October 10, 2008

Scientists Confirm Virgin Shark Birth

The second-known instance of "virgin birth" in a shark has been confirmed through DNA testing, scientists said.

A pup carried by a female Atlantic blacktip shark named Tidbit in the Virginia Aquarium & Marine Science Center contained no genetic material from a male.

Beth Firchau, the curator of fish at the Virginia Aquarium & Marine Science Center in Virginia Beach said Tidbit was born in the wild and lived there for eight years with no males of the same species.

The first documented case of parthenogenesis, or asexual reproduction, among sharks involved a pup born to a hammerhead at an Omaha, Neb., zoo.

Demian Chapman, a shark scientist and lead author of the second study, said the first case was no fluke. "It is quite possible that this is something female sharks of many species can do on occasion."

While some shark species can produce litters numbering in the dozen or more, the aquarium sharks that reproduced without mates each carried only one pup. The scientists cautioned that the rare asexual births should not be viewed as a possible solution to declining global shark populations.

"It is very unlikely that a small number of female survivors could build their numbers up very quickly by undergoing virgin birth," Chapman said.

Firchau said the 5-foot (1.5-meter) shark died after being removed from the tank for a veterinary examination, and a subsequent necropsy revealed that Tidbit was carrying a fully developed shark pup nearly ready to be born.

Tidbit's pup was nearly full term, and likely would have been quickly eaten by "really big sand tiger sharks" that were in the tank, Chapman said in a telephone interview from Florida.

Chapman said parthenogenesis has also been documented in Komodo dragons, snakes, birds, fish and amphibians.

In the type of parthenogenesis seen in sharks, the mother's chromosomes split during egg development.

Chapman said how the sharks do it is unclear. "They may use a hormone to trigger eggs to develop in this manner in the absence of males. Or perhaps if eggs remain unfertilized with no males around, a certain fraction develop into embryos."

"It's a finding that kind of rewrites the textbooks a little," Chapman said. "It just goes to show how the ocean keeps its secrets very well. And the sharks in particular."

"It is possible that parthenogenesis could become more common in these sharks if population densities become so low that females have trouble finding mates," said Mahmood Shivji, one of the scientists and director of the Guy Harvey Research Institute at Nova Southeastern University in Florida.


On the Net: