Virgin Birth: Shark Expert Comments On Parthenogenesis

Birds do it, bees do it, and now there is evidence that female sharks are able to do it on their own -- without the contribution of male DNA. A recent report from a team of American and Irish researchers has concluded that the mysterious appearance in 2001 of an infant female bonnethead shark at Omaha’s Henry Doorly Zoo in a tank that held only two adult female sharks was the result of parthenogenesis (Gr. virgin birth.) Parthenogenic reproduction takes place without fertilization by a male through the process of cell division, when the mother’s egg fuses with a degenerative cell called a polar body, producing a new individual.

What does this mean for lonely-heart sharks on a Saturday night, or for that matter, the evolution of the species? “Parthenogenesis appears to be a rare phenomenon in sharks, and it is unlikely to have an impact on the evolution of a particular lineage,” said Saint Joseph’s University Professor of Biology Eileen Grogan, Ph.D., a noted expert in shark evolution and research associate at both the Academy of Natural Sciences and the Carnegie Museum of Natural History. “However, one might conceive that this mode of reproduction could have a significant impact on small populations because there is less genetic diversity in small, isolated populations.”

While parthenogenesis can ensure the short-term survival of the species, for the long term it is advantageous to keep male DNA in the mix. “The newborn shark derived from this phenomenon would have only half the genetic diversity of the sexually reproduced form because it is based entirely on the mother’s genome,” said Dr. Grogan. “In terms of evolution, it is preferable to have a greater diversity of genes, because that offspring is more likely to have ‘what it takes’ to survive.”

The collective sigh of relief you just heard came from male sharks relieved to learn that they are still necessary to the survival of the species. Male and female sharks that migrate along shorelines this summer will follow their biologic imperative by reproducing the old fashioned way, and may even choose to come into shallower waters to mate or give birth. “Most people don’t realize just how close -- normally and regularly -- sharks come to the beaches as they migrate and follow whatever they are feeding on. Patterns of water flow, temperature of the water, and where typical prey are found can help predict where sharks might be found in local waters,” added Dr. Grogan.

Sharks are fascinating, magnificent animals that draw crowds at aquariums and zoos. “Some sharks are quite reserved, and the smallest shark, the pygmy, might fit in your hand. The largest sharks, the whale and basking sharks, are filter feeders; others prefer grazing on shelled animals rather than acting as an apex predator,” noted Dr. Grogan.

So this summer as we head to the shore for fun in the surf, Dr. Grogan advises us to remember that we are invading their world, and that sharks vary in their aggressiveness. To die-hard surfers and others in search of the perfect wave: “Logic informs us to stay out of waters in which sharks have been recently sighted. Research has shown that shark attacks are more likely at certain times of the day, based on the feeding regime of the animal. In particular, dusk and dawn have been associated with a higher incidence of attacks, so it is recommended that people stay out of the water at these times. If one is in the ocean with sharks nearby, do not thrash around, as this sort of activity can attract the animals much as a struggling fish could attract its predator.

“If someone encounters a shark in its natural environment, it needs to be respected,” she added. “Hopefully it can be appreciated from a distance, but if it is seen in a bathing area, report the sighting to the beach patrol or other local authorities. Even if the animal turns out to be relatively harmless, it is important to confirm the type of shark visiting the area and to take appropriate precautions. If a shark aggressively approaches someone, its snout is a very sensitive sensory area; one should try to hit the snout to fend off an attack.

“Of course, the buddy system is critical for recreational enthusiasts going into or on the water. If something unfortunate happens, it is crucial to have someone to help you and to seek emergency support,” she continued. “Of boaters, surfers, divers, and snorkelers in general, the surfer may be at highest risk since they are more likely to be mistaken for prey.”