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Great white shark with suspected cookie-cutter shark scars. Credit: Gerardo del Villar/Pacific Science

Like Shark Week, But With Actual Facts

It’s Shark Week, and the Discovery Channel have already jumped the shark with a fake documentary, asking if a giant prehistoric shark Megalodon is actually still alive. It’s not, and the show was filled with lies, fabrications and actors playing scientists.

Here’s a radical alternative idea: I thought I might celebrate Shark Week by telling you some actual facts about sharks. This, therefore, is a collection of my earlier Not Exactly Rocket Science posts on sharks (and a couple of rays, for good measure).

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Cookie-cutter shark, by the NOAA Observer Project

On 25 August 2010, diver Gerardo del Villar, saw a great white shark off Guadalupe Island with two odd wounds on its head. One was a crescent-shaped scar. The other was a round crater, still open and bloody. Both were just behind the corner of the young male’s fearsome mouth. Del Villar took photos of the animal and sent them to a team of scientists, including Yannis Papastamatiou from the Florida Museum of Natural History.

He had seen wounds like these before. “A wound from a hook should leave more of a hole and would not be as smooth,” he says.  Instead, Papastamatiou thinks that they were the bite-marks of another shark, just a sixth of the size—a cookie-cutter. “I dont know of any other animal that leaves a bite like that.”

Thresher-shark

For most sharks, the front end is the dangerous bit. Thresher sharks are the exception. They’re deadly at both ends, because they’ve managed to weaponise their tails. The top halves of their scythe-like tail fins are so huge that they can be as long as the rest of the shark. For around a century, people have been saying that the threshers lash out at their prey with these distended fins—hence the name. But no one had ever seen them do so in the wild.

In 2010, one team showed that they can lash out at tethered bait under controlled conditions. But Simon Oliver has done better. His team spent the summer of 2010 in the Philippines, watching and filming wild pelagic thresher sharks—the smallest of the three species—hunting large shoals of sardines. The videos are spectacular and unambiguous: threshers really do hunt with their tails.

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Credit: Jeff Kubina from Columbia, Maryland Sand tiger shark, by Jeff Kubina

Inside its mother’s womb, an unborn sand tiger shark is busy devouring its brothers and sisters. It’s just 10 centimetres long but it already has well-developed eyes and a set of sharp teeth, which it turns against its smaller siblings. By the time the pregnant female gives birth, it only has two babies left—one from each of its two wombs. These survivors have already eaten all the others. They’re the bloody victors of a pre-birth battle.

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Credit: Joshua Drew, Columbia University.

When life gives you lemons, make lemonade. When life sticks you on an isolated island surrounded by shark-infested waters, make utterly badass weapons out of shark teeth.

This is what the people of the Pacific Gilbert Islands have been doing for centuries. Sharks are a central part of their lives. Many social customs and taboos revolve around the finer points of shark-hunting. Young boys go through initiation rites where they kneel on a beach, looking towards a rising sun and slice their hairlines open with shark teeth, letting the blood run into their eyes until sunset. And with no metal around, they used shark teeth to adorn their weapons.

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Credit: Diliff

If you ever saw a sawfish, you might wonder if someone had taped a chainsaw to the body of a shark. The seven species of sawfish are some of the wackier results of evolution. They all wield a distinctive saw or ‘rostrum’, lined with two rows of sharp, outward-pointing ‘teeth’. But what’s the saw for?

Barbara Wueringer has an answer: the saws are both trackers and weapons. They’re studded with small pores that allow the sawfish to sense the minute electrical fields produced by living things. Even in murky water, their prey cannot hide. Once the sawfish has found its target, it uses the ‘saw’ like a swordsman. It slashes at its victim with fast sideways swipes, either stunning it or impaling it upon the teeth.

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Credit: Terry Goss

In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.

When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.

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Credit: Barry Peters

The hammerhead shark’s head is one of the strangest in the animal world. The flattened hammer, known as a ‘cephalofoil’, looks plain bizarre on the face of an otherwise streamlined fish, and its purpose is still the subject of debate. Is it an organic metal detector that allows the shark to sweep large swathes of ocean floor with its electricity-detecting ability? Is it a spoiler that provides the shark with extra lift as it swims? All of these theories hypotheses might be true , but Michelle McComb from Florida Atlantic University has confirmed at least one other -the hammer gives the shark excellent binocular vision.

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Credit: Spotty11222

Thanks to Hollywood, the jaws of the great white shark may be the most famous in the animal kingdom. But despite its presence in film posters, the great white’s toothy mouth has received very little experimental attention. Now, Stephen Wroe from the University of New South Wales has put the great white’s skull through a digital crash-test, to work out just how powerful its bite was.

A medium-sized great white, 2.5m in length and weighing in at 240kg, could bite with a force of 0.3 tonnes. But the largest individuals can exert a massive 1.8 tonnes with their jaws, giving them one of the most powerful bites of any living animal.

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Credit: Mark Conlin, SWFSC Large Pelagics Program

In the middle of the Pacific Ocean, Gonzalo Mucientes has discovered an invisible line in the sea that separates male mako sharks from females. The line runs from north to south with the Pitcairn Islands to its west and Easter Island to its east. On the western side, a fisherman that snags a mako will most probably have caught a male. Travel 10 degrees of longitude east and odds are they’d catch a female. This is a shark that takes segregation of the sexes to new heights.

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Juvenile bamboo shark by Steve Childs

Sharks can sense their prey’s minute electric fields, such as those produced when muscles twitch or nerve cells fire. This super-sense is part of what makes sharks such formidable hunters—you can’t hide from them if the very act of living can give you away.  But the tables are turned when sharks are young. As they begin life, they’re as vulnerable as any other fish, and their electric sense helps them to hide instead of hunt.