Back in 2012, a baby brownbanded bamboo shark was born in tank even though the mother hadn't been exposed to a male for 45 months. Scientists have now figured out how this so-called "virgin birth" was possible.

To photo: Pup of Chiloscyllium punctatum born at the Steinhart Aquarium of the California Academy of Sciences. Photograph by Timothy Wong.

This wasn't the first time that marine biologists have observed an apparent "virgin birth" in sharks. Several years ago, a captive zebra shark produced babies four years in a row. This species of shark, like several others, is known to give birth via "facultative parthenogenesis," a process whereby asexual reproduction occurs without fertilization. It's a well-known phenomenon documented in many species of amphibians and reptiles.

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Marine biologists assumed the same thing was happening in brownbanded bamboo sharks, but this now appears to be incorrect. A new study in the Journal of Fish Biology says it's the result of long-term storage of sperm by one of the females. And in this case, very long long-term storage. An article from NBC News explains:

Researchers analyzed DNA samples from the pup and the three females, and determined which one was the probable mother. But the pup also had genetic material that wasn't found in any of the females, meaning it could only have been inherited from the father. That ruled out parthenogenesis. The last time the three females could have been in contact with a male was at the Aquarium of the Pacific, from which they were acquired in September 2007, the researchers said. That makes for what researchers concluded was a case of very long sperm storage.

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The researchers conclude thusly:

Previous studies on sharks have reported sperm storage periods of 13 (Storrie et al., 2008) to 28 months (Castro et al., 1988). Considering females had not been in contact with males for at least 45 months, this is the longest documented case of sperm storage and successful fertilization for any species of shark.

Why the long-term storage? The researchers say it's an advantageous strategy because fertilization can occur even if females are not ovulating at the time of insemination.

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Read the entire scientific study here.