The fate of a species is resting on the shells of two turtles at China's Suzhou Zoo.

In June, researchers collected eggs from the last mating pair of the critically endangered Yangtze giant softshell turtle (Rafetus swinhoei) in the hopes that at least one will be fertile.

The 220-pound (100-kilogram) freshwater giant, which spends most of its life burrowing in mud, was once common in its namesake Yangtze River, China's Lake Taihu and Yunnan Province, and parts of Vietnam.

By the late 1990s, however, human encroachment and poaching for use of the shells in Chinese traditional medicine rapidly depleted the population. Now, a total of four animals are known—two wild males in Vietnam and the mating pair at Suzhou Zoo.

It's the team's sixth year of breeding the turtles at the zoo, which is not far from Shanghai. So far, none of the eggs have hatched.

Researchers can't pinpoint the reason for the infertility, but they suspect a combination of factors, including poor sperm quality due to the male's age—roughly a hundred—an improper mating posture, and stress on the female.

Because the turtles are the last in captivity and too much human interaction could kill them, sperm samples cannot be taken nor tests run. Still, scientists are hoping that this year will be the lucky one. (Related: "Pictures: Turtles Hunted, Traded, Squeezed Out of Their Habitats.")

"The resurrection of this iconic species in the wild, the largest freshwater turtle in the world, would be a symbol of hope," said Gerald Kuchling, founder of the Australia-based group Turtle Conservancy and a turtle-reproduction expert.

"Miraculous" Find

As is the case with many near-extinct species, by the time scientists realized the extent of the turtle's decline, the species was almost gone.

In 2006, the U.S. nonprofit Turtle Survival Alliance asked Kuchling to establish the sex of the last three captive giant softshell turtles in China, which at the time lived at the Shanghai Zoo, Suzhou Zoo, and Suzhou's West Garden Buddhist Temple. (Related: "6 of Nature's Loneliest Animals Looking for Love.")

When Kuchling landed in China in 2007, the Shanghai Zoo and Buddhist Temple individuals had already died. The Suzhou Zoo male was the last known Chinese survivor. Researchers sent an all-points bulletin to every zoo in the nation in the off chance a turtle had been misidentified.

Their call was answered: A photograph of a turtle at the Changsha Zoo looked promising. Kuchling, along with Lu Shunqing, China director for the Wildlife Conservation Society, traveled to Changsha, where they confirmed it was a Yangtze giant softshell—and a female to boot.

"It's a bit miraculous we found her," said Emily King, the Suzhou Zoo breeding program's field assistant.

Breeding Roadblocks

Although moving the Changsha Zoo's female—the younger of the pair at then 80 years old—to the Suzhou Zoo was risky because of the stress it would cause the animal, zoo officials and researchers had no choice.

Surveys in the wild consistently had turned up no Yangtze giant softshells aside from the two males already known in Vietnam. These individuals haven't been captured because catching and transporting them could be fatal.

Either the Suzhou Zoo pair would mate, or the species would go extinct.

In May 2008, after much red tape, the female finally arrived in Suzhou. Just over a week later, the turtles mated-despite the fact that the female had likely never met a male. (Also see "Mating Turtles Fossilized in the Act.")

A month later, the female laid her first clutch of 45 eggs on the zoo enclosure's beach, 32 of which were incubated.

To determine if an egg is fertile, the scientists candle them, or hold a candle behind the egg to look for a developing embryo.

The initial batch yielded no hatchlings. Later that month, a second batch was equally infertile.

The turtles mated each of the following years, but with the same result.

Di Min, a zoologist at the Suzhou Zoo, said when the program started there was talk about assisted reproductive techniques, a kind of "turtle IVF."

"But the best and safest is they breed naturally. There's only this pair—if we lose one, especially the female, we don't have any chance."

The team doesn't know how much longer the zoo turtles will live or continue to mate, but scientists suspect Yangtze giant softshells can live well over a hundred years. (See more photos of aquatic species.)

Turtle Team Optimistic

Despite these setbacks, scientists are staying optimistic about saving the turtle.

"We have these two [Suzhou] animals, and hopefully in the very near future, as opposed to far distant, we'll have baby Rafetuses on our hands," added field assistant King.