There’s a terrible B-movie from 2010 called Megapiranha, about giant, genetically modified piranhas that wreak havoc upon Floridian tourists. One of them leaps out of the water and bites a helicopter. It’s all rather silly, but if you look on Wikipedia’s entry for the film, you’ll see these words at the top: “For the prehistoric creature, see Megapiranha.”

Yes, Megapiranha existed. At around 71 centimetres long, it wasn’t big enough to attack a chopper, but it was still three times the size of its modern meat-eating cousins.

Now Justin Grubich from American University in Cairo has found that Megapiranha and its modern relatives have some of the strongest bites of any fish, relative to their size. Pound for pound, they chomp down with more force than bigger icons like the Megalodon, the whale-killing monster shark.

The piranha family are known as serrasalmids, a word that, delightfully, means “serrated salmon”. Only some of them are flesh-eaters like the famous red-bellied species. Many are vegetarians—they’re known as pacu. And one species, called the wimple piranha, feeds only on fish scales.

View Images Silver piranha by David Bright; pacu by Julia Dorn, courtesy National Geographic Channel

You can tell their dietary habits by the shape of their mouths and teeth. The meat-eaters have a thuggish underbite and single row of triangular blades, each one serrated like a steak knife. But a pacu’s teeth are squarer, straighter, and eerily human—all the better for crushing nuts and seeds.

Megapiranha sat somewhere between the two. It lived between five and ten million years ago in Argentina, and a team of palaeontologists led by Alberto Luis Cione announced it to the world in 2009. So far, they have only recovered part of the animal’s upper jaw, but that is enough to diagnose it as a big piranha. The teeth look like the serrated triangular gnashers of modern meat-eaters, but they also had thick broad bases. These were multi-purpose tools that could crush hard food as well as slice soft flesh.

View Images Megapiranha teeth, from Grubich et al, 2012.

Grubich estimated how strongly Megapiranha could bite by first studying its largest modern relative, the black piranha. Together with Steve Huskey from Western Kentucky University, he caught 15 individuals and let them chomp down on a force gauge.* The piranhas bit with forces that ranged from 67 to 320 Newtons (15 to 72 pounds)—substantial for such a relatively small mouth.

Relative to body size, this is the strongest bite ever recorded for a fish, and three times stronger than for an alligator of the same size. As Grubich says, the piranha “can bite with a force more than 30 times its weight, a remarkable feat yet unmatched among vertebrates”.

View Images The jaw muscles of Megapiranha. From Grubich et al, 2012

Its secret is a huge group of jaw-closing muscles. These take up virtually the entire space behind the fish’s eye and mouth (labelled AM and A1 in the diagram), and make up 2 percent of its weight! They also attach to a point far up the lower jaw, allowing the fish to transmit as much of the muscle’s force as possible into the tip of its jaws. Combine this with aggressive biting tactics, and “it should come as no surprise that black piranha, whether large or small, can rapidly and efficiently excise large chunks out of their prey,” says Grubich.

But Megapiranha was even more efficient. The black piranhas that Grubich collected varied in length from 20 to 37 centimetres long, and even a small increase in size translated to a huge increase in bite strength. By extrapolating from his living fish to the larger Megapiranha**, he estimated that the prehistoric giant could have bitten with a force of 1,240 to 4,749 Newtons (279 to 1,068 pounds). That’s at least four times more force than the largest black piranha.

View Images Bite force plotted against size, for modern piranhas and Megapiranha. Best. Diagram. EVER. From Grubich et al, 2012

If you compare different predators, and correct biting forces for size, Megapiranha is the undoubted champion. For example, Tyrannosaurus rex could bite with a force of over 13,400 Newtons, more than three times Megapiranha’s top efforts. Then again, T.rex was 100 times heavier!

And even without any scaling, the prehistoric piranha was still pretty impressive. Its bite would have been as strong as that of a small, 400 kilogram great white shark, even though it would have weighed just 10 kilograms.

View Images BFQ = Bite Force Quotient, a measure of bite strength, corrected for size. Black and white bars are the lowest and highest BFQs for small and large individuals. From the top, the fish are: black piranha, Megapiranha, great barracuda, blacktip shark, bull shark, great hammerhead, Dunkleosteus, great white shark, megalodon. Piranha images from Grubich et al, 2012. Barracuda and shark images from Gray Taxidermy. Dunkleosteus terrelii image from Karen Carr.

What did Megapiranha actually do with such a strong bite? As I mentioned, the shape of its teeth suggests that it could crush hard food and slice soft flesh. But rather than crunching through hard nuts, as pacus do today, Grubich thinks that it used its slice-and-crush teeth to break into the bones of its prey, or through the defences of turtles and armoured fish.

Grubich even checked that it could do so by creating a metal-alloy replica of the fossilised jaw and testing it. The replica, which had the same strength and hardness as a piranha’s jaw, was strong enough to cause “catastrophic punctures” in a cow’s femur, a turtle’s shell, and an armoured catfish’s scales. So we know that Megapiranha could have eaten such foods. Of course, we can’t conclude that it did until we find fossils with the right tooth marks.

Reference: Grubich, Huskey, Crofts, Orti & Porto. 2012. Mega-Bites: Extreme jaw forces of living

and extinct piranhas (Serrasalmidae). Scientific Reports. http://dx.doi.org/10.1038/srep01009

* The paper is full of the delightfully dry language of academia: “In-vivo experiments in the field to elicit and record ecologically realistic biting behaviours for predatory species are rare, dangerous, and difficult to perform.” That is, no one wants to work with things that could take your hand off.

** Cione estimated that Megapiranha weighed 73 kilograms and was 1.3 metres long, but by comparing the jaw fragments with the jaw of the black piranha, Grubich thinks it was smaller – 10 kilograms, and just 0.7 metres long.