The marine iguana is the only lizard on Earth that forages in the sea, a striking evolutionary product of the Galápagos Islands and one of the archipelago's most emblematic reptiles. Restricted entirely to the Galápagos, it depends on cool, productive coastal waters for food and is highly sensitive to ocean warming and to introduced predators [MacLeod et al. 2020]. This profile examines the species' remarkable biology, the threats it faces, and the conservation measures protecting it.
Biology and Identification
Marine iguanas are dark-colored, robust lizards with blunt snouts, laterally flattened tails, and a crest of pyramid-shaped scales along the back. Body size varies markedly among islands and between sexes; large males can approach 1.3 m in total length, while females are considerably smaller [Wikelski & Trillmich 1997]. The flattened tail propels the animal through water, and long claws grip wave-swept rock.
The species is the world's only marine-foraging lizard. It feeds almost exclusively on marine algae, grazing intertidal rock at low tide or diving to reach subtidal beds; larger adults dive deepest, sometimes well below the surface [MacLeod et al. 2020]. Because feeding in cold water lowers body temperature, individuals haul out afterward to bask and rewarm. To cope with the high salt load of a marine diet, marine iguanas possess specialized nasal salt glands and periodically expel concentrated salt by "sneezing," leaving characteristic white crusts on the head.
Marine iguanas display an unusual physiological response to food scarcity: during El Niño events, when warm water suppresses the algae they depend on, individuals can shrink in body length and then regrow when conditions recover [Wikelski & Thom 2000]. This flexibility helps the species survive recurring climatic stress.
Habitat and Range
The marine iguana is endemic to the Galápagos archipelago of Ecuador, occurring on the rocky coastlines of most of the islands [MacLeod et al. 2020]. It is tied to the intertidal and shallow subtidal zones where marine algae grow, hauling out onto coastal lava rock, beaches, and mangrove fringes to bask, rest, and nest.
The species shows pronounced genetic and morphological differentiation among islands, and the global assessment recognizes multiple subspecies confined to single islands or island groups; several of these island-restricted forms carry higher extinction risk than the species as a whole [MacLeod et al. 2020].
In accordance with NRWL sensitive-species policy, specific site locations, nesting beaches, and seasonal movement details are not disclosed in this article.
Conservation Status
The marine iguana is listed as Vulnerable on the IUCN Red List under criterion B2ac(iv), reflecting its restricted area of occupancy and extreme fluctuations in population size linked to El Niño events [MacLeod et al. 2020]. The species is included on CITES Appendix II, regulating international trade [CITES 2023].
The assessment notes that while the species overall qualifies as Vulnerable, several island subspecies face higher risk, and the global population is subject to large climate-driven swings rather than a steady trend [MacLeod et al. 2020]. As a Galápagos endemic, the marine iguana is protected under Ecuadorian law and within the Galápagos National Park and Galápagos Marine Reserve, which together cover essentially the entire range [Galápagos Conservancy 2024]. Accurate population monitoring remains challenging, and recent work has tested drone-based methods to improve count reliability [Varela-Jaramillo et al. 2023].
Threats
Introduced predators are the foremost direct threat. Non-native cats, rats, and dogs prey on eggs, hatchlings, and juveniles, which lack defenses against terrestrial mammalian hunters, and have depressed or eliminated populations on some islands [MacLeod et al. 2020].
Climate variability and ocean warming. Because the species depends on cold-water algae, El Niño events and longer-term ocean warming reduce food availability and can cause widespread mortality; more frequent or intense warming events would increase this risk [Wikelski & Thom 2000].
Marine pollution and oil spills. Coastal and marine pollution, including the risk of fuel spills around the islands, threatens the intertidal habitats on which the iguanas feed and rest [MacLeod et al. 2020].
Human disturbance. Growing tourism and coastal activity can disturb basking and nesting areas, while plastic and other debris accumulate in foraging habitat [Galápagos Conservancy 2024].
What Is Being Done
Protected-area management. The Galápagos National Park and the surrounding Marine Reserve regulate access, tourism, and fishing across the marine iguana's entire range, providing a strong legal and management framework for the species [Galápagos Conservancy 2024].
Invasive-species control. Eradication and control of introduced cats, rats, and other predators on Galápagos islands directly benefit marine iguanas by relieving predation on eggs and young, building on the archipelago's long record of island-restoration work [MacLeod et al. 2020].
Population monitoring. Researchers are refining survey methods, including drone-based aerial counts validated against ground surveys, to track population sizes more accurately across the many islands the species occupies [Varela-Jaramillo et al. 2023].
Specialist assessment. The IUCN SSC Iguana Specialist Group maintains Red List assessments for the marine iguana and its subspecies, prioritizing the island forms at greatest risk for conservation attention [Iguana SG 2024].
How Readers Can Help
Citizen science. Photograph and log wildlife observations through platforms such as iNaturalist. Verified occurrence records contribute directly to range-mapping efforts and IUCN assessments.
Policy engagement. Support funding for Galápagos invasive-species control, marine-reserve enforcement, and biosecurity measures that prevent new predator introductions to the islands.
Responsible travel. When visiting the Galápagos, follow National Park rules, keep a respectful distance from basking and nesting iguanas, and choose operators that adhere to recognized conservation standards.
Education outreach. Share accurate, science-based information about the marine iguana's unique sea-foraging ecology and its sensitivity to ocean warming and introduced predators.
References
[CITES 2023] CITES. (2023). Appendices I, II and III. Convention on International Trade in Endangered Species of Wild Fauna and Flora. https://cites.org/eng/app/appendices.php
[Galápagos Conservancy 2024] Galápagos Conservancy. (2024). Marine iguana and Galápagos reptile conservation. https://www.galapagos.org/
[Iguana SG 2024] IUCN SSC Iguana Specialist Group. (2024). Recent Red List assessments — Amblyrhynchus cristatus. https://www.iucn-isg.org/
[MacLeod et al. 2020] MacLeod, A., Nelson, K.N. & Grant, T.D. (2020). Amblyrhynchus cristatus (errata version published in 2020). The IUCN Red List of Threatened Species 2020: e.T1086A177552193. https://dx.doi.org/10.2305/IUCN.UK.2020-2.RLTS.T1086A177552193.en
[Varela-Jaramillo et al. 2023] Varela-Jaramillo, A., Rivas-Torres, G., Guayasamin, J.M., Steinfartz, S. & MacLeod, A. (2023). A pilot study to estimate the population size of endangered Galápagos marine iguanas using drones. PeerJ, 11, e14406. https://doi.org/10.7717/peerj.14406
[Wikelski & Thom 2000] Wikelski, M. & Thom, C. (2000). Marine iguanas shrink to survive El Niño. Nature, 403, 37–38. https://doi.org/10.1038/47396
[Wikelski & Trillmich 1997] Wikelski, M. & Trillmich, F. (1997). Body size and sexual size dimorphism in marine iguanas fluctuate as a result of opposing natural and sexual selection: an island comparison. Evolution, 51(3), 922–936. https://doi.org/10.1111/j.1558-5646.1997.tb03673.x