Photo: amanda.pippin (flickr)
In 2001 a hammerhead shark was born at a zoo in Nebraska.
The birth might not have been newsworthy, except that baby appeared in a tank with three female but no male sharks. The birth baffled scientists since the three females had been in captivity for over three years, and had been captured before they had reached sexual maturity.
Scientists wondered if one of the females could have stored sperm from an encounter with a male before her capture. However, the length of their captivity, and their young age when captured made this hypothesis unlikely.
A team of researchers analyzed the baby shark’s genes, and were surprised to find that the genes of the baby perfectly matched those of one of the females. The paternity test found no genetic sign of a father at all.
The development of an embryo from an egg without the contribution of male genes is known as parthenogenesis. Since the discovery of shark parthenogenesis, two more virgin-births in captive sharks have occurred. A reef shark in Hungary, and a bamboo shark in Detroit both had produced offspring under the same circumstances. Scientists suspect these mystery births were also from parthenogenesis.
The ability of females to produce an offspring without a male could have implications for shark conservation, as sharks are threatened by over-fishing.
Although we don’t know if wild sharks ever reproduce this way, it may be that females could resort to parthenogenesis if populations became critically low, making mates hard to find. However, over time a population of offspring produced this way will have reduced genetic variation, which limits a populations’ chance of survival when environmental conditions change.