Jaws drop at Burj Al Arab aquarium's virgin birth
A female zebra shark in Dubai has successfully spawned pups - without the presence of a male.
Zebe, who was introduced to the Burj Al Arab's aquarium as a pup in 2001, has been laying eggs that have successfully hatched every year for the past four years.
It is the first time the species has been documented reproducing without being fertilised by a male, through a process called parthenogenesis.
Also known as virgin birth, parthenogenesis takes place when the female's egg cells double their genome and then split into two.
One of the egg cells takes on the role of the male sperm and effectively fertilises the other egg. They then merge back together to become an embryo with two female chromosomes.
Commonly witnessed in insects and some species of fish and reptiles, parthenogenesis is rarely seen in sharks and has been observed in other shark species only five times in the past decade.
"It was already known that a shark had done this before, but they were of a totally different lineage than zebra sharks; so, this is very exciting," said David Robinson, one of the marine biologists who has studied Zebe and her offspring.
The first such birth to be documented was in 2001 by a hammerhead shark at a zoo in Nebraska. The pup was killed by a stingray a few days later.
In another example, two white-spotted bamboo sharks were born in an aquarium in Detroit, but mysteriously disappeared from the tank.
Mr Robinson, who works as the assistant aquarium operations manager at the Burj Al Arab, said the pups were the first of Zebe's to have survived. The eldest hatched in 2008.
"These are the first animals that are alive and well, reproduced by parthenogenesis," he said. "And they're so pretty too - that's the best thing."
Mr Robinson and the aquarium's operations manager, Warren Baverstock, first discovered eggs laid by Zebe in 2007.
According to Mr Robinson, eggs are often laid by females even when there is no male present to fertilise them, but are normally discarded.
"We had heard about the other incidents, so we decided to leave the eggs alone to see what would happen, and three months later we discovered embryos inside some of the eggs," he said.
Together with Dr Kamal Khazanehdari, the head of molecular biology and genetics at the Central Veterinary Research Laboratory in Dubai, the process of parthenogenesis was confirmed by DNA analysis.
The test results were published in November's Journal of Fish Biology. The same day, the first pup of this year's batch of eggs hatched.
Zebe, who was caught in the Red Sea, has given birth to 21 pups since 2007, eight of which are still alive. The rest died from various complications, including accidents and infection.
"We've learnt a lot since the first year to find the right conditions for the sharks to develop, such as water temperature and pH [acidity] levels, but hopefully we've developed a fine art by now," he said.
Jonathan Ali Khan, a Dubai-based filmmaker who is producing a documentary called Shark Quest Arabia, says the virgin births are an "amazing example of nature's adaptability".
"Some species react to triggers, which cause survival mechanisms to go into action," he said.
According to Mr Khan, the female shark's captivity "triggered something in its genetic coding", enabling it to reproduce without being fertilised by a male.
Sharks have an XY sex-determination system, like humans - meaning females carry two X chromosomes, whereas males have an X and a Y. The result, in sharks, is that only female offspring can be produced by parthenogenesis, because there is no Y chromosome available.
Whether Zebe's offspring will be able to produce pups of their own is yet to be seen, although Mr Khan doubts it.
"These animals are clones, more than actual offspring, so I'm unsure whether they will actually be able to reproduce. They are born sterile, basically," he said.
But Mr Robinson has higher hopes. When the eldest pup reaches the age where it would normally become sexually mature, they will bring a male zebra shark to the aquarium, and see whether nature takes its course.
This article has been amended to correct the title of Dr Kamal Khazanehdari.
Updated: December 8, 2011 04:00 AM