A Mysterious Beginning

Yoko’s egg case was discovered by our husbandry team in June of 2024 in a tank that houses two female swell sharks, Lucy and Ethel and no male sharks in the habitat. While female swell sharks commonly lay eggs even in the absence of males, these typically do not develop into viable embryos. The chances of this particular egg hatching were extremely low — but against the odds, months later, Yoko emerged, thriving and healthy, making this a truly extraordinary event.

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The discovery raised an incredible scientific question: Was it a result of parthenogenesis, a rare form of asexual reproduction, or did she result from delayed fertilization, where stored sperm fertilizes the egg years after mating?

 

What is parthenogenesis?

Parthenogenesis is when the mother has offspring without a male present and the offspring is more or less genetically identical to the mother. It is theorized this occurs because of a lack of a viable mate and the biological need to pass down one’s genetics.

 

What is delayed fertilization?

Delayed fertilization is when the mother does encounter a male but she holds on to the gamete packet until a favorable time to have offspring. This can be the result of unfavorable water quality conditions (i.e. temperature, salinity, etc.) or perhaps a heavy presence of predators. The longest recorded instance of delayed fertilization in a shark species was 45 months.

 

It has been confirmed that neither of the two female swell sharks have been housed with a male during their time at Shreveport Aquarium, where they have lived since July 2021. Prior to that, the sharks resided at the Wonders of Wildlife Aquarium in Springfield, MO from 2014 through mid-2021, where we have verified that no male sharks were present in their habitat during that time either. So that is over a 10 year period without access to a male.

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Our team plans to conduct a karyotype (DNA) analysis once Yoko is large enough for a safe blood draw—a process that will take several months but will confirm for sure that this is parthenogenesis.

 

Growing Strong

As of June 2025, Yoko is thriving!

• Weight: 1.13 ounces

• Length: 7-1/16 inches

• Favorite Foods: Scallops and shrimp

She is being carefully monitored off exhibit to ensure her health and development, and our aquarists are giving her the best care possible as she grows.

 

Why It Matters

This rare event could help unlock new understanding of shark reproduction, genetic diversity, and the adaptive capabilities of marine species. Whether her birth is the result of parthenogenesis or delayed fertilization, Yoko is already contributing to vital scientific conversations around conservation and the future of elasmobranch species.

 

Follow Her Journey

While Yoko isn't on public display just yet, we’re sharing regular “Pup-dates” on our social media channels so you can follow along as she grows, learns, and helps rewrite what we know about shark reproduction.

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Yoko’s story has made waves worldwide, catching the attention of CNN, CBS, MSN, Popular Science, ABC Australia, CBC Radio, People, USA Today, NPR, and more

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Read news release

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Follow us on Facebook, Instagram for the latest on Yoko!

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About Swell Sharks

Swell sharks are a type of cat shark that live the eastern Pacific Ocean, often found over algae-covered rocky bottoms where they hide in crevices during the day. They grow to about 3 feet long and have flat, broad heads with large gold eyes.

 

When threatened, the swell shark has an unusual response. It bends its body into a U-shape, grasps its caudal fin in its mouth and swallows a large quantity of sea water, which makes it swell to twice its normal size. This behavior makes it difficult for a predator to bite or pull a swell shark from its rocky crevice. When letting the water out, the shark makes a dog-like bark!

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Another highly unique ability swell sharks possess is they can absorb sunlight temporarily and then re-emit a green glow! Scientists believe that this BIOFLUORESCENCE potentially functions in communication and assists camouflage.