"New ultrasound data of captive tawny nurse sharks (Nebrius ferrugineus) revealed that their embryos frequently migrate between the right and left uteri during gestation," the abstract said.

"This report is the first reliable evidence of active embryonic locomotion in live‐bearing vertebrates and is contradictory to the concept of 'sedentary embryo,' which has mainly arisen from studies of mammals."

"The tawny nurse shark is unique among orectolobiform sharks, in which the embryo develops by feeding on sibling eggs in utero. Thus, we hypothesized that swimming aids in an efficient search and capture of these eggs in the uterine environment."

What Does All Of This Mean?

The fact that the shark embryos are traveling from one to another is quite amazing. When one shark's embryo decreased, the other one increased.

It's outstanding.

There are at least three shark species that showcase embryos moving only their mouths, according to Live Science.

"Our data also showed that the cervix of the tawny nurse shark sometimes opens," the researchers said. "The embryo exposes its head out of the uterus through the cervix."

The researchers said this process had been only caught on camera once before.

A 1993 Discovery Channel documentary, actually showed some embryos swimming in and out of uteruses by squeezing through a hole that was cut in the side of a pregnant sand tiger shark.