Image caption The spectacular golden toad of Costa Rica vanished in little more than a year

A frog hunt like no other is about to begin, as conservationists scour the world for species thought to be extinct but which may just be hanging on.

Over the next two months, missions will begin in 14 countries searching for species such as the golden toad, the hula painted frog and the scarlet frog.

Amphibians are the most threatened animals on the planet, with one third of species at risk of extinction.

Many have been eliminated by a fungal disease carried in water.

Image caption The African painted frog (Callixalus pictus) is a vanished enigma

The scientist leading the project, Robin Moore, said he believes some of the 100 amphibians targeted in the survey will turn up.

"A couple of years ago when I was in Ecuador with a team of local scientists, we went in search of a species that hadn't been seen in 12 years," he told BBC News.

"We weren't very hopeful that we'd find it, but after a day of searching we uncovered a rock and found one of these little green frogs.

"Similar stories have started popping up of people finding frogs that we thought had gone; so it gives me hope that there are a lot out there that we think may have disappeared but may actually still be alive."

Dr Moore, of Conservation International (CI), is organising the search for the Amphibian Specialist Group (ASG) of the International Union for the Conservation of Nature (IUCN).

Golden goal

The biggest issue for amphibians globally is loss of their habitat, as forests are cleared and wetlands drained.

Image caption Australia's gastric brooding frogs held out promise for medicine

But this survey will target many species that have fallen prey to a newer and starker threat - the fungal disease chytridiomycosis.

There is currently no way of preventing infection in the wild, or of preventing its spread across the world.

Although some species are immune, the chytrid fungus wipes others away suddenly. The iconic golden toad of Costa Rica (Incilius periglenes) went from abundant to extinct in little more than a year.

This spectacular species has become a poster-child for the amphibian crisis, and finding some specimens still alive - about which the team is not optimistic - would be a major coup.

The same applies to the gastric brooding frogs of Australia, which uniquely in the animal world raise their tadpoles in their stomachs.

This involves turning off production of stomach acid. Medical researchers hoped that understanding how the frogs did it could lead to new treatments for stomach ulcers.

But their disappearance in 1985 - probably another victim of chytridiomycosis - put paid to such notions.

If it turns out that a few of them do still exist, or of any of the other species to be surveyed, conservation measures would be implented in full.

"We're limited by our knowledge of many of these species and whether they even exist - if we don't know whether a species exists, we can't protect it," said Dr Moore.

"So it really is a mission to increase our knowledge of what's out there, what's still alive, so that we can follow up and hopefully do some conservation work on species that are found."

Among the other top targets for the survey teams are:

the hula painted frog (Discoglossus nigriventer) of Israel. Last seen in 1955, it probably went extinct because of marsh drainage - an attempt to curb malaria

the African painted frog (Callixalus pictus), formerly found in the Democratic Republic of Congo and Rwanda. It was last seen 1950, and is thought never to have been photographed

the Mesopotamia beaked toad (Rhinella rostrata). Featuring an unusual pyramid-shaped head, the last sighting dates back to 1914.

Teams will spend between a week and two months in the field looking for each of the targeted species.

The results of the search missions should be known before October's meeting of the UN Convention on Biological Diversity (CBD) in Japan, at which governments will review the reasons why they have failed to implement their 2002 pledge to reduce the loss of nature significantly by 2010.

"This [survey] is something that has never been done before, and is hugely significant," said Dr Claude Gascon, CI vice-president and ASG co-chair.

"The search for these lost animals may well yield vital information in our attempts to stop the amphibian extinction crisis, and information that helps humanity to better understand the impact that we are having on the planet."