Galapagos marine iguana (Amblyrhynchus cristatus)

Also known as: sea iguana
French: Amblyrhynche à crête, Iguane marin
Spanish: Iguana Marina
GenusAmblyrhynchus (1)
SizeAverage male length: 0.75 m (2)
Average female length: 0.6 m (2)
Average male weight: 1.5 kg (2)
Average female weight: 0.5 kg (2)

The Galapagos marine iguana is classified as Vulnerable (VU) on the IUCN Red List (1) and listed on Appendix II of CITES (3). Subspecies: Amblyrhynchus cristatus mertensi and A. c. nanus are classified as Endangered (EN) and A. c. albemarlensis, A. c. cristatus, A. c. hassi, A. c. sielmanni and A. c. venustissimus are classified as Vulnerable (VU) on the IUCN Red List (1).

Galapagos marine iguanas are the world’s only marine lizards (4). They inhabit the Galapagos Islands and, in the absence of mammalian predators, have adapted well to the harsh marine environment. Superficially they resemble large lizards, though they have evolved blunt noses for grazing on seaweed, laterally flattened tails to assist swimming, and powerful limbs with strong claws to help them cling to rocks (5). They are grey to black in colour, though during the mating season they may have blotches of coppery green and red on their scaled body which may result from the consumption of a particular seaweed that blooms in the summer months. These iguanas have obelisk-shaped dorsal scales running from the head to the tail (6). Males are considerably larger than females, though the sexes are similar in appearance. Juveniles also look the same, but are generally darker in colour (2).

This iguana is endemic to the Galapagos Islands, which form an archipelago off the coast of South America. These are volcanic islands, and have never been attached to a land mass, so it is thought that the iguanas rafted over water from South America around 10 to 15 million years ago (7).

This species often lives in colonies where shallow reefs occur with an extensive intertidal zone and a rocky coastline (8). They are found basking on stretches of low cliffs, about two to five metres above sea level, but may also climb to heights of 80 metres. They also need access to sandy areas in which to bury their eggs (2). Though the Galapagos Islands straddle the equator, the water is extremely cold from surrounding ocean currents (5).

Galapagos marine iguanas are active during the day and spend the first few hours after sunrise basking in the sun in preparation for activity. The vast majority of individuals in each colony feed almost exclusively on marine algae in the intertidal zones during low tide. Only the largest five percent of individuals dive into the water for food, mostly during the hot midday hours (9). The waters are extremely cold, and cause the iguana to lose heat rapidly when feeding. This forces them to return to the rocks and warm up in the sun again. In fact, an iguana’s size and the way it retains or loses heat determines its method of feeding. Small individuals, which lose heat quickly, forage on rocks at low tide, scraping algae off the surface, and rarely dive into the sea. Larger individuals, however, do not lose as much heat and so they can be active for longer. They graze seaweeds in the shallow water around two to five metres in depth but can dive up to 25 metres down to rocks where there is an abundance of algae, and no competition from other iguanas (5). While feeding they also consume a great deal of salt solution which, in excess, can be toxic. They therefore excrete concentrated salt crystals from a nasal gland by sneezing (4). Activity slows between noon and late evening, and before sunset the iguana retreats into crevices or beneath boulders for the night (2).

This species breeds every year over a three month period, during which the males defend mating territories (4). Individuals breed normally just once every two years (9). Careful not to waste energy, they rely on less energetically expensive bluffs or bites to protect their territory. The nesting months are January through to April depending on the island (5). Females lay between one and six eggs up to 300 meters inland, in sand or volcanic ash burrows that are 30 to 80 centimetres deep. Females often guard the burrows for several days then leave the eggs to finish incubation, which takes approximately 95 days. When the young hatch they look like and act like miniature adults, and have no parental care (7)

Galapagos marine iguanas have evolved anti-predator behaviours towards the native Galapagos Hawk (Buteo galapagoensis) but are also threatened by introduced cats, dogs and rats. These feral animals eat iguanas and their eggs and have decimated hatchling populations in many colonies (1) (2).

This species is also sensitive to environmental fluctuations caused by El Nino (5). This natural phenomenon is caused by a failing of the trade winds resulting in an increase in sea temperature of about 4.3 degrees Celsius along with an increase in sea levels and precipitation (2). On average, El Nino occurs every 12.3 years, although the 1982 to 1983 event was the most severe for around 100 years (2). The environmental fluctuations and the following invasion of an alga (Giffordia mitchelliae) excluded the normal food species of the Galapagos marine iguana, causing the death of 50 percent of the population (8).

Oil spills and marine pollution are also very serious threats as they destroy food reserves and the nesting beaches. A recent oil spill from an Ecuadorian tanker in January 2001 spilled millions of litres of oil and fuel into the waters of the Galapagos Islands. In the following year, around 15,000 iguanas on the Island of Santa Fe alone died; over 60 percent of the entire island population. Scientists believe the oil may have killed the bacteria that the iguanas need to help digest algae, making it impossible for them to absorb nutrients (10). Fortunately, iguanas can increase their reproduction rate when population densities are low (such as after El Nino events), and thus can potentially recover from disasters to some extent (9).

The Galapagos marine iguana occurs in one of the most biodiverse areas of the world. The Galapagos Islands have long been studied and protected and were influential in the formulation of Darwin’s theory of evolution by natural selection (2). Most recently, in March 1998, a 133,000 km² area was designated as the Galapagos Marine Reserve, making it one of the world’s largest protected areas. Detailed conservation and research programmes have been developed which focus on studying the islands’ ecology, the effects of environmental fluctuations on species, and the effects of humans on wildlife (11).

Controlling introduced feral animals on the islands is the most important and urgent measure, and the Galapagos National Park and the Charles Darwin station are tackling this problem (12). Long-term conservation efforts focusing on other aspects are also essential to allow this unique species to recover (12).

For more information on the Galapagos marine iguana, see:

Authenticated (18/06/05) by Prof Martin Wikelski, Assistant Professor, Department of Ecology and Evolutionary Biology, Princeton University.

  1. IUCN Red List (May, 2009)
  2. UNEP-WCMC species page (April, 2008)
  3. CITES (November, 2003)
  4. Byatt, A., Fothergill, A. and Holmes, M. (2001) The Blue Planet. BBC Worldwide Limited, London.
  5. Bright, M. (2000) Andes to Amazon. BBC Worldwide Limited, London.
  6. (April, 2008)
  7. Halliday, T. and Adler, K. (2002) The New Encyclopedia of Reptiles and Amphibians. Oxford University Press, Oxford.
  8. San Francisco State University: The Biogeography of  Marine Iguana (Amblyrhynchus cristatus) (April, 2008)
  9. Wikelski, M. (2005) Pers. comm.
  10. Hecht, J. (2002) Galapagos oil spill devastated marine iguanas. Nature, 417: 607 - . Available at:
  11. Charles Darwin Foundation (April, 2008)
  12. Galapagos Conservation Trust (November, 2003)