The island of Fernando de Noronha, off the coast of Brazil, is teeming with deformed cururu toads.

They invaded the island several decades ago, and now almost half of them have malformed limbs, eyes, or mouthparts.

Some toads lack hands or feet, while others have extra or missing fingers, or misshapen ones. Others have abnormal jaws, mouths or noses.

About 20% of the deformed toads are also partially or completely blind. Some toads lack one or both their eyes, and others have eyes with discoloured irises or no iris at all.

These deformities have profoundly changed how the toads behave.

For instance, most toads use visual cues to find, chase and catch their prey. But the blind toads on the island of Fernando de Noronha have adopted a different, laid-back feeding strategy, according to a study published in the Journal of Zoology.

"Unlike the normal toads, the blind ones literally wait for the insects to walk on to them, before eating them," says Luıs Felipe Toledo, an amphibian biologist from the Campinas State University in São Paulo, Brazil.

Since the blind toads simply eat what's passing by, they are no longer selecting their prey, he adds. "That causes different body conditions."

For example, Toledo found that the blind toads were lighter in weight than the normal toads, and produced fewer eggs. So losing vision has had a cascading effect on the toads, from changing their predation tactics to ultimately affecting their fitness.

Despite these deformities, the toads continue to thrive on the island. One reason is that the toads are an introduced species, and do not have any natural predators or competitors on the island.

Moreover, even though the deformed females produce fewer eggs, each one still pumps out thousands. "That's enough to keep the population going," says Toledo.

The tadpoles also suffer from various deformities. In a 2014 study, Toledo's team found that nearly 53% of the tadpoles they inspected had at least one anomaly. These malformed tadpoles could turn into either deformed toads, or normal ones.

No one knows for sure when or how the toads got there. "There is a story that a priest, about 100 years ago, took some toads from mainland Brazil to the island to control insects on his crops," says Toledo.

It's also unclear why they have since become deformed and blind.

"Everyone asks why the toads are like that," says Toledo. "And that is the one question we haven't answered yet."

Toledo's team, together with researchers at the San Diego Zoo in California, are testing some ideas that might explain the large-scale malformations in the toads of Fernando de Noronha.

For instance, they are investigating if a parasite, bacterium or virus could be a potential culprit, and undertaking genetic studies to see if the toads are inbred.

The team is also testing the island's water and soil for contaminants.

Cane toads on Bermuda Island, for example, have limb deformities caused by petroleum hydrocarbons in the pond sediments.

But the deformities are just the tip of the iceberg, according to Jamie Bacon of the Bermuda Zoological Society. "We've found endocrine disruption, suppressed immune function and impaired reproduction as well - and not just in amphibians," she says. "Therefore, other species might be at risk."

Fernando de Noronha is a National Marine Park and a UNESCO World Heritage Site. As a result, it is crucial to understand what is driving malformations in the cururu toads, says Allan Pessier of San Diego Zoo.

"These same factors may have the potential to impact or spill over into other wildlife species on the island, with the toad acting as an early warning system of problems in the environment," he says.