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Discovery of rare two-headed shark prompts discussion of ocean pollution

By TONY WU | April 4, 2013

While the existence of a two headed creature can only seem possible in a Disney or sci-fi movie, such animals do actually exist in our world. In fact, we even have a scientific name for two-headedness: dicephalia, or axial bifurcation.

This condition usually results after severe developmental deformities. Michael Wagner, an assistant professor at Michigan State University notes that these deformities are rare, and even more rarely recorded. Most of these mutations occur in animals such as snakes and lizards. However, recently, scientists have confirmed a case of dicephalia in bull sharks.

Dicephalia is a rare phenomenon and one that greatly reduces the survival rate of the animal. The mutation starts during the gastrulation period of a fertilized embryo. During the gastrulation period, the anterior of the neural tube is duplicated. The pair of neural tubes then develop into individual heads, resulting in the organism having two heads. Organisms afflicted with axial bifurcation often do not survive outside of the womb. Those that are successfully born lack control of mobility because it is difficult to manage competing signals emanating from two different brains. The hindered mobility affects the ability to chase after prey and escape predators.

The recently discovered two-headed bull shark was obtained from the womb of a captured shark. Fisherman removed the fetuses from the shark, while abiding by standard procedures necessary for proper extraction and treatment of both the infant and the mother. The normal fetuses were released back into the wild, but the mutated fetus died shortly after the fisherman severed the umbilical cord. To preserve the unusual specimen, the fisherman soaked the shark in 70 percent ethanol.

During the examination of the specimen, scientists noted several interesting characteristics. First, the shark with dicephalia is considerably shorter than normal baby sharks. Furthermore, the shark also has multiple dorsal fins, a deviation from the usual, singular dorsal fin. Besides these defects, the researchers found that other, external physical aspects of the shark were normal.

To gain a deeper insight into the condition, scientists at the Florida Keys Community College decided to subject the shark to a radiograph. The radiograph revealed the abnormalities in the vertebrae of the animal. While normal sharks possess 208 to 227 vertebrae, the mutated shark contained 89 on the right and 88 on the left. Moreover, the vertebrates are larger near the bifurcation and some are more densely packed, giving the impression of fused vertebrae.

As they took a look at the internal organs of the shark, the investigators discovered that the shark possessed two heads, two hearts and two stomachs.

“Further down, the shark fused into a single lower torso and tail,” Wagner said. This division into two parallel systems excited scientists because it is even rarer than the typical axial bifurcation.

The severity of the mutation sparked the researchers’ interest in determining the cause. There have been various theories regarding the cause of axial bifurcation. The possibilities range from physical diseases, such as congenital abnormalities, to parasitic infection or pollutants in the environment. Some also theorize that malnutrition may factor into its prevalence.

“This specimen on its own may not lead to any new science...[but] we may eventually be able to identify patterns and gain an understanding of what causes it to occur,” Wagner said. While the range of possible causes is broad, there is a rising belief that the main cause is the pollution that results from the Deepwater Horizon, a large offshore oil drilling rig.

“[This] could lead to a discussion of the health of the world’s oceans,” Wagner said.


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