June 25, 2007
Shark Bite Leads to Reproduction Mystery
NORFOLK, Va. -- Veterinarian Bob George sliced open the dead shark and saw the outline of a fish.
No surprise there, since sharks digest their food slowly.Then George realized he wasn't looking at the stomach of the blacktip reef shark, but at her uterus. In it was a perfectly formed, 10-inch-long shark pup that was almost ready to be born.
George was dumbfounded.
He had been examining the shark, Tidbit, to figure out why she reacted badly to routine sedatives during a physical and died, hours after biting an aquarium curator on the shin. Now there was a bigger mystery: How did Tidbit get pregnant?
"We must have had hanky panky" in the shark tank, he thought.
But sharks only breed with sharks of the same species, and there were no male blacktip reef sharks at the Virginia Aquarium & Marine Science Center in Virginia Beach.
Could Tidbit have defied nature, resulting in the first known shark hybrid?
The other possibility was that Tidbit had conceived without needing a male at all.
A recent study had documented the first confirmed case of asexual reproduction, or parthenogenesis, among sharks: a pup born at a Nebraska zoo came from an egg that developed in a female shark without sperm from a male.
One of the scientists who worked on that study contacted the aquarium, which sent him tissue samples from Tidbit and her pup for testing. If the pup's DNA turns out to contain no contribution from a male shark, this would be the second known case of shark parthenogenesis.
George hopes to receive a preliminary report soon, but conclusive results could take months.
Tidbit had lived at the aquarium for most of her 10 years, swimming with other sharks in a 300,000-gallon tank.
The sharks get yearly checkups. On May 24, workers guided the 5-foot, 94-pound Tidbit from the main aquarium into a smaller corral to be examined out of public view.
Blacktip reef sharks are sensitive to change, so it was standard procedure to give Tidbit a sedative. This time, Tidbit went under the sedation too deeply - maybe because of a combination of the unknown pregnancy and the stress of being handled and of having recently been bitten by another shark, George said.
George and Beth Firchau, the curator of fishes, massaged Tidbit's tail to get her blood flowing and gave her a stimulant to help her breathe.
The shark swam away, bumped into a wall, headed back toward Firchau and clamped onto her left shin. Whether Tidbit meant to attack Firchau or just collided into her and snapped reflexively is hard to know.
The pain didn't hit Firchau right away.
"Oh, you're not supposed to do that. That was weird," she thought as she felt the shark tug on her leg.
Members of the shark physicals team pulled Firchau out of the tank and began administering first aid. She credits their swift reaction with saving her life.
Firchau was taken to a hospital to get stitches while George and other team members tried to revive Tidbit. The shark rallied a couple times but died about 12 hours later.
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.
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.
On the Net:
Virginia Aquarium: http://www.vmsm.com