The marine iguana inhabits the Galapagos Islands which form an archipelago off the coast of South America. The volcanic Galapagos has never been attached to another land mass so it is believed that iguanas rafted over water from South America (Cogger and Zweifel 1998). Some researchers believe that the land iguanas and the marine iguana diverged from a common ancestor at least 10 MY on the former islands of the archipelago which are now below sea level (Rassmann et. al. 1997).
Biogeographic Regions: oceanic islands (Native )
Distribution: Galapagos (Ecuador) cristatus: Isla Fernandina (= Narborough I.) albemarlensis: Isla Isabela (= Albemarle I.) ater: Isla Pinzón (= Duncan I.) hassi: Isla Santa Cruz (= Intefadigable I.) mertensi: Isla San Cristóbal (= Chatham I.), Isla Santiago (= James I.) nanus: Isla Genovesa (= Tower I.) sielmanni: Isla Pinta (= Abington I.) venustissimus: Isla Española (= Hood I.), Gardener Island; Holotype: SMF
Type locality: restricted to (Eibl-Eibesfeldt 1956) Narborough (Fernandina).
Amblyhynchus cristatus is a grey to black iguana with pyramid-shaped dorsal scales. They have shorter more blunt snouts than land iguanas, and they have a slightly laterally compressed tail. The young have a lighter color dorsal stripe (Rassmann et. al. 1997).
Habitat and Ecology
The marine iguana is found on the volcanic islands of the Galapagos. Many of the islands have steep rock cliffs, low rock ledges and intertidal flats. A. cristatus needs access to the ocean and a sandy area to lay eggs. They evolved in a habitat that is limited in predators. On Santa Fe an island in the Galapagos the predator are hawks, short-eared owls, snakes, hawk-fish, and crabs. With so few natural predators the marine iguana is very vulnerable to feral predators such as rats, dogs and cats. The feral animals can affect egg survival and adult mortality. Females are especially at risk of predation when going to the open nesting areas.
The marine iguana feed almost exclusively on marine algae (Cogger and Zweifel 1998). Larger members of the species feed more often by diving at high tide while smaller animals are restricted to intertidal feeding at low tide (Laurie and Brown II 1990). A major change in the marine algal flora occurred between November 1982 and July 1983. This coincided with abnormally high rainfall, sea level, and sea surface temperatures associated with El Nino-Southern Oscillation Event (ENSO). ENSO events are described as a mass of low-salinity nutrient-poor surface water moving south in the eastern tropical pacific. This causes a decrease in biological productivity and decreases survival and reproduction of animals dependant on the effected ecosystem. This was followed by unusually high mortality of maine iguanas (Laurie and Brown II 1990).
Life History and Behavior
Status: captivity: 6.4 years.
Lifespan, longevity, and ageing
Males defend mating territories during the three-month annual breeding season. Females lay one to six eggs in burrows dug 30 to 80 cm deep. The eggs are laid in sand or volcanic ash up to 300m or more inland. Females guard the burrow for several days then leave the eggs to finish incubation, which is approximately 95 days. Nesting months are January through April depending on the island.
Evolution and Systematics
The body length of marine iguanas shrinks in response to low food availability and energetic stress to reduce energy expenditure and increase foraging efficiency
"Change in body length is considered to be unidirectional in vertebrates1, but we have repeatedly observed shrinkage in the snout-to-vent length of individual adult iguanid lizards. In two studies, one lasting 18 years and one 8 years, of two island populations of Galápagos marine iguanas (Amblyrhynchus cristatus), we found that individuals became shorter by as much as 20% (6.8 cm) within two years. This shrinking coincided with low availability of food, resulting from El Niño events. Body length increased again during subsequent La Niña conditions, when algal food was abundant. We found that lizards that shrank more survived longer than larger iguanas during harsh periods because their foraging efficiency increased and their energy expenditure decreased.
Shrinking in marine iguanas may be an adaptive response to low food availability and energetic stress. Measurements of a cohort of adults more than 300 mm long during the strong 1992–93 El Niño event show that individuals that shrank more survived significantly longer (Fig. 2b). The mechanisms that determine whether and to what extent an individual shrinks during El Niño events remain unclear. Reduction in body length has been observed previously, and growth rates set to zero by definition, but to our knowledge this is the first report of shrinkage in adult vertebrates" (Wikelski and Thom 2000: 37)
[Note: The decrease in body size is the main strategy, no matter how it occurs. However, the paper mentions reabsorption of bone as the possible mechanism.]
Learn more about this functional adaptation.
- Wikelski M; Thom C. Marine iguanas shrink to survive El Nino. Nature. 403: 37-38.
IUCN Red List Assessment
Red List Category
Red List Criteria
- Needs updating
- 1996Vulnerable (VU)
- 1994Rare (R)
- 1990Rare (R)
- 1988Rare (R)
- 1986Rare (R)
It is important to conserve the biodiversity of the marine iguana because it is a unique and interesting animal. It is necessary to protect their island refuges from feral pests and human exploitation because they are long lived animals that can not sustain added mortality.
US Federal List: no special status
CITES: appendix ii
IUCN Red List of Threatened Species: vulnerable
Marchena = 4,00010,000; Rabida = 1,0002,000; Santa Fe = 15,00030,000; Baltra = unknown; Daphne = unknown; Darwin = unknown; Pinzon = unknown; Seymor = 300? Sin Nombre = unknown; Wolf = unknown.
Conservation actions recommended for the species include: further surveys of the islands, taxonomic and genetic research, and monitoring of the population.
The seven marine iguana subspecies described to date have been based on morphology. The taxonomic status of the ten subpopulations of A. cristatus is unclear. Taxonomic/genetic research is recommended for the different island subpopulations to establish whether any of them should be reclassified. Additionally, the status of seven of the ten subpopulations is unknown. Populations on different islands face different threats and should be included in future surveys.
Relevance to Humans and Ecosystems
The marine iguana does not affect humans because humans do not inhabit most of the islands they live on. The main food for the marine iguana is algae and that is not resource we compete for either.
The marine iguana (Amblyrhynchus cristatus) is an iguana found only on the Galápagos Islands that has the ability, unique among modern lizards, to forage in the sea, making it a marine reptile. The iguana can dive over 9 m (30 ft) into the water. It has spread to all the islands in the archipelago, and is sometimes called the Galápagos marine iguana. It mainly lives on the rocky Galápagos shore to warm from the comparably cold water, but can also be spotted in marshes and mangrove beaches.
- A. c. albemarlensis Eibl-Eibesfeldt, 1962 – Isabela 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
On his visit to the islands, 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 [60–90 cm]), 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 medium-sized lizards (200–340 mm, adult snout–vent length) and are unique as they are marine reptiles due to their foraging on inter- and subtidal algae only. These iguanas forage exclusively in the cold sea, which leads them to behavioral adaptations for thermoregulation.
Amblyrhynchus cristatus is not always black; the young have a lighter coloured dorsal stripe, and some adult specimens are grey, and adult males vary in colour with the season. 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.
Body Size and Longevity
Marine iguanas vary in body size, which is different depending on the island the individual iguana inhabits. The iguanas living on the islands of Fernandina and Isabela (named for the famous rulers of Spain) are the largest found anywhere in the Galápagos. On the other end of the spectrum, the smallest iguanas are found on the island on Genovesa.
Adult males weigh from a maximum of 12–13 kg on southern Isabela to about 1–2 kg on Genovesa. The reason for this difference in body size of marine iguanas between islands is due to "variability in algal productivity and sea surface temperature." 
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 for 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, in fact, explained through the size difference between the sexes.
Reproduction in the marine iguana begins during the cold and dry season. 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 in mating with bigger males. During the breeding season, males defend the leks, and roughly one month after copulation, females lay between one to six eggs. The eggs "take 3 months to incubate in nests dug 30-80 cm deep in sand or volcanic ash."  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." 
The marine iguana forages exclusively on inter- and subtidal algae. During high low tides, Ulva lobata, also known as green algae, which is usually avoided, is eaten more often since the usual red algae is not available. Usually, however, the 4-5 red algal species are the food of choice for marine iguanas.
This algal diet varies in accordance to the algal abundance, preferences, and foraging behaviour. Only 5% of marine iguanas dive for algae offshore, and these individuals are the large males. This behaviour is advantageous because these males experience less competition for food from smaller males and females, who are restricted to foraging during low tide. Foraging behavior changes in accordance to the seasons and foraging efficiency increased with temperature. It is because of these environmental changes and occasional food unavailability that have caused marine iguanas to evolve by acquiring efficient methods of foraging in order to maximize their energy intake and body size. In fact, 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.
The physical structure of the iguana also facilitates foraging as they have “long claws, tough skin, blunt heads, flattened tails, and well-developed salt glands.” A flat snout and sharp teeth enable it to browse on algae growing on rocks. A nasal gland filters its blood for excess salt ingested while eating, which is expelled through the nostrils, often leaving white patches of salt on its face.
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. Marine iguanas become highly defensive when in this state, biting at potential threats. When at low temperatures, there is a considerably lower ability for these iguanas to move, making them vulnerable to predation. However, this is counteracted with their highly aggressive nature consisting of biting and expensive bluffs when in this disadvantageous state. Their dark shade also aids them in heat reabsorption.
Fights sometime occur during the breeding season but are generally harmless; they will bob their heads as a threat and if the other suitor responds, both will thrust their heads together until one backs away.
Researchers theorize that land iguanas and marine iguanas evolved from a common ancestor since arriving on the islands from South America, presumably by rafting. It is thought that the ancestral species inhabited a part of the volcanic archipelago that is now submerged. The two species remain mutually fertile, and occasionally hybridize where their ranges overlap. The subspecies are identifiable by their distinct colorations, for example the Espanola race is more red while the Santiago iguanas are more so green.
The marine iguanas may appear to have a light colored face, but in fact, this is due to salt from specialised cranial exocrine glands, expelled from the body in a process much like sneezing. This salt becomes encrusted on their faces. This adaptation allows them to excrete excess salt due to foraging on marine algae. Although the marine iguana may resemble a lizard, there are several adaptations it has developed that set it apart. These include blunt noses for efficiently grazing seaweed, powerful limbs and claws for climbing and holding onto rocks, and laterally flattened tails for improved swimming.
The marine iguana has not, however, evolved in ways that could protect it from new, introduced predators. It evolved over time in a setting where it had few natural predators. Some of these new predators include rats, which tend to feed on the eggs, cats, which can feed on the small, young iguanas, and even dogs which can feed on the adults.
On a more historical note, scientists have discovered that the marine iguana diverged from the land iguana some 8 million years ago, meaning that the marine iguana evolved on what is now a submerged island of the Galapagos.
Taxonomy and etymology
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.
Endangered Species List
The Marine Iguana is currently labeled as vulnerable in its conservation status. The iguana is only known to be living in the Galapagos Islands and its population has been gradually decreasing throughout the years. Since the environment in which they live didn't have many natural predators they never developed the defenses needed to help protect them against new enemies. This lack of development makes them more vulnerable to attack and becoming ill due to new bacteria as these islands attract more and more people and animals from different parts of the world.
Although it is unintentional, humans are one of the big threats to this species. The marine iguana has developed over time in a fairly safe environment and thus does not have a very strong immune system. This leads to a higher risk of the iguanas catching infections that they aren't used to and that their bodies are equipped to protect against thus contributing to their endangerment.
Other predators include animals such as pigs, dogs, and cats. These animals, though do not threat the iguana directly, impact its way of repopulating. These animals do not attack the iguana but rather go after the nesting areas to feed off of their eggs. This inhibits their reproduction and the population growth of the marine iguana.
The marine iguana is completely protected under the laws of Ecuador, and is listed under CITES Appendix II. The total population size is unknown, but the International Union of Conservation of Nature estimates that at least 50,000 exist, while estimates from the Charles Darwin Research Station are in the hundreds of thousands.
Studies and research have been done on Galapagos 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 Galapagos 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 physiologically of marine iguanas. Information of glucocorticoid levels are good monitors in predicting long term consequences of human impact.
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