On January 3, the Shreveport Aquarium in Louisiana welcomed a swell shark pup into the world under mysterious circumstances: It was born in a tank that housed only two sharks—and both were female. The two swell sharks have been in a female-only habitat at the Shreveport Aquarium for more than three years.

Yoko the swell shark was born in a habitat of all-female sharks who hadn't seen a male in years. The aquarium has two possible explanations — and hopes to get confirmation in the coming months.