A female white spotted bamboo shark at the Belle Isle Aquarium in Detroit surprised zookeepers in July by giving birth to two babies. Why the surprise? It was a virgin birth: She hadn’t been near a male for six years.
The mother, who has been housed in a tank with a female brown banded bamboo shark for the last six years, laid a clutch of eggs in April.
That isn’t particularly unusual, said Doug Sweet, curator of fishes at the Belle Isle Aquarium. Many animals will lay eggs even without a male to mate with, but the eggs are assumed to be infertile and are discarded.
Up until now it’s been thought that all shark species use internal fertilization through copulation to produce their young. Once fertilized, many shark species birth their young live; others, like the white spotted bamboo shark, lay eggs. Both the females at Belle Isle had laid eggs in the past, despite their celibacy.
But instead of throwing them out as usual, Sweet left the eggs in the tank for a while because he had heard of a bonnethead shark at the Henry Doorly Zoo in Omaha, Nebraska, thought to have given a virgin birth last year. He eventually transferred the eggs to a separate tank, and 15 weeks after they’d been laid, the eggs hatched, and the mystery of the virgin birth was repeated.
Rethinking Shark Reproduction
The births have raised questions among scientists as to whether sharks may be able to reproduce parthenogenetically, a mode of reproduction in which the egg is not fertilized. These so-called virgin births are common in invertebrates like snails, but are unusual in higher vertebrates.
“Parthenogenesis has been documented in many reptiles,” said Sweet. “There are at least five or six species of snakes, and it’s been known in salamanders, lizards, and even a breed of turkeys. But any way you look at it, this is strange.”
There are possibilities other than parthenogenesis. The Belle Isle white spotted bamboo shark may have been fertilized by a male at a very young age. However, although there have been some random reports of shark species storing sperm for a couple of months or more, six years is a long time, and Sweet thinks in this case it’s extremely unlikely.
“We received both of these bamboo sharks from hobbyists, who don’t typically keep breeding groups; for one thing their tanks aren’t big enough. Usually hobbyists can only buy one egg here and there, and the eggs are typically imported as fertilized eggs. Both the hobbyists who donated these sharks only had one individual. I’m almost dead sure we can rule out long-term sperm storage.”
A third possibility is that the Belle Isle bamboo shark is a hermaphrodite, harboring both male and female sex organs, and capable of fertilizing its own eggs.
“Fish are known to have the only self-fertilizing hermaphrodite vertebrate species, so I don’t rule it out, but again, I think it’s unlikely,” said Sweet.
Genetic testing should provide the answers, but it won’t be soon.
The offspring would not be a true clone, said Sweet. “There is some exchange at the chromosomal level, and the genes are reshuffled very slightly, but the DNA match between mother and child should hopefully be very close,” he said.
Rick Brenneman, a geneticist at the Henry Doorly Zoo and colleagues have been studying the bonnethead shark suspected to be the result of parthenogenesis since last December.
“To date, finding the genetic markers that will identify mom and dad in sharks in general has been very difficult,” he said. It’s even more difficult when you suspect there may not be a dad.
The bonnethead shark and bamboo shark are from different families; the bonnethead suspected of a virgin birth gave birth to a live pup, rather than laying eggs as the bamboo shark had. But studying both will almost certainly provide scientists with insights into the genetic make-up of sharks.
“This will be a long term project requiring tenacity and a great deal of effort, but ultimately it should enable us to better understand what it is that allows parthenogenesis, if in fact this is what is occurring, and what the genetic trigger might be,” said Brenneman. The genetics lab at Omaha’s Henry Doorly Zoo will also conduct the genetic testing of the Belle Isle bamboo sharks.
In the meantime, what about all those eggs that were being thrown away? The two sharks at the Belle Isle Aquarium typically go on an egg-laying spree once or twice a year, laying a clutch of four to six eggs.
“We’re definitely holding them now and incubating them,” said Sweet. “If you have one parthenogenetic shark, you may as well have a whole tank of them.”