Upton-by-Chester, UK - Hundreds of tiny snails have been reintroduced to a rocky Atlantic island following a breeding program to save the mollusks from extinction.

Conservationists at Chester Zoo said they had "one shot" to save the snails, which were so rare the global population could have fitted into a single petri dish.

With work to clear their island home of the invasive goats, rabbits, and mice that had pushed the snails to the brink of extinction, it is now hoped the mollusks – no bigger than a pea – will help restore the soils and boost the natural habitat.

Experts at Chester Zoo, along with those at the ZooParc de Beauval in France, had been asked by the Madeira government to help with the conservation of four snail species that are unique to the Deserta Islands, part of the Madeira archipelago in the Atlantic Ocean.

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A handful of snails were sent to each institution, including 21 of the species Geomitra coronula which were thought to be the last of their kind, found clinging underneath a single rock on Bugio Island.

Now more than 300 descendants of the snail that have been bred at Chester Zoo have returned to the wild on the island, alongside 270 individuals from the species Discula lyelliana bred by specialists in France.

Conservationists had to battle rough weather and a scramble from the water up a cliff face in the dark to find snail-friendly cracks in the rocks in order to return them to the wild.

Chester Zoo is now looking after all four species, Atlantica calathoides, Geomitra grabhami, Geomitra coronula, and Discula lyelliana, in biosecure breeding pods mimicking their natural environment.

Tamas Papp, assistant team manager for invertebrates at Chester Zoo, said it had to be a "rapid rescue mission for all four species of snail."

Over several hundred years, people destroyed the islands' woodland and brought in rabbits and goats, which prevented native plants from regrowing, while accidentally-introduced mice used the snails as a source of water on the arid islands, wiping out their populations, he said.

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Papp added: "By the time the Madeira government contacted us for help, one of the species, Geomitra coronula, was so close to extinction that its entire global population could fit inside a single petri dish."

"We knew we had one shot to save them," he continued. "If we missed this chance, that was it – they were gone."

Madeira government conservationists have undertaken work to remove the invasive species and restore the island, enabling the captive-bred snails to return – but it was not plain sailing, according to the team behind the release.

Papp said: "It might be the trickiest reintroduction we've ever done."

"We had a short window due to rough weather, and there's no jetty on the island," he explained. "We had to jump straight into the water, climb out, put helmets and harnesses on, and start scrambling up the cliff face."

"It took us two hours to navigate less than two miles," he said, traveling in the dark to find snail-friendly crevices in the rocks, using red-spectrum torches to avoid disturbing the island's rare seabirds.

Now the snails – which measure no bigger than 8mm in size but are important as they eat vegetation and enrich the soil – are in place, they will be monitored as they settle back into the wild.