Are swell sharks aggressive? All we know as a baby shark nicknamed Yoko mysteriously born without a male present

In an unusual turn of events, a baby swell shark named Yoko was born at the Shreveport Aquarium in Louisiana. The shark was born in a tank that had not seen a male shark for over three years. Marine biologists and aquarium staff are baffled, yet fascinated, by this "virgin birth."

Swell sharks are interesting marine creatures, primarily living in the rocky, kelp-covered waters of the eastern Pacific Ocean. Although most people are afraid of sharks being aggressive predators, swell sharks are a species that is known to be non-aggressive.

Swell sharks avoid divers and snorkelers as much as possible. Most bites that occur due to human interaction happen when a shark is provoked or feels threatened.

In fact, research studies show that most attacks on people by sharks are not predatory, but rather from curiosity or mistaken identity. Swell sharks especially have some behaviors that prove their preference to retreat whenever they feel threatened instead of confronting the threat.

Swell sharks use a rather unique defensive strategy when threatened: they inflate their bodies by gulping in water or air. Inflated, their size doubles, and predators cannot easily bite into or remove them from crevices.

The shark curls its body into a U-shape and takes its tail into its mouth to facilitate this process. Once the threat has passed, they expel the water or air and return to their normal size.

Swell sharks are not the largest sharks around, growing a maximum length of about 110 cm or 43 inches long. Their overall yellow-brown color is quite unique with many dark blotches and white spots that camouflage extremely well in rocky substrates which they prefer dwelling in.

Largemouths and rounded snouts describe their mouth conformation adapted primarily for their feeding on small fishes of reefs as well as several invertebrates like mollusks and crustaceans.

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Birth of a swell shark- Yoko

According to Newsweek, Yoko hatched from an egg on January 3, 2025, when she was first found in a tank that consisted of only two female swell sharks. The staff at the aquarium had been monitoring the egg for nearly eight months before it actually hatched.

Greg Barrick, who is the curator of live animals at the aquarium, was also excited about this event and stated:

"This situation is incredible and shows the resilience of this species"

"It really proves that life. uh. finds a way."

According to the news source, aquarium staff have suggested two main hypotheses to account for Yoko's birth: parthenogenesis and delayed fertilization.

Parthenogenesis is a form of reproduction used by some species, in which females can produce offspring without fertilization by males. In this type of reproduction, polar bodies–– small cells that are produced during the development of an egg–– can sometimes fertilize the egg instead of being reabsorbed.

This is a phenomenon that has been reported in other animals, such as some reptiles and fish.

The second explanation is delayed fertilization, whereby sperm from an earlier mating remains in the female's reproductive tract and fertilizes the eggs weeks or even months after mating has taken place.

Both explanations are possible, but parthenogenesis is more probable since no males have been present in Yoko's tank for a long time.

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Checking Yoko's condition

Yoko is not on public display currently as the aquarium staff closely monitors her health. Young sharks born under such unusual circumstances often face significant developmental challenges.

The aquarium plans to conduct DNA testing on Yoko once she is large enough for a blood draw to confirm the nature of her birth. This analysis will help determine whether she is a product of parthenogenesis or delayed fertilization.

Meanwhile, Yoko is doing well under the care of dedicated aquarists who are committed to ensuring her well-being. The aquarium has emphasized that regardless of her lifespan, Yoko's existence will contribute valuable insights into shark reproductive studies and conservation efforts.