In a really astounding discovery, it was confirmed that a 2001 birth by a female Hammerhead shark was achieved in the complete absence of males, in a process called parthenogenesis. While at first it was believed that the female shark mated before being captured, and stored the sperm for three years (ew), testing on the resulting offspring showed that their DNA only matched the mother and showed no evidence of a father. This phenomenon, while seen in some other vertebrates like birds and amphibians, has never been demonstrated in such a major vertebrate line as sharks. Mammals are now the only major vertebrate group that has yet to show evidence of parthenogenesis.
Co-author Dr Mahmood Shivji, who led the Guy Harvey Research Institute team, said: “We may have solved a general mystery about shark reproduction – our findings suggest that parthenogenesis is the likely explanation behind the anecdotal but increasing observations of other species of female sharks reproducing successfully in captivity despite not having contact with males.
“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.”
During parthenogenesis, a female organism’s eggs develop without fertilization. This likely evolved as a mechanism to ensure population persistence in the absence of a fertile male (sharks are often solitary for long stretches, and the sea is a big place.) However, the process does have drawbacks, ie the reduction of genetic diversity in populations.
The study is published in Biology Letters. (Chapman et al. 2007. Virgin birth of a hammerhead shark. Biology Letters. doi:10.1098/rsbl.2007.0189)