Lone Blacktip shark was pregnant

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Aquarium officials from the Virginia Aquarium and Marine Science Centre have revealed that a lone female Blacktip shark that died during a routine health check-up was pregnant.

The lack of a male Blacktip shark within the exhibit, in which the female was an inhabitant for 10 years, has attracted great scientific interest in the pregnancy.

DNA testing is now underway to find out how the pregnancy could have occurred " whether it was a result of asexual reproduction, or hybridisation with another species of shark within the enclosure.

ParthenogenesisThe female Blacktip shark, known as Tidbit, had reacted badly to anaesthetics during its annual health check-up. Post-mortem results revealed that the shark was close to giving birth to a 25cm/10 pup.

A form of asexual reproduction, also known as parthenogenesis, would appear to be the strongest possibility for the cause of the pregnancy. Up until very recently, however, parthenogenesis had not been documented in sharks.

In fact, a report confirming the first occurrence of parthenogenesis in sharks was published just the day before Tidbits death. The phenomenon was discovered through the DNA analysis of a Hammerhead shark that was born in 2001 at the Henry Doorly Zoo in Omaha.

Although it had not been previously documented, Bob Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Sarasota, believes that parthenogenesis could actually be quite a common occurrence among sharks in captivity.

"This is probably something that does happen in aquariums, more often than we realise," Heuter told the Washington Post. He believes that these recent cases are merely shedding light on the phenomenon.

HybridisationAnother possible cause of the pregnancy could be hybridisation between the Blacktip shark and another species of shark within the exhibit. Hybridisation between species had not previously been recorded in sharks, and so should this be the case then it would be a world-first.

Bob George, the vet who carried out the post-mortem, told the Washington Post that although pathogenesis would be "a spiffy, interesting thing", he hopes that the tests do show hybridisation.

Tissue samples of Tidbit and the unborn pup have been sent to the same research team that analysed the DNA of the Hammerhead shark, a joint venture between scientists from Northern Ireland and the USA, in the hope of unravelling the mystery.

A preliminary report is expected to be delivered to the Virginia Aquarium soon. Should it show that the pup was a hybrid between two species, the scientists hope they will be able to identify the paternal species.