Two-headed shark
Scientists are unsure about the reasons behind the spate of two-headed shark sightings

It may sound like a creature from a story you tell your kids when they're misbehaving, or a strange creature out of a B-grade horror flick (with our apologies to the Sharknado franchise), but believe it or not, two-headed sharks are becoming increasingly common.

Back in late October, the BBC reported that researchers had stumbled upon the world's first recorded instance of an egg holding a two-headed shark embryo.

Other sharks that give birth to live young have had instances of two-headed offspring in the past, the BBC said, but the image of this particular egg freaked people out, with some asking whether this was simply a hoax.

Apparently not.

On Nov. 5, National Geographic published an article saying that not only do two-headed sharks really exist, there seem to be more and more instances of them on the record.

Most of these two-headed sharks are merely embryos or infants, because such fish are unlikely to survive long due to their genetic abnormalities. But there have been several instances since 2008, according to the Daily Mail.

According to National Geographic, the blue shark is the species most likely to produce two-headed offspring because females can carry up to 50 babies at a time. That, combined with overfishing, has led to an increase in genetic abnormalities as rates of inbreeding increase, some scientists have theorized.

Regardless, even if these two-headed animals never grow into adults, social media has already gone wild with the idea.

According to the Daily Mail, what prompted this rising trend in two-headed shark discoveries currently remains a mystery to science.

While their numbers are rising, sightings are few and far between, making it difficult for researchers to pin down exactly what triggers the mythical mutation.

More recently, Spanish researchers have now found a two-headed Atlantic sawtail catshark embryo while rearing hundreds of sharks for human-health research.

Study leader Professor Valentín Sans-Coma's team suggest that genetic mutations may be behind their catfshark finding.

Their embryos are grown in a lab with almost 800 other specimens, meaning they were unlikely to have exposure to any mutating infections, chemicals or radiation.

Wild sharks' rising mutation rates could come from a variety of factors, including viral infections or pollution.

Some researchers have suggested that over-fishing may be the culprit.

As shark population numbers dwindle, their gene pool shrinks, giving rise to more inbreeding which carries a high risk of passing on crippling genetic abnormalities.

Marine scientist Nicolas Ehemann recently discovered the first two-headed shark ever found in the Caribbean Sea.

Ehemann speculates that the high prevalence of two-headed sharks in nature points to over-fishing as the likely origin, Daily Mail reported.

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