Zielo Adventures
A Galápagos marine iguana on volcanic rock

Galápagos wildlife · Reptile · Endemic

The Galápagos marine iguana

Amblyrhynchus cristatus · Iguanidae

Short answer

The marine iguana is the only lizard on Earth that forages in the sea. It grazes algae off submerged rock, holding its breath for up to thirty minutes, then hauls out to reheat on black lava. The salt it swallows is expelled through a nasal gland — the sneeze that leaves a white crust on its head. Darwin called them "imps of darkness" and did not like them. He was wrong about very little else.

1 m 2 m A person · 1.75 m 1.5 m

Actual size

nose to tail tip, Isabela males.

Length
1.5 m
Weight
to 1.5 kg on most islands; Isabela’s are the largest
Lifespan
Around 12 years
The thing people repeat afterwards

It shrinks. In a bad El Niño year a marine iguana can lose up to 20% of its body length — bone included — and grow it back when the algae returns.

Where you actually see one

This is the part a general encyclopedia cannot tell you: not where the species occurs, but which sites you can stand on, and what each one is like.

THE EQUATOR EspañolaFloreanaSan CristóbalSanta CruzFernandinaSantiagoIsabelaMarchenaPinta 1 Tortuga Bay 1 Puerto Ayora 2 Las Tintoreras 2 Concha de Perla 3 Punta Espinosa 4 Playa Mann
  1. 1

    Santa Cruz — Tortuga Bay and the Puerto Ayora shoreline

    Large colonies on the black rock, often piled on each other for warmth.

  2. 2

    Isabela — Las Tintoreras and Concha de Perla

    Isabela holds the largest marine iguanas in the archipelago.

  3. 3

    Fernandina — Punta Espinosa

    The densest colonies anywhere — hundreds on one lava flow. Reachable only by cruise.

  4. 4

    San Cristóbal — Playa Mann and Punta Carola

    Easy walking distance from town.

Coastlines are real. Darwin and Wolf sit 1.7° further north and are left off so the rest stays legible.

When to go for this one

Year-round. Breeding colour peaks December to March, when Española’s males turn red and green.

The archipelago has two seasons and they are good at different things — we break both down here, month by month.

How to tell it apart

  • Blunt, flat-fronted snout — built for scraping algae off rock, not for biting leaves.
  • A white salt crust on the head, from the nasal glands.
  • Laterally flattened tail, used like an oar.
  • Colour varies by island: black on most, red and green on Española in breeding season.

What it does, and why

  • Basks for hours before and after feeding: it cannot digest until its body temperature comes back up.
  • Larger adults dive; smaller ones graze the intertidal zone, because the sea is too cold for a small body.
  • Shrinks in length during El Niño years when algae fails — and regrows afterwards. It is one of the few vertebrates known to do this.

Why people miss it, and who it is not for

This is the part we can write and an encyclopedia cannot: what actually goes wrong when someone comes looking for this animal. It is from running these islands, not from a field guide.

The famous red-and-green ones are on one island, in one season

Every photograph you have seen of a scarlet marine iguana was taken on Española during the breeding season. Everywhere else, and the rest of the year, they are black. Visitors who booked expecting the colours and did not go to Española are disappointed, and they were never going to see them.

You will walk past a hundred before you notice one

They are black, on black lava, and motionless. The park rule is two metres and people break it constantly by accident, simply because they did not see the animal. Look before you put a foot down, especially at Tortuga Bay.

The best thing they do happens underwater

The large ones graze algae on the seabed, holding their breath for minutes. Almost nobody sees it because almost nobody snorkels the shallow rocky shoreline where it happens. If you want that, ask — it needs the right site and calm water.

Conservation status

IUCN Red List: Vulnerable

Listed Vulnerable. The main threats are introduced cats and dogs on inhabited islands, oil spills, and El Niño events that wipe out the algae they depend on. The 1982–83 El Niño killed an estimated 70% of some island populations.

Questions people ask about the marine iguana

Are marine iguanas dangerous?

No. They are herbivores with no interest in people and no defensive aggression toward them. Park rules still require two metres of distance, and on a busy shoreline that mostly means watching where you put your feet.

Why do marine iguanas sneeze?

They are expelling salt. Grazing algae in seawater loads them with more salt than a kidney can handle, so a specialised nasal gland concentrates it and they sneeze it out. The white crust on their heads is that salt.

How long can a marine iguana stay underwater?

Large adults can stay down around thirty minutes, though most feeding dives are far shorter. Their heart rate drops to about half to conserve oxygen.

Where are the biggest marine iguanas?

Isabela and Fernandina. Body size tracks how much algae an island’s coast produces, so the western islands, fed by the cold Cromwell Current, grow the largest animals.

What else is on the same islands

These share at least one island with the marine iguana. On a real day out you are likely to see several of them on the same walk.