Everything You Know About Piranhas Is Wrong

Ajouté : 2026-06-04

[00:00:00]In the early 1900s, a group of Brazilian fishermen pulled off arguably the greatest nature scam in history. And the poor soul pulled it on was none other than the former US president, Theodore Roosevelt, who at the time was fresh off of a failed presidential run and looking for some adventure in the Amazon. And the locals, well, they wanted to put on a show for our famous American. So, they got to work weeks in advance, netting off a section of the river and stuffing it with piranhas, all the while making sure not to feed them a single thing. so that by the time Teddy rolled around, every single fish in the pen was starving and ready to jump on anything that dared move in their general direction. And so, of course, when Roosevelt showed up, like clockwork, the fisherman dragged out a live cow and shoved it into the water, which turned red, frothy, and bloody in seconds, while Teddy stood there watching what he was 100% sure was the most terrifying fish on the planet. Roy then proceeded to recount the fish in his bestselling book, Through the Brazilian Wilderness, and calling it, I quote, the most ferocious fish in the world. with her malignant eyes and cruy armed jaws. And yet, despite me telling you this, this story is in fact false, even though it's been spread around the internet for years, including by some more reputable sources. So, yeah, there were no locals conniving to show false brutality of the Amazon. No piranhas, no cow. It simply didn't happen. The book part was real, but not this embellished quote unquote adventure, if you will. And that, my friends, is incredibly fitting considering that basically sums up what we think we know about piranhas as a whole. So, in other words, basically everything you know is wrong, or at least wrongish. And so, despite Hollywood building entire horror franchises around these guys and convincing kids around the world that falling into the wrong river meant being reduced to a skeleton in under 60 seconds, or dare I even say 10 seconds, this is all really just not true. And so, to understand what panas actually are, you have to start with a family they belong to, the Sarasalmans. And the easiest way to picture this family is as a spectrum with a true prawn sitting at one end and then the pacus at the other and roughly 100 species in between. The family name itself comes from Latin for saw salmon which is a nod to the distinctive sawike keel running along the belly of every fish in the group.

[00:02:07]And they all share a laterally compressed body that kind of looks like someone took a normal fish and squeezed it from both sides until it was a little too flat for its size. But here's the first very interesting part, which is that this whole family descends from a common herbivorous ancestor, specializing not in meat flesh, but plant flesh, and only later splintering out to different branches that included more, well, interesting diets. So, what you've got today is around 60 species of piranhas along with dozens of other species split between pacus and silver dollars, all sharing somewhat of the same body plan, but doing very different things with it. And when I say different, I really mean different. Take the black pu for example, which can reach close to 1 meter or 3 feet in length and push past 40 kg or 88 lbs, making it the largest of all the saw salmon, including obviously all types of prawns. But when you take a look at it head-on, you'll quickly realize that something very different is going on.

[00:03:01]Because instead of a mouth full of serrated triangle blades, it's packing broad flat merl-like teeth that also look kind of uncannily humanlike and would honestly, if I saw it in the wild, probably make me nope out just as fast as piranhas would. But the reason it has says teeth is actually pretty straightforward. Because when the Amazon floods every year, inundating tens of millions of hectares of forest, the paku follows the rising water right into the flood plains where it cruises through now down forest, eating fallen fruit and cracking open nuts and also generally doing something that's quite underappreciated and honestly a bit surprising too, which is acting as one of the Amazon's most important seed dispers. And yeah, you heard that right.

[00:03:40]This is a fish that lives in well the water and yet is key to growing trees which well live on land. In a 2011 study tracked them and found that these fish hold on to seeds in their guts for up to 12 days before depositing them, with some individuals carrying seeds as far as 5.5 km or 3.4 miles from the mother tree. The team also went through the stomach contents of 230 different fish pulling out nearly 700,000 seeds from 22 different species of trees and lyas, which is uh very impressive. And to put this into perspective, only the Asian elephants and African hornbills are known to disperse seeds as far in their guts. So, here we have a fish beating most of the animal kingdom despite being quite tiny in comparison and a non-aterrestrial. And to top it all off, a well-fed 10 kg or 22lb individual can have over 1 kg or 2.2 lb of seeds in his guts. So, yeah, my guy is literally a mobile seed pouch. Now, when we get to the so-called real piranhas, things get more complicated, but in a totally different direction. You see most recognized piranha species fall into a handful of genera primarily pygoentric sarasalisto bryson pyopristus and then people sometimes also including the likes of catoprion and within this group the dietary diversity is bananas pun intended the redbellied piranha pygoentric and terry is the dreaded species most responsible for Hollywood's whole reputation of these fish and its pygoentric relatives all share that convex bulldog shaped head/face and a proportionally massive of lower jaw packed with razor-sharp triangular teeth with each tooth fitting into a socket on the adjacent tooth and thus forming what is essentially a continuous serrated blade that runs the entire length of the jaw. And so the whole setup looks like it's been designed for the worst possible purpose. And honestly, it kind of is depending on your perspective, seeing that it is a jaw optimized for cutting and rapidly at that. But the real question comes down to what it's actually cutting. And the answer is a lot more peculiar than you might think because the redbellied piranha is not the red meat lover you might think it is. Instead, it mainly feeds on insects, worms, crustaceians, fish, and then also fallen fruit and seeds. So, shocker, but unlike what Hollywood might have you believe, omnivory is the rule with these guys, not carnivory. And then other pras still go for that umami goodness, but are pacifists at heart kind of. And what I mean by this is take the wimple prana caterpron mento for example which has evolved one of the weirder feeding strategies in freshwater biology and that's eating wait for it scales. Now you're probably like what? How does it go about doing that? Well it's odd because like I said they don't really have a vested interest in murder but rather just the shiny stuff i.e. scales which are removed by executing a high-speed ramming strike to literally knock the scales out. In a 2005 study used high-speed footage to capture the strike and it's uh something else with the fish achieving a gape angle of nearly 120° which is unusual enough on its own even when putting aside the whole eats shiny things bit. And so the end result is that the host fish survives albeit probably traumatized.

[00:06:44]The scales regenerate and the wimble prawn gets a meal that essentially renews itself. And interestingly, the fish can actually switch tactics depending on what it's going after.

[00:06:53]Because when chasing elusive prey or grabbing loose scales floating in the water, it uses a more typical suction strike, much like a normal predator. But when going after a specific target fish for scale removal, it deploys the full force gape attack. So, in other words, this is a flexible weirdo, not just a onetrick weirdo. And the wimple isn't even the only freaky guy in the family because several other Sarasal species are what ichthyologists call terragoists, meaning they specialize in nipping fins rather than scales. There's even one species, Sarasalmus marginatus, that has been observed sometimes acting as a cleaner fish for its larger red-bellied prawn relatives, picking parasites off of them. So, here we have a piranha grooming another prana with no bloodthirsty to fleshing involved. Who would have thought? And running through all of this dietary diversity is one share trait that's quite peculiar to say the least and that's their dental system. See, generally speaking, most vertebrates replace their teeth one at a time throughout their lives or at certain points in their lives. But piranhas and pacus do something different in that they shed and regrow the entire row of teeth on one side of the jaw simultaneously. So basically the whole right or left side just suddenly loosens, falls out as a unit and then the row of replacement teeth that has been developing beneath the surface snap right into place after which the opposite side does the same thing. And interestingly this is not an adaptation for meat eating as one might assume.

[00:08:15]Instead with one study finding that every single Sarasal shares this trait with just one exception. And so rather the real reason why this matters is due to the fact that they all share interlocking teeth which simply would not work or work as well if a tooth was missing because then it would break up the continuous edge so to speak. And unsurprisingly the tools used to use their tools, in other words, how they find food are just as specialized as their teeth. Though unsurprisingly, they get way less attention. Now, like I said, the body is laterally compressed, and they have these relatively large eyes, with some studies having referred to them in certain species as being exceptionally well-developed, likely being adaptation for the low-light environment of the flooded forest. They also have what seems to be pretty big nostrils, considering the fact that, well, you can see them. I mean, how often can you think of seeing a fish's nostrils, which might be out of me as someone who doesn't really know his way around a fishing pole? But regardless, sniffing is definitely a skill of theirs. And one early study on them noted they had highly differentiated olfactory systems showing a strong degree of specialization. There's even a pop science fact that's been going around for a while that piranhas are capable of detecting roughly one drop of blood in 200 L or 50 gall of water which would definitely be blood hound territory. Now while this does show up in both a BBC and Smithsonian article, I admittedly have not been able to track this down. So who knows? But what I can say though is that their nose is clearly a strength of theirs. And considering that they have to navigate in what is often murky waters, the ability to detect any potential food through a non-visual system is quite the game changer. And once a tasty morsel is found, absolutely no mercy is shown.

[00:09:47]With a black piranha, Sarasmus Rambus, holding the record for the most powerful bite of any bony fish ever recorded, measured at 320 newtons at the front of the jaw. And for reference, that force nearly three times what an American alligator of comparable size can produce. is so incorrect for its body mass, it's putting out force more than 30 times the fish's own weight. A feat that is not only unmatched among bony fish, but also amongst vertebraes, period. And the comparisons just keep going because when you scale for body size, the black prana actually outclasses great white sharks, hammerhead sharks, barracudas, and get this, also the megalodon. Yes, the freaking megalodon, as well as a nearly 4,000lb or 2,000 kg armored deonian nightmare, which you might have heard of, known as the Dunlioas. And the researchers measured all this by catching wild prawns and getting them to clamp down on a force measuring device.

[00:10:41]An experiment that sounds easy enough on paper, but I imagine in practice was a tad bit terrifying. Unsurprisingly, the muscle complex for the bite takes up quite a bit of weight at 2% of the fish's total body mass, which might not sound like a lot, but to put that into perspective, there has never been a bony fish recorded at such a high proportion.

[00:10:59]But here's the ironic bit. This power is not typically used to attack large animals, but is instead often more deployed on plants, fallen fruits, and animals much smaller than themselves, like insects and small fish, and sometimes scales or the tasty fin or two when they get the chance. So, basically, the jaw is way overbuilt for the job it usually does, which kind of makes you wonder what evolution would do if it took the same setup and then just pushed it even further. And uh oh wait, we actually have a great example of that.

[00:11:26]Enter megapana parinencis, a fish which lived in what is now Argentina roughly 9 to 6.8 million years ago and was something else entirely, literally and metaphorically. This absolute specimen of a specimen measured about 71 cm, nearly 2.5 ft long, weighing somewhere around 10 kg or 22 lb, which already makes it several times the weight of a modern prana. And this, by the way, is just the lower estimate with some studies giving it a weight of potentially 73 kg or 161 lb. But what's kind of strange is that the holotype, which by the way is just a premaxilla, shows the teeth arranged in a zigzag pattern with a tooth morphology that looks like an intermediate between true piranhas and their pacu cousins, which suggests that perhaps this animal set either at a transition point in the family's evolutionary history or more likely at a more intermediate diet. But one thing that certainly wasn't no intermediate level was their bite force.

[00:12:21]Because remember how I said that the black piranha had the highest adjusted bite force of any known bony fish? Well, scratch that because one has a beat and that's of course the mega prana who could have possibly topped out at a bite force at 9,498 newtons. Which means that these numbers a 73 kg merona would have bitten with the same amount of force of a 3,000 kg great white shark. 3,000 freaking kilos.

[00:12:46]The same study also tested whether the megapana's teeth shape combined with a force could fracture the bones and hard tissue of large animals. And the answer was a resounding and obvious yes with the teeth being able to pierce through a cow femur and go through turtle shells like butter. So basically here you have a giant prana or should I say mega prana that could open turtles like a can opener crunch through genuine megapana femurss which is for the record one of the strongest bones in the body. So uh good thing it's been dead for several million years. Am I right? Now, speaking of megapana, despite their reputation, modern piranhas don't actually attack large vertebrates regularly, with much of its negative reputation coming from records like Roosevelts, as well as its habit of consuming dead bodies, which can make it look like the piranhas were the original killer and not just a scavenger. And interestingly, and contrary to what you might think, studies that have actually looked at necroofasages consumption in the ecosystem, i.e. the eating of dead things. I found that it's actually various species of catfish that are responsible for most of the consumption of drowned people, not prawns. And if you've ever seen river monsters, you might have seen the one episode where Jeremy Wade took redbellied piranhas, put them in a pool, a small pool, that is, proceeded to put blood and meat in said small pool, and then literally the mad lad jumped in the water with them.

[00:14:01]And guess what happened? The end of River Monsters. Just kidding. Absolutely nothing. Nothing at all. Well, actually something did happen, which was that they moved to the edge of the swimming pool away from the scary man thing. And the fact that they did this also speaks towards another myth of these guys, which is that people believe that groups or shoulds of piranha's main purpose is cooperative hunting. And yet, there's actually no evidence of this at all with a sholing or grouping of these animals actually instead being an anti- predator behavior, not a predator behavior, with a sholing really just being a strength and number kind of thing. And this makes a lot of sense when you think about it, especially since, well, they're not that big. And also because their predators are no joke either, as they include caymans, giant river otterters, birds, other fish, and river dolphins. And so to survive in a river full of things that will eat you, prawns, shaw, and the composition of that shell is not random either. One study exposed redbellied piranhas to a fake corrant, which is a type of bird, in order to simulate a predator attack. And what they found was is that fish in groups of eight return to their resting breathing rate much faster than fish in just groups of two with the smaller groups showing elevated ventilatory rates for much longer after the simulated threat which just like when you hyperventilate before having to give a public presentation is a physiological measure of sustained stress. So basically in a large group the perceived risk per individual drops and the fish can chill more quickly. The show, by the way, is also socially stratified with the older, bigger individuals occupying the central positions where exposure is lowest, while the younger, smaller little guys get pushed to the edges, i.e. the worst location where predation risk is higher.

[00:15:38]So, basically how human society works, too. Just kidding, kind of. But overall, looks like Hollywood is taking another L as they got this pretty much exactly backwards because that quote unquote menacing deathcloud sh you see in movies is in reality just a giant huddle of nervous fish trying not to die. And another wrinkle in the big eat you alive scary fish picture is their reproduction which adds another behavioral layer that directly explains many prawn attacks.

[00:16:04]Now, you have to understand prawners are generally seasonal breeders with the males during the naughty season excavating bull-shaped nests in soft sediment with a female then depositing her eggs in adhesive clumps that cling to submerged vegetation in the nest. And during this time, the soon tobe parents guard their starter home with all the aggressiveness you come to expect of prawns, attacking pretty much anything that enters the nest zone. And yes, that includes people, mainly their feet, that are trespassing on their home. So fun not so fun fact many of the documented prana bites on humans seems to be associated with defensive behavior during the breeding season not deliberate hunting or the likes with one 2025 review looking at 711 documented pronabites over a 10-year period and finding that roughly 82% of them were classified as mild with single bites instead of repeated ones which is exactly the pattern you'd expect from a defensive bite not a feeding strike and guess what about 30% of attacks happened during the breeding season, lining up perfectly with a nest defense hypothesis. While then another 26% was associated with improper food disposal in rivers, meaning that people were basically chumming up the water without realizing it. And so a good chunk of these incidents, in other words, prana attacks, trace back to humans either stepping into nests or just baiting the fish themselves, which is, you know, on us. Now, obviously, I'm not going to sit here and tell you that these guys are harmless because, well, they're uh not as serious wounds from piranhas have been documented. And while exceedingly rare, human fatalities attributed directly to prawns, have also been noted. But at the same time, there are many cases that seem to be a bit muddy in the sense that it's not clear if someone died due to drowning and then were partially eaten by piranhas, i.e. scavenging, or if it was the pras that actually took them out. But considering how extensive their range is and how abundant they are within said range, the fact that human deaths are so exceedingly rare does say a lot. But even with that said, the prana does rightly earn some hate. Perhaps not so much from us humans, but rather from other fish. Because like I said, prawns do sometimes feed on the fins and scales of their cousins. And while that doesn't kill them, obviously not everyone appreciates being forcibly stripped by an annoying big tothed fish. And so in response to this public indecency epidemic, some of them have developed some pretty interesting defenses to it, such as potentially the Arapima, one of the largest freshwater fish on Earth who can reach a staggering 3 m or 10 ft in length and weigh more than 200 kg or 440 lb. And what does big chungus do to prevent this? Well, it's built different. Not just from a size perspective, but also its scales. You see, each scale is made up of a hard mineralized outer layer and a collagenrich inner layer whose long fibers are arranged in rotating sheets in a configuration called a bullan pattern, which basically looks like a spiral staircase of collagen and allows the material to deform and absorb force rather than break under it. And researchers at UC San Diego and Berkeley tested our pima scales against actual prana teeth. And the results were quite shocking as not only could the scales easily withstand simulated pronabytes, but they were also capable of withstanding pressures up to 1.7 million pounds per square in which for perspective is close to twice the pressure that US nuclear submarine holes can withstand. So yeah, built different is perhaps even an understatement.

[00:19:19]There's even some hypothesis around the idea that our pima scales evolved specifically as anti- prana armor. But then again, there's also been others who refute it, saying that pranas are much too small to drive such evolutionary pressure among the Arappima. But it's certainly a cool idea, the idea that one of the world's largest freshwater fish had to develop some of the most complex scales ever seen just to survive. So yeah, piranhas pretty wild. Thanks for watching and until next time.