Sharks in an Italian aquarium keep having 'virgin birth' after years without males

Captive female sharks at an aquarium in Italy keep reproducing on their own with no males involved, indicating that it could be a vital survival mechanism, according to a new study.

In the study published July 26 in the journal Scientific Reports, the scientists documented repeated asexual reproduction in two common smooth hound sharks (Mustelus mustelus), which are classified as an endangered species by the International Union for Conservation of Nature. The sharks are usually found in the Mediterranean Sea and other warm waters but are threatened by illegal fishing.

The two female sharks are in captivity at Cala Gonone Aquarium in Italy and have spent the last 14 years away from males. Since 2020, both females have reproduced asexually, which indicates facultative parthenogenesis.

Parthenogenesis means "virgin birth," in Greek and describes when an egg develops into an offspring without fertilization. While uncommon in vertebrates, this phenomenon has been observed in reptiles such as crocodiles and water snakes, amphibians and some fish.

Related: 8 animals that have virgin births

However, this is the first documented case of facultative parthenogenesis — the ability to reproduce both sexually and asexually — in this shark species. The researchers found that the two females could reproduce parthenogenetically roughly once a year. That rules out the possibility of sexual reproduction because of stored sperm, according to the researchers.

"Remarkably, this finding reveals that parthenogenesis can occur annually in these sharks, alternating between two females, and conclusively excludes long-term sperm storage as a cause," the authors wrote in the study.