The Fascinating Evolution of Piranhas

You're probably familiar with the red-bellied piranha, which you can see in the Amazon River and at Ripley’s Aquarium of Canada .

But how much do you really know about these magnificent creatures? Let's dive in for the full scoop on piranhas!

Teeth and Bite Force

Behind their large, curled lips on their blunt-shaped heads hide a single row of teeth on both their top and bottom jaws.

These fish have remarkable teeth described as tricuspid; a molar with three points, or cusps. Like sharks, they constantly lose and regrow their teeth throughout their entire lives.

These serrated teeth are designed in a way to allow perfect interlocking with one another, and with the jaw.

The red-bellied piranha has incredible force to bite down and shear food. It can exert a bite force equivalent to thirty times their body weight, topping any living animal on earth!

A related species under the same family, the black piranha ( Serrasalmus rhombeus ), outmatches even the tyrannosaurus rex when it comes to bite force to body weight equivalency. If looking at the numbers here, the black piranha has a bite force of 320 Newtons, three times greater than that of the American Alligator!

This is due to piranhas having greater sized jaw muscles to body ratio with a specialized jaw lever closing system. Piranhas top even crocodiles, sharks, hyenas, and the prehistoric megalodon.

Debunking Misconceptions

Living piranhas today give off the impression that they chase after and eat large animals swimming in the Amazon waters. However, it is quite the contrary.

The red-bellied piranha hunt in groups for safety, also called a shoaling, and typically nip off the fins and tails of other fish, rather than consuming them whole.

They are more likely to go after and rip flesh off already deceased or weakened animals. What may surprise many is that they are omnivores, readying at the opportunity to swim over to the sound of fallen nuts in the water that their jaws are specially adapted to break open, or even graze off vegetation on rocky surfaces.

But Why Are Their Teeth So Sharp?

The answer lies in an ancestor that has a bite force exceeding 1200 to 4700 Newtons.

This ancestor lived in the Miocene Epoch (24 to 5 million years ago). It is known as the megapiranha ( Megapiranha paranensis ). Unlike the red-bellied piranha that grow up to 30 centimetres (one foot) and weigh up to 3.2 kilograms (7 pounds), the megapiranha is estimated to grow over 70 centimeters (2.3 feet), weighing over 10 kilograms (22 pounds).

It is predicted that these prehistoric fish had their jaws adapted to eating larger oceanic animals with hard protection on them such as sea turtles, catfish, and larger terrestrial animals.  

Geological Changes Played a Role

Geological changes, especially to the Atlantic Ocean over 5 million years ago, greatly influenced the environment.

The ocean flooded into the Amazon, increasing salinity and forcing freshwater fish to move upstream. Once the ocean receded, the fish returned downstream. Fossil records show that modern piranhas have ancestors dating back 25 million years, but the piranhas in the Amazon today have been around for only about 3 million years.

The Missing Link

The megapiranha is the missing link in explaining how piranhas came to have a single row of teeth on both jaws, and also connects them to one of their living ancestors, the Pacus ( Piaractus brachypomus ), which also inhabit the Amazon rivers. Pacus and Piranhas fall under the same family, Serrasalmidae, and megapiranha is an extinct member of this family.

Pacus have squarer flatter teeth like that of humans to grind up vegetation, fruits, and seeds as they are primarily herbivorous. The Pacus have two rows of teeth on both the top and bottom jaw. Both Pacus and Piranhas constantly lose and regrow teeth, but since all teeth are interlocked on one half of their face, they simultaneously replenish a whole half set at a time.

The piece of megapiranha evidence we have today is just the upper right jawbone that shows a zig-zag like tooth formation, which indicates how two rows of teeth moved to become one row of teeth.

Behaviour of Possible Ancestor

Researchers found fossils in South Germany, in the same limestone as Archaeopteryx, of a piranha-like fish from the Jurassic Period, over 150 million years ago . This extinct fish, part of the Pycnodontiformes order, had teeth similar to modern piranhas, capable of tearing flesh. Fossils of smaller fish with missing fin pieces were found alongside it.

This suggests that, like today's piranhas, this ancient fish nipped fins rather than eating whole fish, a strategy useful when food is scarce since fins can regrow. Scientists believe this fish would pretend to be harmless to ambush prey, a strategy that may have influenced modern piranhas' hunting behaviors.

The Story Doesn't End Here

While there is still more to uncover through fossils and fitting the evolutionary pieces together, there is evidence of lineage between piranhas, pacus and their ancestor, the megapiranha.

One can only make scientific guesses on how geological changes over the years have really shaped how these animals evolved. Nonetheless, the adaptations they have are all unique which help them successfully hunt and survive in the Amazon habitats and are like no other animal on earth. 

Sensing You Want More?

Swim on over to Ripley's Aquarium of Canada and see the red-bellied piranha up close like never before!