SHARK PREGNANCY BAFFLES AQUARIUM
NORFOLK, Virginia – 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 Tidbet 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 and 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 Tdibit 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. (AP)
In 2001 a bonnethead, a type of small hammerhead shark was thought to have produced a pup, born live on the 14th December 2001, in captivity in a tank containing three female hammerheads but no males; thought to be through parthenogenic means at Henry Doorly Zoo in Nebraska. The shark pup was apparently killed by a stingray within three days of birth.[16] The investigation of the birth was conducted by the research team from Queen's University Belfast, the Southeastern University in Florida, and Henry Doorly Zoo itself and concluded after DNA testing that the reproduction took place under parthenogenic circumstances. The testing showed the pup's DNA matched only one female that lived in the tank, and that no male DNA was present in the pup. The pup was not a twin or clone of the mother, but rather contained only half her DNA ("automictic parthenogenesis"). The type of reproduction exhibited had been seen before in bony fish but never in cartilaginous fish such as sharks.In 2002, two white-spotted bamboo sharks were born at the Belle Isle Aquarium in Detroit after hatching 15 weeks after laying. The birth baffled experts as the mother shared an aquarium with only one other female shark. The female bamboo sharks had laid eggs in the past. This is not unexpected, as many animals will lay infertile eggs even if there is not a male to mate with. Normally, the eggs are assumed to be infertile and are discarded. This batch was left alone by the curator as he had heard about the previous birth in 2001 in Nebraska.
Other possibilities had been considered for the birth of the Detroit bamboo sharks included thoughts that the sharks had been fertilized by a male and stored the sperm for a period of time and also the possibility that the Belle Isle bamboo shark is a hermaphrodite, harboring both male and female sex organs, and capable of fertilizing its own eggs.
The repercussions of self fertilization in sharks, which reduces the genetic diversity of the offspring, is a matter of concern for shark experts, taking into consideration conservation management strategies for this species, particularly in areas where there may be a shortage of males due to fishing or environmental pressures.
Unlike Komodo dragons, which have a WZ chromosome system and produce male (ZZ) offspring by parthenogenesis, sharks have an XY chromosome system, so they produce only female (XX) offspring by parthenogenesis. As a result, sharks cannot restore a depleted male population through parthenogenesis, so an all-female population must come in contact with an outside male before normal sexual reproduction can resume.
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