More than 1,300 pea-sized, critically endangered snails that were bred in a zoo have been set free to wander (very slowly) on a remote Atlantic island.

The release brings two species of Desertas Island land snails back to the wild. Prior to this they were believed to be extinct - neither species had been spotted for a century.

When a team of conservationists found a small population surviving on the rocky cliffs of Deserta Grande island, close to Madeira, they mounted a rescue effort.

The snails were brought to zoos in the UK and France, including Chester Zoo, where a home was created for them in a converted shipping container.

The tiny molluscs are native to the windswept, mountainous island of Deserta Grande, just south-east of Madeira. Habitat there has been destroyed by rats, mice and goats that were brought to the island by humans.

It was thought that all these invasive predators had eaten the tiny snails to extinction. Then a series of conservation expeditions - between 2012 and 2017 - proved otherwise.

Conservationists discovered just 200 surviving individuals on the island.

Those snails were believed to be the last of their kind, so they were collected and brought into captivity.

At Chester Zoo, the conservation science team made a new home for 60 of the precious snails. The right food, vegetation and conditions were recreated in miniature habitat tanks.

1,329 snail offspring, bred at the zoo, have now been marked with identification dots - using non-toxic pens and nail varnish - and transported back to the wild for release.

"[It's a] colour code," said Dinarte Teixeira, a conservation biologist at Madeira's Institute for Nature Conservation and Forests. "This will allow us to spot them and track where they disperse to, how much they grow, how many survive and how well they adapt to their new environment."

A wild refuge has been restored for the snails on Bugio, a smaller neighbouring island in the Ilhas Desertas (Desert Islands) archipelago.

Bugio is a nature reserve and invasive species have been eradicated there.

Gerardo Garcia from Chester Zoo said that the reintroduction was "a major step in a species recovery plan".

"If it goes as well as we hope, more snails will follow them next spring. It's a huge team effort which shows that it is possible to turn things around for highly threatened species."

"These snails are such an important part of the natural habitat [on the islands they come from]," explained Heather Prince from Chester Zoo. As well as being food for other native species, she explained, snails break down organic matter and bring nutrients to the soil.

"They help plants grow. All of that is dependent on the little guys - the insects and the snails that so often get overlooked."