On January 3, a baby swell shark hatched at a Louisiana aquarium. Everyone is thrilled with the 15-centimetre hatchling, nicknamed Yoko. But staff at Shreveport Aquarium have questions.

When they spotted the egg eight months ago, it came as a big surprise. Why? Only two swell sharks live in that habitat. And Ethel and Lucy are both female.

So who is the father? Is there a father?

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There are a couple of possibilities. The most likely? Ethel or Lucy fertilized her own egg.