When a fisherman caught a bull shark recently off the Florida Keys, he came across an unlikely surprise: One of the shark's live fetuses had two heads.
The fisherman kept the odd specimen, and shared it with scientists, who described it in a study published online today (March 25) in the Journal of Fish Biology. It's one of the very few examples of a two-headed shark ever recorded -- there about six instances in published reports -- and the first time this has been seen in a bull shark, said Michael Wagner, a study co-author and researcher at Michigan State University.
Technically called "axial bifurcation," the deformity is a result of the embryo beginning to split into two separate organisms, or twins, but doing so incompletely, Wagner told OurAmazingPlanet. It's a very rare mutation that occurs across different animals, including humans.
"Halfway through the process of forming twins, the embryo stops dividing," he said.
The two-headed fetus likely wouldn't have lived for very long in the wild, he said. "When you're a predator that needs to move fast to catch other fast-moving fish ... that'd be nearly impossible with this mutation," he said. [See the two-headed shark.]