14 DAY TRIAL // Sharks have many issues of their own in the reproductive domain, from double penis in case of males to intrauterine cannibalism.
Now, another odd trait adds to their reproductive peculiarities: female sharks can give birth without fertilization.
The new mix Northern Ireland-U.S. research investigated the DNA of a hammerhead shark born in 2001 in the Henry Doorly Zoo (Omaha, Nebraska).
The tank in which the shark was born contained just three females of hammerhead sharks, and none had mated for at least three years. Few hours later, the offspring was killed by a stingray in the same tank and when the researchers made the DNA analysis, they discovered no foreign chromosome coming from a male partner.
This is the first shark case of parthenogenesis ("virgin birth" in ancient Greek). Parthenogenesis is a rule in some mite, crustacean and insect species and rarer in reptiles (but some lizard species are completely parthenogenetic) and fish, while no mammal was found to display this. "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," said co-author Paulo Prodohl, marine biologist at Queen's University of Belfast, Northern Ireland. "As is typical with scientists, we doubted our own results and so we did it again, and then a third time using a new technique with new genetic approaches. This confirmed there was no DNA of any male," said Prodohl, an expert in fish genetics.
Before the investigation, many shark experts put the event on the female shark's well known ability to store sperm for months but none considered that the female shark was immature when delivered to the zoo. "This phenomenon has now been demonstrated in all major vertebrate groups except for mammals. Birds do it, reptiles do it, amphibians do it, fishes do it, and now sharks are known to do it," said Bob Hueter, director of the Center for Shark Research at the Mote Marine Laboratory in Sarasota, Florida, not involved in the project.
The analysis "may have solved a general mystery about shark reproduction," showing that sharks can "switch from a sexual to a non-sexual mode of reproduction." said co-author, Mahmood Shivji of the Guy Harvey Research Institute in Dania Beach, Florida.
But offspring produced through parthenogenesis have less genetic diversity, being less fit and more vulnerable to threats like disease and climate change. If this occurs in the ocean as female sharks cannot encounter males due to the rapid shark decline, it would be "an evolutionary dead end that compromises the survival of the species", said Prodohl.
This could be "an evolutionary strategy to keep the population and species going when all else fails. But as a short-term alternative to extinction, it has its benefits," added Prodohl.