The female shark may have just beaten Rosie the Riveter for the title of Ultimate Feminist Empowerment Symbol. Published today in the Journal of Fish Biology, scientists at Stony Brook University have confirmed the second-ever case of a virgin shark birth, proving that—literally—“We can do it!” (Okay, so by “we,” I mean some of our sister amphibians, reptiles, birds, and bony fish. But hey, sisterhood defies the boundaries of the biological class system.)
Eight-year-old Tidbit, a blacktip shark, had lived in captivity at the Virginia Aquarium since shortly after her birth when caretakers found her pregnant during an autopsy. Tidbit’s death was unexpected, but more surprising was her condition; there had been no male sharks in the tank the entire eight years she had lived there.
Image courtesy Matthew D. Potenski
Whether a higher power was involved or not, most would call this a pretty miraculous conception. Scientists, however, have a different name for it: parthenogenesis.
In the case of Tidbit, as well as the other known virgin shark birth (that one was a hammerhead), the exact term is automictic parthenogenesis. Let’s break it down: in parthenogenesis, reproduction occurs without male fertilization of the seed or egg. Automictic parthenogenesis happens when an egg cell essentially fertilizes itself.
How can that be? In normal reproduction, the cell that is to become the egg splits into four separate cells and only one develops into an egg cell, while the other three are discarded. With automictic parthenogenesis, one of the three other cells acts as the second set of chromosomes and binds with the egg. Both sets of chromosomes come from the mother and are near replicas of each other. The end result in this case was Tidbit conceiving a half-clone daughter… Lil’ Bit.
The shark pup did not survive its mother’s death, but Demian Chapman, a scientist with Stony Brook’s new Institute for Ocean Conservation Science who helped confirm the case, reported no significant physical differences between Tidbit’s pup and a normal shark offspring. In other words, said Chapman, there weren’t twelve fins or three eyes (darn).
So is this a new solution to overfishing? Scientists are quick to caution that this phenomenon should not encourage fishermen to get all spear-happy. Parthenogenesis often results in a smaller litter than usual: Both Tidbit and the hammerhead produced only one offspring while a normal litter for sharks can be anywhere from a few to a hundred. Besides, the fact that the baby shark has only one source of DNA puts it at a disadvantage in the wild, where genetic diversity is king.
Past reported cases of virgin shark births were thought to be the result of long-term sperm storage, from when the shark was in the wild. Now it seems that those, too, could be instances of parthenogenesis. Chapman is currently working with Kevin Feldheim of the Field Museum in Chicago to analyze the DNA of another shark offspring to see if it, too, was a virgin birth.
Otherwise, other virgin births have been seen with birds, amphibians, and reptiles, but are thought to be impossible for mammals. This is because mammalian reproduction requires certain chromosomes from the father to “turn on” specific traits in the offspring, and vice versa with the mother.
Sigh. Looks like we do still need men for something.
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