Hail Mary: Chicago Shark has Virgin Birth

A baby shark has been born in a Chicago aquarium asexually, with her mother having not mated with any of the males present in her tank.

A study published in the Journal of Fish Biology in November revealed a female endangered zebra shark at Shedd Aquarium in Chicago gave birth to offspring without having mated, despite the presence of healthy males of the same species in the same tank.

Asexual reproduction—creating offspring that are genetically identical to the parent without mating—is common in many invertebrate organisms, but is less frequently seen in vertebrates like sharks unless there is an absence of partners to reproduce sexually with, as a kind of biological "Hail Mary."

Asexual reproduction is a common method of making new individuals across all of life: bacteria, plants, fungi and less complex animals frequently use this method of reproduction. Virgin birth, or parthenogenesis, is the process in which organisms, usually vertebrates (animals with backbones), reproduce asexually, by giving birth to young using an unfertilized egg cell in a female animal. All of these offspring are female.

The researchers found out that the Chicago zebra sharks were giving birth via parthenogenesis after genetically testing the offspring, which was originally meant to help the successful breeding of the endangered species. However, one clutch of eggs confused the scientists, as it only contained the genetic information of one of the sharks, not two.

"These pups didn't match any of the mature males that were in the enclosure. But they did match the female that laid the eggs," Kevin Feldheim, a researcher at Chicago's Field Museum and the lead author of the study, said in a statement.

Another case of a shark "virgin birth" occurred in an Italian aquarium in 2021, where a female smoothhound shark gave birth to a baby despite having spent 10 years sharing her tank with only one other female shark.

Parthenogenesis is surprisingly common. Various types of reptile, including species of whiptails, geckos, rock lizards and snakes have been observed to reproduce occasionally via parthenogenesis. Some species, including the New Mexico whiptail, reproduce exclusively by parthenogenesis, meaning that they have not had males for millions of years. Despite the absence of males, the female lizards still undergo mating behaviors with each other to stimulate hormonal cycles prompting egg-laying.

"They only have DNA from her, but it's recombined differently in the offspring, which is why parthenogenesis offspring are often unhealthy, even if the mom is fine," Kate Golembiewski, the PR and science communications manager at the Field Museum, told Newsweek.

While this phenomenon has previously been seen in sharks, it is rare, and even rarer to be seen when males are available to mate with.

"This is only the second case that we know of where sharks have been born by parthenogenesis even when there were healthy mates available," Feldheim said. "This discovery throws a wrench in what we thought we knew about how and why parthenogenesis happens, and it illustrates a key aspect of science: We're continually learning."

The other instance of shark parthenogenesis with males present was at the Aquarium of Pacific, Feldheim said.

Another case of females reproducing via parthenogenesis despite the presence of males was seen in late 2021, when a female California condor was found to have laid two eggs that were genetically unrelated to any of the males in the enclosure.

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