A lone stingray mysteriously became pregnant in an aquarium. Scientists now think a male shark could be responsible.

• A stingray in a North Carolina aquarium became pregnant without a male in the tank.
• Scientists believe that a male shark could be the father.

Staff at an aquarium in North Carolina were left confused after a stingray became pregnant despite there not being a male in her tank.

Scientists have a few theories about how this might have happened — and one involves a male shark.

The Aquarium and Shark Lab by Team Ecco in Hendersonville was initially concerned that the stingray had cancer when she began to swell. However, an ultrasound later revealed that the growths were eggs, the lab’s executive director wrote in a blog post.

The director, April Smith, said that one possible explanation was parthenogenesis, a process in which a female can produce an embryo asexually without an egg being fertilized with sperm.

She said that this was a survival mechanism that allows for the preservation of a species and typically happens in situations where no males are present, such as in a zoo or aquarium or even in a secluded area of the deep sea.

However, scientists developed another theory when they noticed bite marks on the stingray. The stingray had been sharing her tank with two white-spot bamboo male sharks.

“Then our light bulb went off — sharks bite to mate — did one of our young males mate with her?” Smith wrote.

She said that the most recent ultrasound revealed that the stingray was pregnant with two, possibly three pups.

She explained that their theory would need to be confirmed after the birth unless they first found any visual clues about a mixed breed.

Sharks and stingrays are closely related taxonomically, so interbreeding is theoretically possible.

The animal is due to give birth any day now, and Smith said that updates would be posted on their website and Facebook page.