Sharks with two heads might sound like something from a b-movie (like the one above) but they have actually been occurring in the wild. Back in 2008 a fisherman discovered a two-headed blue shark embryo in the Indian Ocean and in 2013 fishermen in the Gulf of Mexico found a bull shark with a two-headed fetus in her uterus.
While in 2011 scientists found conjoined twins in blue sharks caught in the Gulf of California and northwestern Mexico. In fact it's thought blue sharks produce more two-headed embryos than other species because they can carry up to 50 babies at one time.
The most recent incident comes from Spanish scientists who were rearing hundreds of sharks for medical research. A two-headed Atlantic sawtail catshark embryo was found among the 797 stored for their research. "Both sharks had a mouth, two eyes and even two brains." says the BBC. "They also had two hearts, stomachs and livers, but shared an intestine. The two were attached below the gills." It was also notable because it was the first recorded two-headed shark born from an oviparous species, which is one that lays eggs.
However it's unlikely that this particular two-headed shark would have survived had it hatched, in fact many of the two-headed sharks discovered in utero would unlikely survive birth.
Because of this two-headed sharks are very rare which means it's difficult for scientists to pin down exactly what's been causing it. For the two-headed shark found in the lab in Spain, the scientists say genetic mutation was most likely the cause. But for those found in the wild all number of reasons have been posited—these include viral infections, metabolic problems, pollution, and shrinking gene pool due to over fishing which leads to inbreeding and abnormalities.
And although it appears sightings and recordings of two-headed sharks seems to have increased one scientist, Dr Felipe Galván-Magaña from Mexico, thinks that the number of two-headed sharks hasn't necessarily risen—it's just that there are more journals to publish in so more scientists are writing about them.
Still, this hasn't stopped people from having nightmares about 8ft long two-headed sharks though.