'Virgin' shark birth in Virginia
The first time it happened, scientists thought it might be a fluke. A female hammerhead shark residing at a zoo in Omaha, Neb., had not been in contact with male sharks for at least three years and yet experienced a "virgin birth." She delivered a single pup.
But it has happened again, according to today's issue of the Journal of Fish Biology. This time, a blacktip shark, similar to the one pictured above, had spent nearly her entire eight years at either the Virginia Aquarium without any male companionship from her kind. And again, in what some religions might call a miracle, and what science calls "parthenogenesis," she produced a single pup. Using DNA fingerprinting techniques used in human paternity tests, scientists have determined that in this case, as well as the hammerhead in Omaha, the solitary offspring contained no genetic material from a father.
"It's reasonable to assume that female sharks can do this on occasion," said Demian Chapman, a shark scientist with the Institute for Ocean Conservation Science at Stony Brook University in New York. "I'm sure this happens in the wild, but haven't been able to prove it yet. There's no reason that keeping a shark in captivity would cause a fundamental change in the reproductive system."
Sharks have suffered steep declines in all of the world's oceans, either inadvertently caught by fishing nets and hooks or targeted for shark fin soup marketed as a delicacy in China. Some scientists have suggested that this may be a last-ditch way for severely depleted populations to reproduce if their numbers fall so low that males cannot find females.
Yet parthenogenesis, derived from the Greek words for "virgin birth", has limits. First, he said, female sharks can have large litters of young, but in these documented cases only produced one pup. In addition, the offspring have reduced genetic diversity, putting it at a disadvantage in the wild.
Parthenogenesis has been observed in dozens of species, in some birds, amphibians and fish. This asexual reproduction occurs when an egg cell is triggered to develop as an embryo without the addition of any genetic material from a male sperm cell.
Before these recent discoveries, scientists presumed that sharks reproduced exclusively by internal insemination. Indeed, some sharks can store sperm for months, but not long enough to be responsible for these cases.
Sharks are among a group of species with ancient origins dating back 400 million years. (An earlier version of this post erroneously referred to sharks as a species.)
"On the face of it, sharks were the first vertebrates to invent what we call sex, penetrative insemination," Chapman said. "We can learn a lot from studying them."
Chapman is trying to rewrite the book on shark sex, often a brutal affair that involves biting with sharp teeth and sometimes death. "It's taken us a long time to figure out that a female doesn't need a male," Chapman said. "You couldn't blame them for reproducing asexually because the sex is often quite violent."
-- Kenneth R. Weiss
Photo: Blacktop shark. Credit: Matthew D. Potenski, MDP Photography