But when they decided to take a closer look in 2010, they found that two of the egg casings were housing healthy shark embryos. One of them survived, and in 2012 a healthy shark pup was born.
But who's that shark's daddy? The only male in the whole tank was a Javenese cownose ray, and the pup born was definitely no shark-ray hybrid.
The researchers knew it could be a case of parthenogenesis, where a female that usually reproduces sexually is able to create offspring on her own. It's not common, but it's been observed in four shark species before, as well as in other animals. So they examined the pup's DNA to see if it showed signs of being a clone or half-clone of its mother.
Instead the Chiloscyllium punctatum (commonly known as a brownbanded bamboo shark) was shown to have two parents, leading scientists to conclude that the pup was simply the result of a very patient mom.
The last time any of the pup's potential mothers had contact with a compatible mate was in 2007 -- nearly four years before the pup's birth.
This is an unusually long example of sperm storage in captivity (and the first for this particular species), but this is a trick sharks are known to pull. Some seem to be able to store sperm just outside of their ovaries after mating, allowing them to mate successfully even if they're not ovulating at the time, or if their environmental conditions or health aren't optimal.
Just how sharks manage to stave off reproduction until the right moment -- and how some of them manage to do it without male intervention -- is something that conservationists are eager to understand.
"Questions remain," Moisés A. Bernal, a doctoral candidate and researcher in the academy's Ichthyology Department, said in a statement. "We know that several species of sharks have reproductive tricks like storing sperm or reproducing by parthenogenesis in the absence of males, but we need to know when and how these alternate techniques are triggered. Understanding these mechanisms -- and how they impact genetic diversity -- could be vital for the future of shark conservation."