Zebra shark surprises scientists by having babies without a mate

A zebra shark like this one (above) has produced babies, despite years without a mate.
A zebra shark like this one (above) has produced babies, despite years without a mate. PHOTO: IDIBAL/FLICKR

A female zebra shark separated from her long-term mate for over three years has astounded scientists in Australia by producing offspring.

The shark, named Leonie, has developed the ability to have babies on her own - without the need for a male partner.

Leonie met her mate at an aquarium in Townsville, Australia, in 1999, according to a report in the New Scientist.

They had more than two dozen offspring together before he was moved to another tank in 2012.

From then on, Leonie did not have any male contact.

But in early 2016, she had three baby sharks.

Biologist Christine Dudgeon at the University of Queensland in Brisbane and her colleagues began fishing for answers.

One possibility was that Leonie had been storing sperm from her former partner and using it to fertilise her eggs, the New Scientist said. But genetic testing showed that the babies only carried DNA from the mother - indicating they had been conceived via asexual reproduction.

While scientists have previously observed "virgin births" in vertebrates such as sharks, rays and reptiles - a reproductive strategy thought to aid survival during periods of isolation - this is the first time a female shark has ever been observed reproducing asexually after previously mating with a male.

It is only the third documented case of a vertebrate of any species switching its reproductive strategy from sexual to asexual, said a CNN report. An eagle ray and a boa constrictor, both held in captivity, are the only other species known to have undergone this unusual biological shift.

“In species that are capable of both reproductive modes, there are quite a few observations of switches from asexual to sexual reproduction,” Russell Bonduriansky at the University of New South Wales in Sydney told the New Scientist.

“However, it’s much less common to observe switches in the other direction.”

Dudgeon published a report on Leonie's unusual display of sexual behaviour in Scientific Reports.

In sharks, asexual reproduction can occur when a female’s egg is fertilised by an adjacent cell known as a polar body, Dudgeon said, according to the New Scientist.

This also contains the female’s genetic material, leading to “extreme inbreeding”, she says.

“It’s not a strategy for surviving many generations because it reduces genetic diversity and adaptability.”

Nevertheless, it may be necessary at times when males are scarce. “It might be a holding-on mechanism,” Dudgeon says.

“Mum’s genes get passed down from female to female until there are males available to mate with.”

It is possible that the switch from sexual to asexual reproduction is not that unusual; we just haven’t known to look for it, Dudgeon said.

Bonduriansky agreed, the New Scientist said.

“It would seem to be highly advantageous,” he said. “It could be much more common than we currently realise.”