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A smooth-hound shark in an aquarium
A lack of mate could be one reason why a shark might reproduce asexuallyImage: Stefan Sauer/dpa/picture alliance

How does a shark reproduce without a mate?

Louisa Wright
September 2, 2021

The news that a female shark at an Italian aquarium gave birth without having mated with a male first was widely reported. But how rare is a "virgin birth" in sharks?

https://p.dw.com/p/3zmur

Scientists at the Cala Gonone Aquarium on the Italian island of Sardinia say a female smooth-hound shark that's been living in an all-female shark tank for 10 years recently gave birth to a baby shark.

The aquarium's press team told DW that they are currently waiting for a DNA analysis to confirm that what happened is a case of parthenogenesis.

To procreate, most species require an egg to be fertilized by a sperm. That's the case with sharks, too. But some animals can produce offspring all by themselves. This is called parthenogenesis.

The term comes from the Greek words parthenos, meaning "virgin," and genesis, meaning "origin."

The case in Italy could be the first time this "immaculate conception" has occurred in smooth-hound sharks, at least in captivity.

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It isn't the first time parthenogenesis has been seen in sharks , and the process has been observed in a number of other shark species.

But scientists still don't know how often it happens, says Kevin Feldheim, a researcher at the Field Museum in Chicago, who researches the mating habits of sharks.

"We don't know how common it is and the handful of cases we have seen have mostly taken place in an aquarium setting," Feldheim told DW.

One study from the Field Museum discovered parthenogenesis in a wild population of smalltooth sawfish, a type of ray. This was the first time a vertebrate (animals with backbones inside their body), which usually reproduces the conventional way with a mate, was found to reproduce asexually in the wild, Feldheim said.

Where do the babies come from?

In sharks, asexual reproduction usually happens via a process called "automictic parthenogenesis," explained Feldheim. During egg development, one egg is produced along with three other products called polar bodies.

Usually these polar bodies are simply reabsorbed by the female. Parthenogenesis occurs when one of the polar bodies has the same amount of genetic material as the egg and fertilizes it.

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Lack of males could be a trigger

In 2017, Australian scientists published a study in the journal Nature that found two female zebra sharks produced pups on their own ― no men involved.

The year after being separated from her male mate, the older female shark didn't lay any eggs. The following year her daughter from her previous mate was added to the tank. Two years later, the mother had three pups on her own, while her daughter had one.

Scientists think the lack of a mate could be a cause for asexual reproduction.

"We think that being without a male certainly triggers parthenogenesis," said Feldheim, "but beyond that, we don't know the mechanism."

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