Shark bite leads to reproduction mystery

George initially was depressed by the events. But something positive emerged out of the negative.

Since Tidbit hadn't looked pregnant — and there was no reason to think she was pregnant — the pup likely would have been born and immediately eaten by another shark, without aquarium employees ever knowing it had existed.

But Tidbit's death led to George stumbling upon a mystery of nature.

In normal reproduction, an egg is fertilized by sperm, producing an embryo that contains a set of chromosomes with half coming from the mother and half from the father.

In asexual reproduction, an egg splits in two and DNA contributed from the mother doubles, so each resulting egg has a full complement of chromosomes from the female. The eggs then fuse, producing a single embryo with no DNA from a father.

Asexual reproduction is common in some insect species, rarer in reptiles and fish, and has never been documented in mammals. Until now, sharks were not considered likely candidates.

But with sharks, "this is probably something that does happen in aquariums, more often than we realize," said Bob Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Sarasota, Fla.

Asexual reproduction in captivity
He said the phenomenon is coming to light with the joint Northern Ireland-U.S. research that analyzed the DNA of a hammerhead shark born in 2001 in the Henry Doorly Zoo in Omaha, Neb. The study was published in the Royal Society journal Biology Letters on the day before Firchau was bitten.

Asexual reproduction among sharks is more likely to happen in captivity, when there is no other option for reproduction, than in the wild, Hueter said.

Crossbreeding, on the other hand, is not known to happen at all among sharks, said Heather Thomas, aquarist at the John G. Shedd Aquarium in Chicago.

"It's not natural," Thomas said. "If you've got a shark that needs to swim to breathe and cross it with a shark that can lay on the bottom to breathe, what are you going to get? Are you going to get these weird mutations?"

If the pup indeed turns out to be a hybrid, DNA testing should be able to identify the species of the father. The most likely candidate would be a sandbar shark, the most similar shark to a blacktip reef in the aquarium, George said.

While parthenogenesis "is certainly kind of a spiffy, interesting thing," George hopes the tests confirm crossbreeding, since that would be a first among sharks.

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