Marine iguanas – the ocean’s forgotten victims


By Sarah Eager

Overfishing, pollution, and climate change are devastating the once diverse ecosystem of the Galapagos Islands. This unique archipelago is home to some of the highest levels of endemism (species found nowhere else) on the planet, including marine reptiles like the marine iguana. Once considered to be one of the most abundant creatures on earth, marine iguanas are seeing their numbers rapidly decline.

“Nowhere on Earth are the combined impacts of climate change and overfishing more clearly defined than in the Galapagos Islands.” 
 Sylvia Earle.

Globally, there aren’t many marine reptile species. Of around 12,000 known reptile species, fewer than 100 are classed as marine reptiles. Marine reptiles were prolific in the Cretaceous and Palaeozoic periods and, in 2015, scientists attributed the mass extinction of prehistoric marine reptiles, including mosasaurs and ichthyosaurs, 94 million years ago in part to global warming. Today, the living species of marine reptiles are sea turtles, sea snakes, saltwater crocodiles, and marine iguanas.

Marine iguanas are classified into 11 subspecies, including the Godzilla marine iguana (Amblyrhynchus cristatus Godzilla) who’s named after everyone’s favourite fictional lizard.

Looks can be deceiving. When he visited the Galapagos Islands in 1835, Charles Darwin made some unforgiving assessments of marine iguanas. He described them as “hideous looking creatures”, “stupid and sluggish”, “imps of darkness”, and “disgusting, clumsy lizards”. He couldn’t be more wrong. They’re simply misunderstood.

Despite their fierce appearance, marine iguanas are gentle, curious herbivores who flock to greet visitors to the Islands’. Not at all like their bigger fictional cousin.

They’re gregarious, living in large colonies and huddling together in groups at night for warmth. By day they bask in the sun to absorb heat until they have enough energy and body heat to head out to sea to forage. Upon entering the water, they swim agilely and gracefully, and slow their heartbeat to half its normal pace to conserve energy and feed for longer.

In fact, marine iguanas are so good at slowing their heartbeat that they can stop their hearts for up to an hour to avoid sharks, their main ocean predator, who can hear a heartbeat from up to 13 feet. On land, that’s not so effective; cats and dogs introduced to the Islands’ prey on the iguanas. Darwin was the last person to officially record a sighting of marine iguanas on Santiago Island. They were decimated by invasive species introduced by mariners and early settlers to Galapagos’ largest island.

Marine Iguanas are the capybara of the marine world. They’re friendly, docile creatures who nonchalantly go about their lives unfazed by humans. But maybe they should be concerned about us. After all, the human impact is having destructive effects on the Galapagos Islands and marine reptile populations. From pollution to climate change, invasive species to habitat loss through land development, we’re the biggest threat to the marine iguana.


The unspoiled perception of the Galapagos Islands gives the impression that the islands are exempt from plastic pollution. The Galapagos Marine Reserve, at 133,000 square kilometres, is one of the largest marine protected areas in the world. So, it must be working, right? Here’s the thing about plastic – it trespasses. Despite our idyllic mental image, marine reptiles, including the marine iguana, are at risk of severe injury or death from plastic ingestion and entanglement.

Primarily algae eaters who spend much of their time at the sea surface, marine iguanas are greatly impacted by floating plastic. Like sea turtles, they’ll often mistake plastic for food.

A study by the University of Exeter, the Galapagos Conservation Trust, and the Galapagos Science Center found up to 800 microplastics per square meter, with an average of 449, around Galapagos coastlines. Each year, over 8 tonnes of plastic is removed from Galapagos beaches.

Underwater – Sodacan via Wikimedia Commons

Only 2% of the plastic found in Galapagos comes from the Islands’. Most plastic pollution arrives on ocean currents. East-facing beaches have been hit hardest by plastic pollution. These beaches are exposed to debris carried from the Great Pacific Garbage Patch (GPGP), the world’s largest offshore plastic accumulation zone. The GPGP is three times the size of France.

In a 2019 study, researchers worked to determine if:

  • Marine iguanas were ingesting plastics
  • Location increases micro and macroplastic exposure and ingestion
  • Plastics negatively impact marine iguana health.

The answer was a resounding yes. Synthetic particles ingested by marine iguanas were found to be primarily from discarded fishing equipment, fishing gear, and clothing. At least 38 different species in the Islands’, including sea turtles and marine iguanas, have been recorded entangled by plastics, particularly fishing lines.

Climate Change

In 1983, El Nino caused over 60% of the marine iguana population to die. 

Events like El Nino can be good news for land animals. Increased rainfall causes more food availability. It’s bad news for the marine iguana.

Marine iguanas feed on red and green algae in subtidal and deeper, cooler water. Rising sea temperatures cause marine nutrient levels and phytoplankton concentrations to drop. Thus, the marine iguanas’ food source, red and green algae, dies and is replaced by brown algae which they can’t digest. The environmental changes can cause up to 90% of marine iguanas to die of starvation or overheating due to rising ocean temperatures.

“Climate change is increasing the frequency of El Nino events.” 
Proceedings of the National Academy of Sciences

To combat the effects of climate change and food scarcity, some marine iguanas are getting smaller. A small iguana expends less energy, requires less food, and their odds of survival increase. Shrinking for survival is unheard of in vertebrates.

Marine iguanas aren’t just losing weight. They’re getting shorter.

Iguanas can decrease in size by up to 20% within two years. For reference, that’s like a 6’2” person shrinking to 4’11″.

They don’t just shrink. The Galapagos Islands aren’t full of pocket-sized iguanas. They’ll grow back to their original size once food becomes available again.

Climate change doesn’t just mean less food. It means less iguanas. Sea turtles are struggling to develop eggs as temperatures rise, so it’s possible this is also affecting marine iguanas.

Rising sea levels caused by climate change contribute to coastal erosion which, coupled with coastal areas being lost to hotels and tourist facilities, reduces nesting areas, and impacts the iguanas’ ability to regulate their body temperature.


Despite being one of the most pristine ecosystems in the world, the Galapagos Islands are not immune to the issues facing marine life in the rest of the world. The Galapagos Marine Reserve bans commercial fishing in the waters surrounding the Islands’, yet marine reptiles routinely ingest or become entangled in commercial fishing gear.

Given the levels of plastic and rising sea temperatures found in the Islands’, we need to stop damage to our oceans at its source. We can’t just take a local approach; we need to take a global approach to our oceans and marine life. If we don’t, the marine iguana is likely to join its ancestors, the mosasaur and ichthyosaur.

Blue Planet Society is a global pressure group campaigning to protect the world’s ocean. You can help our work by donating here.

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Blue Planet Society is a global pressure group campaigning to protect the world’s ocean. By utilising effective activism, minimising the use of resources and applying the highest ethical standards, we believe our approach is the future of marine conservation advocacy.