No Sex Please, We're Female Sharks

An international team of researchers from Queen’s University Belfast, the Guy Harvey Research Institute at Nova Southeastern University in Florida and the Henry Doorly Zoo in Nebraska has found evidence that sharks can reproduce asexually by an unusual method known as “parthenogenesis”. This is the first scientific report of asexual reproduction in sharks.

Head of the Queen’s research team and study co-author, Dr Paulo Prodöhl, from the School of Biological Sciences, said: “The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals.”

“The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate group where this form of reproduction has not been seen.”

The long-term study was prompted by the unexpected birth of a baby hammerhead shark in an aquarium at the Henry Doorly Zoo in December 2001. The astonishing thing about the birth was that none of the three candidate mother hammerheads in the tank, all of whom been caught in Florida waters as babies themselves, had been exposed to any male hammerhead sharks for the three years since their captivity.

Initial thinking was that the mother had mated with a male before capture, and then somehow stored the sperm for over three years before finally fertilizing her eggs in the aquarium, or alternatively, perhaps the hammerhead female had mated with a male shark of a different species that was in the tank.

By using DNA profiling techniques to examine the genetic makeup of the baby hammerhead and the three candidate mothers, the researchers were able to identify which of the three females was the actual mother. Surprisingly, the baby shark’s DNA only matched up with the mother’s – there was no DNA of male origin in the baby shark! This finding eliminated the possibilities of earlier mating with a male hammerhead followed by sperm storage, or hybridization with another shark species in the tank.

Females of only very few vertebrate species can give birth to fully formed young without requiring their eggs to be first fertilized by a male’s sperm. This unusual reproductive ability, known as “parthenogenesis”, is only very occasionally seen in some vertebrate groups such as birds, reptiles and amphibians. However, it has never before been seen in other major vertebrate lines such as mammals or sharks.

Co-author Dr Mahmood Shivji, who led the Guy Harvey Research Institute team, said: “We may have solved a general mystery about shark reproduction – our findings suggest that parthenogenesis is the likely explanation behind the anecdotal but increasing observations of other species of female sharks reproducing successfully in captivity despite not having contact with males.

“It now appears that at least some female sharks can switch from a sexual to a non-sexual mode of reproduction in the absence of males. Unfortunately, this occurrence is not benign because it results in reduced genetic diversity in the offspring since there is no new genetic variation introduced from the paternal side.”

The researchers found that the most likely form of asexual reproduction that had occurred was a specific type called “automictic parthenogenesis” that leads to less genetic diversity in the offspring compared to even the mother.

“During this process the unfertilized egg, which contains about half of the mother’s genetic diversity, is activated to behave as a normal fertilized egg by a small, genetically nearly-identical cell known as the sister polar body. The resulting baby shark therefore gets a double-dose of genetic disadvantage”, says lead author Dr Demian Chapman, who took part in the study while he was a graduate student at the Guy Harvey Research Institute.

The discovery raises concerns about the genetic and reproductive health of dwindling shark populations.

Now Head of Shark Research at the Pew institute for Ocean Science, Dr Chapman continued: “Not only does it experience reduced genetic diversity because it has no father, but around half of the genetic variation present in the mother is not passed on to the offspring.”

“Female sharks might reproduce like this more often when they have difficulty finding mates at low population densities. This could hasten the erosion of population genetic diversity and perpetuate the production of genetically disadvantaged offspring.”

Dr Prodöhl added: “The bottom line is that we have to include a whole new dimension to our thinking about shark reproduction and its influence on population health. If the ability of female sharks to switch from sexual to asexual reproduction is widespread under conditions of low encounter rates with males, incorporating this new information into our management and conservation efforts will be imperative to prevent further declines in genetic diversity for an intensely, and many instances over exploited, group of fishes.”

The research team’s paper was published in the Royal Society journal, Biology Letters on 23 May 2007.