Two-Headed Shark Found by Fisherman

We’ve written about a cyclops shark, freshwater sharks, and whorl-toothed sharks, but we have to add a new curiosity to the digital curio: a two-headed shark.

A fisherman working off the Florida Keys recently caught a bull shark, then opened it up to find that it contained two live fetuses, including one highly unusual one with two heads. The fishermen gave it to scientists, who wrote about it in an article published in the Journal of Fish Biology this week.

The scientists, led by C. Michael Wagner of Michigan State University, said it was the first known case of the phenomenon in bull sharks, and one of only about a half dozen recorded cases of a two-headed shark anywhere.

“In and of itself, this single natural history observation does not tells us anything Earth-shattering about the health of the world’s oceans or populations of bull sharks,” Wagner told Ocean Views via e-mail today.

“It’s simply a rarely observed phenomena that we recorded. Yet, it does capture public attention, and what a great opportunity for journalists like yourself to shine the light on some interesting information that does bear on that very important question."

As further reading, Wagner pointed to the recent Ocean Health Index published in the journal Nature by Benjamin Halpern and colleagues (which we covered here).

Why Two Heads?

The two-headed bull shark displays a process technically called “axial bifurcation,” in which the embryo doesn’t finish splitting into two separate individuals (twins). This mutation has been seen in other animals, including humans.

According to Wagner’s team, such individuals rarely survive in the wild, since they are at a big disadvantage when it comes to finding food and avoiding predators. In this case, the two-headed bull shark also ended up with a very small body, since so much energy went into growing two heads.

Wagner said the fishermen who found the animal told him it died shortly after being removed from its mother. “It likely would not have survived very long had it been born naturally,” said Wagner.

Studying such rare organisms may help us better understand developmental processes, Wagner added.

Update: Wagner pointed out, in response to a reader question about sharks not having bones: “It is true that shark skeletons are formed of cartilage, not bone. Shark cartilage is sufficiently dense to be imaged by x-ray. It is a commonly used technique for examining museum collection specimens that are rare, and when there is a desire to keep the specimen undamaged (e.g., see the Smithsonian Institution exhibit on fish diversity).”