Virgin shark gives birth without a male, baffling researchers

The activity was reported in zebra sharks at Shedd Aquarium in Chicago.
Jijo Malayil
An adult zebra shark at Shedd Aquarium's Wild Reef exhibit.
An adult zebra shark at Shedd Aquarium's Wild Reef exhibit.

Shedd Aquarium/Brenna Hernandez 

A study has brought to light interesting facets regarding virgin births in sharks using a process known as parthenogenesis, which lets some female animals fertilize an egg with their genetic material. In 2021, a smooth-hound shark gave birth to a baby shark without male interference at Cala Gonone Aquarium in Sardinia, Italy. 

Asexual means for reproduction are typical for animals like starfish, deep-sea worms, and stick insects, but it's rarely observed in vertebrates. The miracle childbirths in sharks without male interactions have baffled scientists for some time. Researchers had assumed that vertebrates that usually reproduce sexually turn to parthenogenesis as a "last-ditch effort at reproduction when there aren’t enough mates to go around," according to a news release

The study, published in the Journal of Fish Biology, focused on endangered zebra sharks at Shedd Aquarium in Chicago. Contrary to earlier understandings, the study revealed an example of a female zebra shark reproducing via parthenogenesis, even though there were healthy, reproductive males in the same enclosure.

Virgin births in spite of the presence of healthy males

The efforts started in 2004 with genetic testing of zebra sharks at the aquarium to confirm which of the sharks were the parents of the offsprings present. By doing so, the researchers could decide on "future breeding efforts to maintain maximum genetic diversity," said Lise Watson, assistant director at Shedd Aquarium and an author of the study. 

Genetic analysis of offspring born to these sharks in 2018 revealed that they didn't match any mature males in the aquarium. "In addition to having genetic markers in common with their mother but none of the potential fathers, the pups had identical homozygous copies of some alleles," said Kevin Feldheim, a researcher at Chicago’s Field Museum, the study’s co-author. According to the team, homozygous meant the presence of both DNA strands from one parent, their mother, rather than from two different parents. 

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This was the second case where sharks were born by parthenogenesis even when healthy mates were available. The first incident was reported at the Aquarium of Pacific in Chicago.

Such offsprings tend to have a shorter lifespan

Studies have shown that offsprings tend to have short life expectancies due to their susceptibility to rare recessive genetic conditions. The pubs born during this study only survived for a few months.

"This study is just the beginning of our understanding of the occurrence of this genetic phenomenon in zebra sharks,” said Watson. Researchers say such efforts will help in the conservation of species like zebra sharks, which are nearly extinct in some parts of the world.

Abstract 

Parthenogenesis has been observed in several elasmobranch species, primarily in public aquaria. The majority of cases of parthenogenesis have occurred either when females were held without males or once a male was removed from a female's habitat. Here we report a second instance of parthenogenesis in a zebra shark female that was housed with conspecific mature males. This study calls into question the conditions under which elasmobranch females undergo parthenogenesis.