Discovery of rare two-headed bull shark stuns scientists

This image, courtesy of SharkDefense, shows the two-headed bull shark fetus.

It's the stuff of nightmares for anyone with a shark phobia, but for scientists the discovery of a two-headed bull shark is a unique opportunity to study one of nature's rare glitches.

The two-headed shark was found inside the womb of an adult bull shark caught by a fisherman off the coast of Florida on April 7, 2011 and passed on to SharkDefense, a Florida company that specializes in shark repellants.

Results of an analysis of the shark were published this week in the Journal of Fish Biology.

"External examination, Radiography and magnetic resonance imaging revealed a case of monosomic dicephalia where the axial skeleton and internal organs were found to divide into parallel systems anterior to the pectoral girdle resulting in two well-developed heads," said the abstract from the study, written by scientists from Michigan State University

In other words, the shark had two hearts and stomachs, with the remainder of the body merged into one structure with a single tail.

Each of the shark's two heads have five pairs of gills and gill openings, a single set of eyes, and a mouth and teeth that appear well-developed, according to the study. The shark also had three dorsal fins and a highly-deformed caudal fin.

But, contrary to expectations, this was not a case of conjoined twins. Rather, the fetal bull shark has been positively identified as the first known case of dicephalia -- a single fetus with two heads -- in a bull shark.

The shark's condition was caused by "axial bifurcation," where the embryo begins to split in two, but stops short of complete separation. It can occur in many species, including humans.

Though two heads, and therefore two sets of teeth, may seem to represent an advantage for an ocean predator, experts say the shark likely would have perished shortly after birth due to the stunted nature of its body.

The shark embryo, which was about the size of a Frisbee when it was found, died soon after being removed from the mother's womb. However, some of the shark's other unborn pups were reportedly released alive.

According to reports, roughly six two-headed sharks have been documented over the years, but this was the first multi-headed bull shark to ever be officially recorded and documented.