Don’t Lose Your Head Over Two-Headed Sharks

Polycephaly, the condition of having more than one head, is relatively common in nature.
November 7, 2016, 7:25pm
Trailer for 2-Headed Shark Attack. Video: Trailers+RedHeadGirl/AsylumFilms/YouTube

The year 2016 seems intent on going down in history as an extended roller-coaster of absurd events, from mortifying political antics to troubling clown sightings. With this in mind, it should come as no surprise that encounters with two-headed sharks appear to be on the rise, according to National Geographic. This year is so far-gone that jumping a normal shark simply wasn't good enough. To get the kind of ratings 2016 is after, you need to jump a two-headed shark.

As disconcerting as this news may seem, there's no reason you should fear getting attacked by mutant bicephalous sharks that can eat you more efficiently, as depicted in the inspired 2012 film 2-Headed Shark Attack.

Trailer for 2-Headed Shark Attack. Video: Trailers+RedHeadGirl/AsylumFilms/YouTube

For starters, there have only been a handful of verified cases of two-headed sharks. Reports of them date back to 1838, according to ecologist Michelle Heupel of the Australian Institute of Marine Science, but there's no evidence to suggest any of these genetic oddities survived infancy, let alone grew to adulthood.

"Survival after birth may occur, but would likely be very brief," Heupel told David Shiffman in Hakai magazine. "It is unclear whether the two heads will preclude swimming and prey capture, and whether joined internal organs will function adequately."

Case in point: The two-headed sharks that are popping up in the news recently were all discovered in utero. For instance, in 2008, longline fishermen Christopher Johnston discovered a two-headed blue shark fetus during an expedition in the Indian Ocean.

"We pulled up a pregnant blue shark, cut it open, and there was the two-headed one," Johnston told National Geographic. "It was about two-thirds the size of the rest of the pups in length. I put it in the tank on the deck. It swam a little while, but it couldn't swim properly, it just swam in one spot as if it were on a treadmill. I tried feeding it squid but it wasn't interested." The pup died shortly afterwards, and Johnston returned it to the sea.

Two-headed sharks are becoming more common, no one knows why
— The Independent (@Independent) November 7, 2016

In 2011, researchers led by marine biologist Felipe Galván-Magaña of the Instituto Politécnico Nacional, Mexico City identified yet another two-headed blue shark embryo, and published their results in Marine Biodiversity Records.

"Abnormal sharks showed a symmetric bicephaly that could be caused by the high number of embryos found in the uterus of the blue shark, which is the most fecund species of shark in the world," Galván-Magaña and his team noted in the study. "The abnormality probably began during the embryonic development."

READ MORE: Can Sharks Survive Humans?

In 2013, another team discovered the first instance of dicephaly in a bull shark fetus, and published those findings in the Journal of Fish Biology.

The 2013 bull shark fetus. Image: Courtesy of Patrick Rice, Shark Defense/Florida Keys Community College

The eighth example of a two-headed blue shark and the first example of a smalleye smooth-hound shark were described by a Caribbean-based team in this 2016 bulletin.

Just a few weeks back, on October 9, the another group published research on a two-headed Atlantic sawtail catshark fetus, which marks the first time this abnormality has been observed in an egg-laying shark species.

Two-headed catshark fetus. Image: Valentín Sans-Coma et al/Journal of Fish Biology

There's no doubt that these repeated discoveries make it seem as if two-headed sharks are becoming more common, and recent headlines have seized upon that clickbait-friendly assumption. Some speculative explanations for a potential uptick in the disorder have been put forward, including marine pollution, metabolic problems, or inbreeding due to a decreased, overfished gene pool. But there's no way to back up these hypotheses without a bigger sample size to study.

Indeed, as Galván-Magaña points out, increasingly frequent sightings may simply be due to the fact that the oceans and their inhabitants are being poked and proded by humans more than ever before, providing more opportunities for fishermen and marine scientists to come across anomalies like two-headed sharks.

It's also worth mentioning that sharks experiencing polycephaly, which is the condition of having more than one head, is not that weird considering that this genetic malfunction has been observed in a diverse range of animals, including humans.

There have been past cases of two-headed cats, crocodiles, snakes, turtles, cows, pigs, and sheep, among other many other species. The fossil record has even been so kind as to preserve an example of the birth defect rearing its two heads in the Cretaceous marine lizard Hyphalosaurus lingyuanensis, which lived 120 million years ago.

In other words, polycephaly is a well-documented bug in tetrapod DNA, and sharks are one of many lifeforms that have to live and die with it. That is normal, fine, and probably not a herald of apocalyptic times.

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