The discovery has resulted in the first scientific report of parthenogenesis - or 'virgin birth' - in sharks, leaving mammals as the only major vertebrate group where this form of reproduction has never been recorded.

The researchers discovered that the rare form of reproduction had taken place when they investigated the surprise birth of a baby hammerhead shark in an aquarium at the Henry Doorly Zoo in Nebraska in December 2001.

None of the three sharks in the tank - all caught as babies - had been exposed to any male hammerheads in the three years since their captivity.

Using DNA profiling techniques to examine the genetic make-up of the baby hammerhead and three potential mothers, the researchers, made up of a team from the zoo, Queen's University Belfast and the Guy Harvey Research Institute at Nova Southeastern University in Florida, found the infant had no DNA of male origin - eliminating those alternatives.

Dr Paulo Prodohl, head of the Queen's research team, said: "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.

"The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate where this form of reproduction has not been seen."

Parthenogenesis - where females give birth to fully formed young without their eggs being fertilised by a male - has very occasionally been seen in some vertebrate groups such as birds, reptiles and amphibians, but has never before been seen in major vertebrate lines such as mammals or sharks.

The most likely form of parthenogenesis leads to less genetic diversity in the infant than the mother, leading to fears that genetic diversity could be eroding in shark populations if females have difficulty finding mates.