Female sharks can store a male's sperm for a long time before fertilizing and laying eggs, but one shark species has set a record by holding on to sperm for almost 4 years.

Researchers at the California Academy of Sciences say a brownbanded bamboo shark laid a viable egg -- from which hatched a healthy shark pup -- and being secluded from any contact with males for an incredible 54 months.

The successful birth of the pup at San Francisco's Steinhart Aquarium in January 2012, in a tank holding only female bamboo sharks, represents the longest confirmed example of sperm storage for any shark species, they report in the Journal of Fish Biology.

It's an encourage finding, a sign of hope for the fate of sharks in the wild threated by habitat loss and overfishing, says Luiz Rocha, the academy's curator of ichthyology.

"Long-term sperm storage -- where a female can delay fertilization for months or even years after mating -- is a remarkable adaptation that helps promote genetic diversity," he says. "Exploring the bamboo shark's ability to store sperm gives us hope that wild sharks can help protect their population's genetic diversity when mates are scarce and serious threats arise."

Populations that lack genetic diversity are susceptible to widespread die-off in the face of significant environmental threats, the researchers point out.

While they where unable to determine which of the female sharks in the aquarium's tank had laid the viable egg, DNA examination of the pup showed it was not a case of parthenogenesis -- in which a female that usually reproduces sexually is able to create offspring on her own, observed before in some shark species -- but that the pup had the normal two parents.

None of the females had had any contact with male bamboo sharks since they had been transferred to Steinhart from the Aquarium of the Pacific in Southern California in 2007.

The DNA of the pup showed evidence of genetic material inherited from the father, an unknown bamboo shark male sharing that Southern California tank almost 4 years ago.

Long-term storage of sperm is an advantageous adaptation seen in other sharks as well as in many other species, including marsupials, frogs and insects. However, this was the first instance of the technique being observed in bamboo sharks, Chiloscyllium punctatum, the researchers said. They plan on further research.

"We know that several species of sharks have reproductive tactics like storing sperm or reproducing by parthenogenesis in the absence of males, but we need to know when and how these alternate techniques are triggered," says study lead author Moisés A. Bernal. "Understanding these mechanisms  - and how they impact genetic diversity -- could be vital for the future of shark conservation."

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