A female blacktip shark in a Virginia aquarium got pregnant despite the fact that no male blacktip sharks shared her tank in her eight years of residency, researchers say. This is the second documented case of asexual reproduction, or parthenogenesis, among sharks; the first example of a shark "virgin birth" occurred with a hammerhead shark in a Nebraska zoo. The new findings suggest that the previous event wasn't a marine miracle.
"This first case was no fluke," Demian Chapman, a shark scientist and lead author of the second study, said in a statement. "It is quite possible that this is something female sharks of many species can do on occasion" [AP].
Now scientists are debating whether asexual reproduction is a backup method for female sharks that can't find a mate, or whether the pregnancy was a developmental aberration that occurs from time to time. Chapman argues for the first camp:
“The reason this has happened in captivity isn’t because there’s a change in their reproductive biology,” he explains. “It is more likely to happen if female sharks aren’t having enough dates,” he says. “These females did it because they were in captivity and ovulating” [Science News].
But others say that since the shark didn't produce a normal litter of four to six pups, the pregnancy could have been an anomaly.
"The fact that only one shark embryo was formed may suggest that this is more a case of an egg developmental aberration rather than a physiological response to the lack of a mate," said [shark expert Robert] Hueter [National Geographic News].
The pregnancy came to light in a mournful way. The shark, named Tidbit, died when she was tranquilized for a routine trip to the vet, and a subsequent autopsy revealed that she was carrying one shark pup that was nearly ready to be born. A genetic test revealed that the pup contained no DNA from a male. Chapman explains
that during egg production, a female shark produces four cells. Only one of these becomes the egg. Another of the four is called "the sister polar body," which is a close genetic match to the egg. During parthenogenesis, according to Chapman, "the sister polar body fuses with the egg and injects its chromosomes into it. Therefore, it acts like a sperm and triggers the development of an embryo," he said [Discovery News].
The finding was written up in the Journal of Fish Biology [subscription required], and researchers say they hope to answer some of the remaining questions about the process soon; they're currently investigating another reported case of parthenogenesis in a white spotted bamboo shark in Chicago. While the researchers are excited to learn more about the shark's asexual trick, they caution that the ability shouldn't be counted on to save shark populations, which are plummeting around the globe. Parthenogenesis reduces genetic diversity, and the resultant offspring may have more risk of genetic defects, Chapman says.
“This is not the great white hope for the shark population.... This is not a solution to overfishing,” he says [Science News]
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Image: Matthew D. Potenski
