Marine Iguanas are the only lizard species that forage in the ocean. Adults and juveniles occur on rocky coasts and intertidal zones. Adult females can be found nesting up to 2 km inland and adult males can be found foraging in marine waters, up to depths of 30 m. They feed almost exclusively on marine algae, primarily three red genera (Centroceras, Gelidium, Pterocladia; Rubenstein and Wikelski 2003) and the green algal genus Ulva when exposed during low tides (Shepherd and Hawkes 2005). The majority of iguanas graze intertidally, though the largest iguanas of each colony also dive beneath the sea surface to forage on offshore algal beds (30 m offshore, 2–30 m depth; Buttemer and Dawson 1993, Wikelski and Trillmich 1994). Some iguana populations have been known to supplement their algal diet with highly salty land plants, primarily Saltwort (Batis maritima), but also other coastal succulents such as Sesuvium portulacastrum (Wikelski and Wrege 2000).

Marine Iguanas exhibit high morphological diversity, including substantial body size and shape variation among populations and sexes (Chiari et al. 2016). Males are larger, with different head morphology than females and more pronounced spines on the head and along the back. Average body size (snout-to-vent length) is 260 mm (range 120–490 mm; Miralles et al. 2017). Adult males weigh approximately 70% more than adult females (Laurie and Brown 1990a). Differences in maximum body size across islands is likely due to variability in algal productivity and sea surface temperature between islands (Wikelski and Trillmich 1997, Wikelski and Romero 2003, Wikelski 2005).

Reproduction occurs once a year during a month-long mating season, coinciding with the highest abundance and best nutritional quality of food (Rubenstein and Wikelski 2003). Nutrient-rich upwellings from the Humboldt and Cromwell currents affect all islands differently, so mating occurs at different times across the archipelago, varying somewhat from year to year. During this time males tend to become more colourful.

Marine Iguanas are the only lizard known to use a lek mating system, in which dominant males defend small territories (without resources beyond themselves) from other males. Male mating success is highly skewed to a few individuals and depends on body size, condition, and head-bob courtship behaviour. A single territorial male may be responsible for 35% of copulations on a lek (Trillmich 1983; Wikelski et al. 1996, 2001). Non-territorial males roam the periphery and try to force copulations with females, although they are rarely successful (Wikelski and Bäurle 1996). “Sneaker” males (physically small and resembling females) occupy the territories and also try to force copulations with females, which is also almost never successful.

Around four weeks after mating, females seek out and defend suitable sandy areas for digging burrows (Rauch 1988). Females lay 1–6 eggs (most commonly 2–3), which is highly dependent on female body condition and food availability (see subspecies cristatus, hassi, and trillmichi for additional data). Eggs are left to incubate for three months (Laurie 1990; Laurie and Brown 1990a, 1990b). Individual female iguanas typically reproduce once every two years (Vitousek 2009), or annually during periods of high food abundance (Laurie 1990). Hatchlings emerge at a body mass of ca 50 g (Trillmich and Trillmich 1986).

The Marine Iguana occurs on the large islands of Española, Fernandina, Floreana, Genovesa, Isabela, Marchena, Pinta, San Cristobál, Santa Cruz, Santa Fé, and Santiago, the mid-sized islands of Baltra, Bartolomé, Pinzón, Plaza Norte, Plaza Sur, Rábida, and Seymour Norte, smaller key populations on Darwin, Roca Redonda, and Wolf, as well as many satellite islets of the Galápagos Archipelago, Ecuador. This species has an extent of occurrence of 56,647 km² by minimum convex polygon enclosing all subpopulations, that includes the far distant small islands of Darwin and Wolf. A minimum convex polygon enclosing just the central islands (without Darwin and Wolf) of occurrence is 42,155 km². Marine Iguanas are found within 2 km of the coast with a total area of occupancy of 4,368 km², using a 2x2 km grid overlay in this coastal zone. They are found near (< 200 m asl) and below sea level at depths up to 30 m (large adult males).

The entire Marine Iguana distribution is included in three protected areas: Galápagos National Park and National Marine Reserve, Galápagos Islands Man and Biosphere Reserve (UNESCO), and Galápagos Islands World Heritage Site. Because the Galápagos is a high tourism area, it has attracted large numbers of migrants from the Ecuadorian mainland seeking tourist industry jobs. To curb population increase, Ecuador enacted a new law (Ley Orgánica de Régimen Especial para la Conservación y Desarrollo Sustentable de la Provincia de Galápagos: LOREG) requiring a special visa to visit the Galápagos, and there are strict limits on who can move there permanently (Strahm and Patry 2010).

Several eradication programmes have occurred in the Galápagos, including feral goats from several islands, cats from Baltra and Venecia, and Black Rats from Seymour Norte and Bainbridge #4 (Phillips et al. 2012). Increased efforts to eradicate or control cats, dogs, rats, and pigs to benefit all subpopulations of Marine Iguanas is sorely needed. The impact of rats is less well understood, however, it was documented that more hatchlings than normal were observed following a nearly complete eradication of Black Rats from Pinzón in 1988 (Cayot et al. 1994).

Conservation and research actions recommended for the species include an improved knowledge of population size, trends, distribution, factors limiting population size, habitat trends, and invasive species management. Some populations are of particular conservation concern and further research on the impact of low effective population size is recommended. Research to understand the impact of warming waters (climate change and El Niño) on individual algae species, along with other factors affecting algae community composition and iguana foraging dynamics, is recommended (Shepherd and Hawks 2005, Vinueza et al. 2006, Tomkins and Wolff 2017). An increased understanding of the effect of widespread oceanic pollution, including oil spills and micro-plastics, on Marine Iguanas and their micro-biota (which enable effective digestion) is also needed. Plastic debris ranks as one of the most abundant and persistent solid wastes at sea and shorelines, representing 25% of the total marine debris collected during beach cleanups in the Galápagos (Alava et al. 2014). Improved biosecurity measures to prevent disease introductions, such as West Nile Virus, avian malaria, and other are also recommended. Ongoing monitoring for introduced pathogens in Galápagos reptiles and other fauna is needed (Bataille et al. 2009; Eastwood et al. 2013, 2014).