Charlotte the stingray due to give birth within weeks despite no male ray company for years
The pregnancy of a round stingray in a North Carolina Aquarium has bamboozled staff, who say she hasn't shared a tank with a male of the species in more than eight years.
Charlotte, who is native to southern California coast and is the size of a dinner plate, could give birth to as many as four pups in the next two weeks.
At first, staff thought she had a tumour when they noticed a lump growing on her back that was "blowing up like a biscuit".
But an ultrasound revealed she was expecting.
"We were all like, 'shut the back door, there's no way'," said Brenda Ramer, executive director of the small aquarium in Hendersonville.
"We thought we were overfeeding her. But we were overfeeding her because she has more mouths to feed."
An expert on stingrays said it would have been impossible for Charlotte to have mated with one of the five small sharks that share her tank, despite news reports suggesting that was the case after Ms Ramer joked about a possible interspecies conception.
Besides being different sizes, the animals wouldn't match up anatomically.
Neither would their DNA.
“We should set the record straight that there aren’t some shark-ray shenanigans happening here,” said Kady Lyons, whose graduate work focused on the species.
A more likely explanation
But she said there was another explanation for the surprise pregnancy — parthenogenesis, which is a type of asexual reproduction.
It happens when offspring develop from undertilised eggs, meaning there is no genetic contribution by a male.
The mostly rare phenomenon can occur in some insects, fish, amphibians, birds and reptiles, but not mammals.
Other kinds of sharks, skates and rays — a trio of animals often grouped together — have had these kinds of pregnancies in human care.
"I'm not surprised, because nature finds a way of having this happen," she said.
To be clear, Dr Lyons said, these animals were not cloning themselves.
Instead, a female's egg fuses with another cell, triggers cell division and leads to the creation of an embryo.
The cell that fuses with the egg is known as a polar body.
They are produced when a female is creating an egg but these usually aren't used.
"We don't know why it happens," Dr Lyons said. "Just that it's kind of this really neat phenomenon that they seem to be able to do."
Dr Lyons said the species were fascinating.
For example, embryos in the womb are bathed in uterine milk that provides nutrients to help them develop.
"I'm glad the round stingray is getting the media attention that it deserves," Dr Lyons said.
"It's not necessarily as sexy as a white shark, but they do a lot of really neat stuff."
