In 2001, zoo workers at the Henry Doorly Zoo in Omaha, Nebraska, were surprised to find a baby shark in a tank with only female sharks. For three years, there had been no male shark in the tank, but a new pup was swimming. Scientists were confused. They first thought the female sharks might have stored sperm from a previous mating. Some shark species can store sperm for over three years. However, DNA tests showed that the baby shark had only its mother's genetic material. There was no father involved.
This process is called parthenogenesis. It happens when an egg grows into a new organism without sperm. In sharks, this occurs through a process where the egg combines with a similar cell from the same mother. Scientists knew this happened in other animals but had never seen it in sharks before.
Experts thought this was only a last resort for female sharks that could not find a mate. However, a zebra shark named Leonie in Australia changed this idea. She had previously mated with a male, but after he was moved, she gave birth to pups without a male. DNA tests confirmed she had reproduced asexually. Later, another female zebra shark in Chicago reproduced asexually even though she lived with male sharks.
These discoveries are interesting, but they are also worrying for sharks. Sharks born this way have less genetic diversity, which can lead to health problems and shorter lives. This lack of variation makes them weaker. Researchers have also found baby sharks born this way in the wild. With fewer sharks in the ocean due to overfishing, more sharks might have to reproduce this way, making the surviving populations less healthy and less able to adapt to changes.