Second virgin shark birth confirmed

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The second ever case of a virgin birth in sharks has just been confirmed by scientists in Virginia, ironically.

The shark, a nine-year old Blacktip reef shark, Carcharhinus limbatus, named Tidbit, lived at the Virginia Aquarium until she died following a routine veterinary inspection.

An autopsy revealed a single, well-developed female embryo measuring about 25cm long, despite the fact that the Blacktip had been brought into the aquarium at the age of one and had been isolated from any sharks of the same species for the last eight years.

The Blacktip had been kept with a male sandbar shark Carcharhinus plumbeus, although staff had never observed mating between the pair or observed any physical evidence of mating wounds.

Various theories were suggested as to the origin of the pup; including sperm storage from when the female was a wild juvenile and mating with the male sandbar shark.

Fin clips were taken from the mother and juvenile shark and analysed using a type of DNA fingerprinting by scientists from oceanographic institutes in Florida and from the aquarium itself.

They established that there was no genetic evidence that this could be a hybrid of the two shark species and that the probability that this was a case of sperm storage was also so small as to be negligible.

The analysis did show an extreme genetic likeness to that of the pup s mother.

It was not identical however, suggesting that this is a case of ~automictic parthenogeneis (also known as automixis).

This is a process where the egg and a polar body, which is a type of discarded germ cell formed at the same time as the egg, fuse to produce a single viable fertilised cell which then goes on to develop into an embryo.

This is also the mechanism proposed for the first confirmed case of parthenogenesis in a shark.

This was seen in 2001 in a female hammerhead shark Sphyrna tiburo from a zoo in Nebraska, which gave birth to a female pup after a period of over three years in captivity without access to conspecific male sharks.

There have also been a number of cases where captive female sharks have produced normal offspring despite the absence of females. These have not been verified but they look increasingly likely to be instances of parthogenesis. What is not known is whether this phenomenon is due to an occasional aberration in the egg or whether sharks have developed asexual reproduction as a mechanism to cope with an absence of suitable mates as seen in some reptile species.

For more information see: Chapman DD, Firchau B and MS Shivji (2008) - Parthenogenesis in a large-bodied requiem shark, the blacktip Carcharhinus limbatus. Journal of Fish Biology (2008) 73, 1473"1477, doi:10.1111/j.1095-8649.2008.02018.x.