The female, held at Henry Doorly Zoo in Nebraska, gave birth to the pup in 2001 - but only now, after genetic analysis of the pup's tissues, have scientists been able to state categorically that the pup was born with only maternal and no paternal DNA.
Unfortunately, the pup was killed by a sting ray in the aquarium tank before husbandry staff could get to it. But it was still possible to preserve its remains.
At the time of the pup's birth, it was thought that the mother may have mated while still in the wild, as it is known that the females can hold sperm for a period of time before embryos develop. But the mother had been in captivity for three years when the pup arrived, making this unlikely.
Scientists suspected what is now proven - that parthenogenic reproduction, in which an egg develops into an embryo asexually, had occurred. Parthenogenesis has been observed before in bony fish, but not in cartilaginous sharks.
The research team was made up of scientists from the zoo, Florida's South-Eastern University, and from Queen's University in Belfast, Northern Ireland.
They surmised that parthenogenesis may be occurring also in the wild, if females have difficulty in finding a mate because of overfishing or other pressures on populations.
Parthenogenesis may keep the species going - but, warned the researchers, it is not a particularly healthy form of reproduction as it eliminates genetic diversity, important in enabling species to adapt to changing habitats or, say, new diseases.
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