Female sharks can reproduce without having sex, scientists in the North revealed today. The discovery could have solved a mystery which has baffled experts studying the species in captivity.
An international team of researchers based in Northern Ireland and the United States made the breakthrough after a hammerhead shark gave birth without mating. No traces of any paternal DNA were detected in the offspring.
The researchers, from Queen's University Belfast, the Guy Harvey Research Institute at Nova Southeastern University in Florida, and Henry Doorly Zoo in Nebraska, found evidence that sharks can reproduce by parthenogenesis - the development of an egg without fertilisation.
It is the first scientific report of such a development.
Dr Paulo Prodohl, of Queen's School of Biological Sciences, headed the university's research team and co-wrote the study said: "The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating.
"The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate group where this form of reproduction has not been seen."
Researchers began their investigation following the unexpected birth of a hammerhead pup in an aquarium at the Henry Doorly Zoo in December 2001.
Astonishingly, none of the three potential mothers in the tank - all caught in Florida waters as pups - had been exposed to any male sharks during their three years in captivity.
At first, it was thought the mother mated before capture, and then somehow stored the sperm for over three years before finally fertilising her eggs in the aquarium.
An alternative theory was that the hammerhead female had mated with a different species in the tank.
Using DNA profiling techniques to examine the genetic makeup of the new-born hammerhead and the three candidate mothers, the researchers were able to identify which had given birth.
But its DNA only matched up with the mother's - there was none of any male origin. This eliminated the possibilities of earlier mating or hybridisation.
Females of only very few vertebrate species can give birth to fully formed young without requiring their eggs to be first fertilised by a male's sperm, the researchers stressed.
This unusual reproductive ability is only very occasionally seen in some vertebrate groups such as birds, reptiles and amphibians. But it has never before been seen in other major vertebrate lines such as mammals or sharks.
Co-author Dr Mahmood Shivji, who led the Guy Harvey Research Institute team, said: "It now appears that at least some female sharks can switch from a sexual to a non-sexual mode of reproduction in the absence of males."
The team established the most likely form of asexual reproduction that had occurred was a specific type called automictic parthenogenesis, which leads to less genetic diversity in the offspring compared to even the mother.