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
- UNESCO-IOC Register of Marine Organisms
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
- 1994Rare(Groombridge 1994)
- 1990Rare(IUCN 1990)
- 1988Rare(IUCN Conservation Monitoring Centre 1988)
- 1986Rare(IUCN Conservation Monitoring Centre 1986)
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 = 4,000â10,000; Rabida = 1,000â2,000; Santa Fe = 15,000â30,000; 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
Economic Importance for Humans: Positive
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 live and 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.
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.
Breeding-season adult males on the southern islands (Española, Floreana and nearby islets) are the most colourful and will acquire red and teal-green colours, while on Santa Cruz they are brick red and black, and on Fernandina they are brick red and dull greenish.
Another difference between the iguanas is 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 are up to 1.7 metres (5.6 ft) long, females 0.6–1 metre (2.0–3.3 ft), males weigh up to 12 kilograms (26 lb).
The marine iguana lacks agility on land but is a graceful swimmer. Its laterally flattened tail and spiky dorsal fins aid in propulsion, while its long, sharp claws allow it to hold onto rocks in strong currents.
Its diet consists of seaweed and algae. 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. During the breeding season males assemble large harems of females, which they guard aggressively against rivals. Fights sometime occur 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.
Marine iguanas have been found to change their size to adapt to varying food conditions. 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 iguanas actually shorten as shrinkage of connective tissue could only account for a 10% change in length. Research suggests iguanas secrete a stress hormone that induces decreased skeletal size.
El Niño conditions also increase mortality among larger-bodied iguanas, which take longer after foraging trips to warm up and digest algae consumed than smaller-bodied iguanas. Thus the latter are able to make more feeding excursions in a given day.
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.
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.
The marine iguana is completely protected under the laws of Ecuador, and is listed under CITES Appendix II. Decreases in food supply due to El Niño cause periodic major declines in population. The species is threatened by predation by introduced species such as cats and dogs, which prey particularly upon its young. Due to the lack of natural predators in the area, they are defenseless against the new species and most of the population are matured. The total population size is unknown, but is, according to IUCN, at least 50,000, and estimates from the Charles Darwin Research Station are in the hundreds of thousands.
- Rothman, Robert, Marine Iguana Galapagos Pages. Rochester Institute of Technology. Retrieved 19 April 2009.
- Nelson, K., Snell, H. & Wikelski, M. (2004). "Amblyrhynchus cristatus". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. Retrieved 2012-09-26.
- Amblyrhynchus cristatus, Reptile Database
- Darwin, Charles (2001). Charles Darwin's Beagle Diary. London: Cambridge University Press. p. 494. ISBN 0-521-00317-2
- "Marine Iguanas". Retrieved 20 December 2013.
- M, Wikelski; Thom, C. (Jan 6, 2000). "Marine iguanas shrink to survive El Niño". Nature 403 (6765): 37–8. doi:10.1038/47396. PMID 10638740.
- Wikelski, M. (2005). Evolution of body size in Galápagos marine iguanas. Proceedings: Biological Sciences, 272(1576), 1985-1993
- Wikelski, M. (2005). Evolution of body size in Galapagos marine iguanas. Proceedings: Biological Sciences, 272(1576), 1985-1993
- Rassman K, Tautz D, Trillmich F, Gliddon C (1997), The micro - evolution of the Galápagos marine iguana Amblyrhynchus cristatus assessed by nuclear and mitochondrial genetic analysis.: Molecular Ecology 6:437–452
- Marine Iguana: marinebio.org. Retrieved 16 August 2006.