]

Status and threats[edit]

Marine iguana on Santa Cruz Island

The marine iguana has a relatively small range and is currently considered vulnerable by the IUCN.[1] On some shorelines they can be very numerous with as many as 2,800 per kilometer (4,500 per mile),[6] and may reach a higher biomass compared to the area than known from any other reptile.[21] The total population for the entire archipelago is estimated to be 200,000–300,000 individuals,[6] although this number is labelled with considerable uncertainty.[28] Most subpopulations have not been surveyed in detail, 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.[1]

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%.[41] When their food algae disappears during El Niños, the areas may be taken over by the invasive brown algae Giffordia mitchelliae.[2][22] With global warming, it is expected that El Niño events will be stronger and occur more frequently.[41]

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.[4] This inhibits reproduction and the long-term survival of the species.[42] Among the few natural land predators are Galápagos hawks, short-eared owls, lava gulls, herons and Galápagos racer snakes that may take small marine iguanas.[6][9][19][43] When swimming, marine iguanas are occasionally attacked and eaten by sharks.[31] Of the native predators, the Galápagos hawk is likely the most important,[19] but marine iguanas have anti-predator strategies that reduce its impact.[2]

Marine iguanas can easily be approached by humans as they are very tame and generally do not attempt to escape.[21] Although unintentional, humans 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.[44]

Chance events also present a threat, at least locally. For example, the Santa Fe population was drastically reduced as a result of the MV Jessica oil spill in 2001.[1]

Protection[edit]

A marine iguana swimming at sea near Puerto Ayora, the most populous town in the Galápagos

The marine iguana is completely protected under the laws of Ecuador, and is listed under CITES Appendix II.[1] Almost all its land range is in the Galápagos National Park (only the 3% human-inhabited sections in the archipelago are excluded[45]) and all its sea range is in the Galápagos Marine Reserve.[1]

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.[46] 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.[47]

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.[48]

See also[edit]

References[edit]

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