[5] They mainly live in colonies on rocky shores where they bask after visiting the relatively cold water or intertidal zone, but can also be seen in marshes, mangrove swamps and beaches.

[8] Although relatively large numbers remain and it is locally abundant,[9] this protected species is considered threatened, primarily from El Niño cycles, introduced predators and chance events like oil spills.

However, a more recent study that included both mtDNA and nDNA indicates that the two split about 4.5 million years ago, which is near the age of the oldest extant Galápagos islands (Española and San Cristóbal).

[17] There is one confirmed record of a marine iguana outside the Galápagos Islands; in 2014 an individual appeared on Isla de la Plata near the Ecuadorian mainland.

In 1798, captain James Colnett of the British Royal Navy wrote: On his visit to the islands in 1835, despite making extensive observations on the creatures, Charles Darwin was revolted by the animals' appearance, writing: Marine iguanas have a thickset body and relatively short, robust limbs.

Males of the relatively small southern islands of Española, Floreana and Santa Fé (subspecies venustissimus and trillmichi) are the most colourful, with bright pinkish-red and turquoise markings.

In comparison, those of the relatively small northern islands of Genovesa, Marchena, Pinta, Wolf and Darwin (jeffreysi, hayampi, sielmanni and nanus) are almost all blackish without contrasting markings.

[8][19] Other subspecies tend to resemble duller versions of venustissimus and trillmichi, or are blackish with markings in pale yellowish, ochre, greenish or grey (sooty to near white).

In comparison, Santa Cruz marine iguanas (hassi) had an average snout-to-vent length of about 35 cm (14 in), and those of Isabela and Fernandina (cristatus) were only slightly smaller.

This difference in body size of marine iguanas between islands is due to the amount of food available, which depends on sea temperature and algae growth.

[35] Reproduction in the marine iguana begins during the last part of the cold and dry season,[22] with breeding from December to March and nesting from January to April.

[28][41] Females prefer male territories that include or are near their own resting places, which they choose based on proximity to the sea, access to shade, low midday temperature and the possibility of sunbathing in the afternoon.

[52] As soon as they emerge from the nest they run for cover,[50] and begin their trip to locations that provides optimum conditions for feeding, temperature regulation and shelter.

[5] Some species with chemical deterrents, such as Bifurcaria, Laurencia and Ochtodes, are actively avoided, but otherwise algal food choice mainly depends on energy content and digestibility.

[49] They may feed on octopuses, crustaceans, insects (such as grasshoppers and cockroaches),[9] fish carrion, and sea lion feces and afterbirth on rare occasions.

[59][60] The population on North Seymour Island will supplement their diet with land plants, primarily Batis maritima, or other coastal succulents like Sesuvium portulacastrum.

[27] In the first months after hatching, the juveniles mainly feed on feces from larger marine iguanas, gaining the bacteria needed for digesting algae.

[61][63][64] Most females and smaller males feed on exposed algae in the intertidal zone during low tide,[5] retreating once the water returns and starts washing over them.

[49] They often scurry back-and-forth repeatedly, running to a patch of algae to take a few bites and then return fast to higher ground to avoid incoming waves.

[55] These environmental changes and the ensuing occasional food unavailability have caused marine iguanas to evolve by acquiring efficient methods of foraging in order to maximize their energy intake and body size.

[12][63] Marine iguanas have powerful limbs with long, sharp claws for climbing, holding onto rocks and pulling themselves forward when at the sea bottom (adding to the propulsion provided by the tail).

[22] Other adaptions in marine iguanas are blunt heads (flat noses) and sharp teeth allowing them to graze algae off of rocks more easily.

Lava lizards may scurry over marine iguanas when hunting flies, and Darwin's finches, mockingbirds and Sally lightfoot crabs sometimes feed on mites and ticks that they pick off their skin.

[61] When in the water and their temperature is falling, their blood circulation is reduced because of a low heart rate of about 30 beats per minute, allowing them to better conserve their warmth.

[5] Individuals that forage further from the shore by diving are large males, which mainly feed during the hot midday (although it may occur from late morning to early evening), are less affected by the cool water because of their body size and are more efficient swimmers.

[74] Introduced predators represent a major problem on the islands of Santa Cruz, Santiago, Isabela, Floreana and San Cristóbal where very few marine iguana hatchlings survive.

It is suspected that the primary cause of death during these events is starvation due to the loss of their sensitive gut bacteria, which they rely on to digest algae.

Marine iguanas show higher stress-induced corticosterone concentrations during famine (El Niño) than feast conditions (La Niña).

The variable response of corticosterone is one indicator of the general public health of the populations of marine iguanas across the Galápagos Islands, which is a useful factor in the conservation of the species.

[7] The development of a captive breeding program (as already exists for the Galápagos land iguana[9]) possibly is a necessity if all the island subspecies are to survive.