The Shreveport Aquarium in Louisiana has witnessed something remarkable—a newborn shark pup in a tank that only houses female sharks. Named Yoko, which means ocean child in Japanese, the pup’s birth has baffled scientists and sparked questions about how sharks actually reproduce.

A mysterious birth

About eight months ago one of the two swell sharks in the tank laid an egg. The aquarium team wasn’t sure which shark the egg belonged to. The mystery deepened even further when the egg recently hatched and little Yoko emerged. 

The biggest puzzle was that the only inhabitants in the tank were female sharks. They also hadn’t encountered a male in the last three years. So then how come Yoko is born? 

When it comes to some plants and simpler organisms (such as amoeba, starfish, and yeast), new life can emerge from a single parent through asexual reproduction. However, complex life forms like humans and sharks only reproduce sexually, requiring a female egg to be fertilized by male sperm for a baby to be born.

There are two main types of reproduction in the animal world. Sexual reproduction involves the combination of genetic material from two parents—a male and a female—through fertilization, resulting in offspring with a mix of both parents’ DNA. This process increases genetic diversity, which helps species adapt to changing environments.

In contrast, asexual reproduction requires only one parent and produces genetically identical offspring (clones). This method is common in simpler organisms like bacteria, starfish, and some reptiles, and allows for rapid population growth but lacks genetic variation, making populations more vulnerable to environmental changes and diseases.

Scientists suggest that in the case of Yoko, there could be two possibilities. The first is delayed sexual reproduction, which means that the shark that gave birth to the pup has stored sperm for years. Swell sharks have a special gland called an oviducal gland that produces the eggshell and can also store sperm released by a male.

The ability to store sperm is a useful one. It allows sharks to delay egg fertilization and birth when conditions are unfavorable for their pups’ survival. In fact, there have been cases where sharks gave birth four years after mating with a male. 

But there’s another, even more tantalizing possibility.

Parthenogenesis

A more extraordinary explanation is parthenogenesis—a rare type of asexual reproduction in which an unfertilized egg develops into an embryo. While common in insects, reptiles, and crustaceans, it is rare in sharks but has been documented in captivity.

For instance, in 2001, a bonnethead shark at the Henry Doorly Zoo in Omaha, Nebraska, gave birth to a pup despite having no male present in the tank for three years. Genetic testing in 2007 confirmed that the pup was produced through parthenogenesis. 

Similarly, in 2017, researchers in Australia documented parthenogenesis in zebra sharks. They found that female zebra sharks, after being separated from males, could switch to parthenogenetic reproduction.

However, currently, the aquarium team can’t confirm whether Yoko was born via parthenogenesis or delayed fertilization. Yet.

The shark pup must live

To determine the exact cause of Yoko’s birth scientists need to conduct some genetic tests that will involve studying chromosomes. These tests will also reveal which swell shark in the tank is the real mother. However, since Yoko is too young for such tests, the aquarium team’s primary focus is currently on caring for the pup and keeping it healthy.

“We are very excited in the coming months to confirm whether this was indeed a case of parthenogenesis or if it was delayed fertilization. It really proves that life…finds a way,” Greg Barrick, a curator of Live Exhibits at Shreveport Aquarium, said.

Another important point to note is that while parthenogenesis is rare in sharks, what is even rarer is the survival of pups that are born this way. In the last 20 years, out of all the parthenotes, only one white-spotted bamboo shark reached sexual maturity, including the zebrafish mentioned in the previous section, which died very young. 

This is not the case with delayed fertilization though, as it is a more natural phenomenon. However, only time will tell how fortunate Yoko truly is.

“Yoko is currently being closely monitored off exhibit to ensure its health and well-being. While visitors cannot see the pup in person just yet, the aquarium is excited to share its progress on social media until the pup is old enough to move to a larger tank,” the aquarium team said.