Study: Female sharks can reproduce without males

DUBLIN, Ireland — Some female sharks can fertilize their own eggs and give birth without sperm from males, according to a new study of the asexual reproduction of a hammerhead in a U.S. zoo.

The joint Northern Ireland-U.S. research, being published today in the Royal Society's peer-reviewed Biology Letter journal, analyzed the DNA of a shark born in 2001 in the Henry Doorly Zoo in Omaha, Neb. The shark was born in a tank with three potential mothers, none of whom had had contact with a male hammerhead for at least three years.

Analysis of the baby shark's DNA found no trace of any chromosomal contribution from a male partner. Shark experts said this was the first confirmed case in a shark of parthenogenesis, which is derived from Greek and means "virgin birth."

The 7-inch-long baby was killed soon after birth, apparently because another fish in the tank attacked it.

Parthenogenesis is common in some insect species and has been observed in certain species of birds, reptiles, amphibians and bony fishes, but the new finding suggests vertebrates' ability to reproduce without sex evolved earlier than scientists had thought. Parthenogenesis hasn't been documented in mammals.

"The findings were really surprising because, as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals," said marine biologist Paulo Prodohl of Queen's University of Belfast, Northern Ireland, a co-author of the report.

Before the study, many shark experts had presumed the Nebraska birth involved a female shark's well-documented ability to store sperm for a long time. Doing this for six months is common, while three years would be exceptional, they agreed.

The lack of paternal DNA in the baby shark ruled out this possibility.

"We were all very skeptical about these reports, about the possibility of a so-called virgin birth in a shark, because sharks have this unusual ability to store sperm for months if not years. So this finding is new and definitely unexpected," said Bob Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Sarasota, Fla., who wasn't involved in the project.

The report's other co-author, Mahmood Shivji, of the Guy Harvey Research Institute in Dania Beach, Fla., said the finding explained growing numbers of reports of male-free shark births in captivity.

Shivji said the research "may have solved a general mystery about shark reproduction," because it suggests sharks can "switch from a sexual to a nonsexual mode of reproduction." But he said this was not necessarily a positive ability because baby sharks produced only by the mother suffer from "reduced genetic diversity."

Genetic diversity makes living creatures better able to adapt to threats.

Material from The Washington Post is included in this report.