Piranhas-WCC STEM Scholars Newsletter October 2023

When Steven Spielberg released his 1975 classic Jaws, often regarded as the first ever summer blockbuster, it’s not surprising that many other movie studios immediately decided to follow in the movie’s footsteps in order to capitalize on the movie’s success. In Jaws’s wake, the 1970s and 1980s had many movies about a police officer and a scientist facing off against carnivorous aquatic creatures. Aside from facing sharks, our heroes have squared off against aggressive killer whales (Orca, 1977), mutant octopi (Tentacles, 1977), and vicious crocodilians (Alligator, 1980). But perhaps the most famous of these knock-offs was the 1978 film Piranha, directed by Joe Dante and executive produced by Roger Corman. The movie focuses on a school of piranhas held in a military facility that are accidentally released into a lakeside summer camp and resort. Unlike Jaws, Piranha has a more self-aware premise, not taking itself as seriously, resulting in a campy experience that has garnered it a huge cult following. This following earned it two sequels, the first directed by a young James Cameron in his debut, and a 3D reboot duology. Steven Spielberg even praised Piranha as his favorite Jaws knock-off, such to the point that it saved the movie from a plagiarism lawsuit from Universal. However, much like the earlier movie that it parodies, Piranha, along with other media, has fueled a lot of misconceptions about the titular fish and has helped establish them as vicious little carnivores capable of stripping a human to the bone in a minute. So, what are piranhas really like?

Piranhas, meaning “tooth fish” in the Tupi language (a mostly extinct indigenous tribe from Brazil), can be found all over the rivers and lakes of South America, particularly in the Amazon region. The name originates from their sharp, triangular-shaped teeth which interlock together almost like pieces of a puzzle, giving them the ability to tear out big chunks of meat. Their teeth are so sharp that many native tribes in the region used them to make scissors by removing their jaws after catching them for food. These tribes have also used just the teeth themselves to make other cutting and carving tools. Much like sharks, piranhas are constantly replacing teeth that have either been lost or worn down, but unlike sharks, which replace their teeth one at a time, piranhas have half a jaw’s worth of teeth replaced. The reason for this is because the teeth need to be perfectly aligned in order to interlock with each other properly.

Having piranhas being gigantic sounds like a major plot point in many films of its ilk, and in light of the 2010 3D remake of Piranha, the notorious mockbuster studio, The Asylum, released such a film called Megapiranha the very same year. As silly and self-explanatory as the title is, such an animal can actually be found in the fossil record. Much like how modern-day mackerel sharks had a massive ancestor with the Megalodon, modern-day piranhas had their own massive ancestor with Megapiranha, estimated to be 3-5 times their size. Unlike modern-day piranhas, which have a single row of teeth, Megapiranha had two rows of teeth arranged in an alternating fashion. Furthermore, the sharp and triangular-shaped teeth of a piranha overall, the bases of the teeth are also broader and thicker, much like those of a pacu (a close relative of the piranha that will be discussed later). This evidence indicates that Megapiranha was the common ancestor linking the two fish together. Using its modern-day relatives as test subjects, a group of scientists from the American University in Cairo, Egypt discovered that Megapiranha probably add the strongest bite force in proportion to its body size of any fish, even bigger than that of the Megalodon. Combined with its unique set of teeth, the massive bite force enabled the fish to both penetrate protective armor and even break their bones of its prey for a faster kill. Megapiranha lived about 8–10 million years ago, but piranhas as we know them did not evolve until two million years ago. Within that time frame, the Atlantic Ocean levels began to rise and fill up the flood plains of South America which caused the water to become salty. To escape this environment, piranhas swam upriver to higher altitudes, eventually evolving into the modern ones we have today.

Nobody knows for sure how many species of piranha there are, but scientists have identified them into four main genera: Pygocentrus, Serrasalmus, Pristobrycon, and Pygopristis. Depending on the species, piranhas can come in many sizes, ranging from six inches to just under two feet in length. Here are some of the most notable kinds:

Probably the first species that people think of when they hear the word “piranha” is the Red-bellied piranha (Pygocentrus nattereri), and the reason why is not surprising, considering that these are the ones that sparked many of the rumors surrounding the fish. Almost always found in schools, Red- bellied piranhas are among the most social piranha species and also inhabit the widest geographic range. They can growing up to a foot in length and weigh almost nine pounds. Although they are best known for being carnivores, they are actually opportunistic omnivores, and are just as willing to feed on insects as well as fruit and other plant matter. One of the most unique qualities of these piranhas is that they can rapidly contract muscles near their swim bladders to make acoustic sounds in order to communicate with each other, either while fighting off rivals, attacking prey, or warning others of dangers.

Though it’s known by a variety of common names, the São Francisco piranha (Pygocentrus piraya), gets its name for being located in the São Francisco River in Brazil. They can grow to be more than a foot long and weigh almost eight pounds, making them the largest species in the Pygocentrus genus. Like other piranhas in its genus, it is rather aggressive when threatened, breeding, or extremely hungry, and can be dangerous to humans during. They prefer breeding in flooded vegetation in deep river areas, seeking cover in foliage.

Growing over a foot-and-a-half in length, the Black piranha (Serrasalmus rhombeus) is the largest species of piranha known today. Despite their name, Black piranhas are usually lighter colored with various blotches as juveniles, and darken when they mature, turning black or even dark gray. However, they always maintain their distinctive red eye color, which is why they are also known as the red-eye piranha. Unlike many other piranha species, Black piranhas are more solitary and are hardly seen among others of their kind except for feeding frenzies and during mating season. The most notorious aspect about this species is its bite force. At about 30 times its own body weight, it has the most powerful bite of any animal living today. In fact, scientists have used the Black piranha as a living reference to test the bite force of its monstrous ancestor, Megapiranha.

The Marbled piranha (Pristobrycon maculipinnis) gets its name from the dark spots that run along its silvery body, giving it a marbled appearance. It can be found in blackwater habitats, which are dark-colored waters resulting from the decomposition of plant matter. This results in the being more acidic and having fewer nutrients and oxygen levels. Like other members of the Pristobrycon genus, Marbled piranhas have not been as extensively researched. All current information has been obtained obtained mainly through preserved or captive specimens instead of being observed in the wild.

Another fairly large piranha in the Serrasalmus genus, the Manuel's piranha (Serrasalmus manueli) can grow to be over a foot long, although some locals claim it can be almost two feet in length, though there is no hard evidence to confirm this. While not quite on the same level as Red-bellied piranhas or Black piranhas, these piranhas are very popular in aquariums. Although some other piranhas are more adaptable to any changes in water quality, Manuel’s piranhas are more delicate and cannot survive major changes in oxygen or temperature. Juveniles have earned the nickname Green Tiger piranhas, due to the vibrant green coloration of their bodies, although they lose this coloration as they mature.

The sole member of the genus Pygopristis, the Lobe-toothed piranha (Pygopristis denticulata) is among the rarest piranhas. It is one of the smaller species, at less than eight inches long. Its most striking feature is that its teeth have five pointed projections (pentacuspids) as opposed to the three (tricuspids) that are held by other piranhas. Because of this, there is debate among the scientific community if Lobe-toothed piranhas are truly considered piranhas.

As a member of the Catoprion genus, the Wimple piranha (Catoprion mento) is not really considered to be a true piranha by many piranha experts. The reason why is because its teeth shape and arrangement are different than those in the other genera. However, if scientists did consider it a true piranha species based on the genetic analysis alone, it would be the smallest species, at only six inches in length. Wimple piranhas are famous for their diet, which consists primarily of nibbling on the scales and fins of other fish. As such, it has one of the narrowest diets of all fish.

With such razor-sharp teeth and strong biting jaws, as well as their notable trait of mostly living among others, it's not unreasonable to believe that piranhas are voracious little predators that work together to take down prey much bigger than themselves. However, this is actually not the case. Piranhas, despite their reputation, are not on top of the food chain; they are common prey for animals like caiman, river dolphins, waterbirds and even other fish. As such, they live in schools as a form of protection, much like herds of caribou as opposed to hunting in packs like wolves. On top of that, piranhas will usually only prey on much larger animals if they have already been sufficiently injured or dead, more scavengers as opposed to predators. Whenever they are active predators, they usually prey upon smaller fish and other aquatic creatures. Some other piranhas, like the aforementioned Wimple piranhas are happy just to be munching on the scales and fins of other, much larger fish in small hit-and- run attacks, allowing their prey time to heal afterwards. Piranhas are also not exclusively carnivores; they are actually omnivores, much like people, and are just as happy to dine on fruit that have fallen into the water. In fact, piranhas have a very close relative that is strictly vegetarian, and interestingly enough, one that has also fallen victim to similar rumors of attacking people.

Being close relatives to piranhas, pacus have a very similar appearance, except they don’t have the underbite that piranhas do. Furthermore, instead of a single row of sharp, triangular teeth, they have two rows of teeth that are squarer and neatly aligned, much like a human. These teeth are ideal for crushing seeds, fruits, nuts, and other vegetation. While they will occasionally eat snails, worms, or very small fish, they are primarily herbivorous. On top of that, pacus are usually much bigger than piranhas, growing more than two feet long as opposed to piranhas which don't often get much bigger than a foot. While they are native to South America, pacus have been introduced to many other areas, becoming invasive species to the United States, Asia, and even Europe. In Asia, pacus have earned the nickname the “ball cutter” or “nutcracker fish” as they gotten a reputation for chomping on another type of nut. There have been several reports of pacus castrating male swimmers and fishermen in Papua New Guinea, resulting in a few cases of death from blood loss. A Danish newspaper reciprocated these fears after a pacu was found in Scandinavian waters. However, there is not enough sufficient evidence to verify the credibility of these accounts in Papua New Guinea and the authors in Denmark even admitted their article was meant to be taken as a joke. Although their crushing teeth and jaws would definitely be hazardous if you stuck any part of your body in their mouth, pacus pose no real threat to people.

So, with all things considered, why have piranhas been given such a bad reputation? A good chunk of it can be blamed on former president Teddy Roosevelt. While he was on an expedition to Brazil in 1913, a group of local fishermen led a cow into the water. The cow was ripped apart within minutes, leaving behind only a skeleton that was picked clean. As he stated in his memoir, Through the Brazilian Wilderness: "They are the most ferocious fish in the world. Even the most formidable fish, the sharks or the barracudas, usually attack things smaller than themselves. But the piranhas habitually attack things much larger than themselves. They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers—in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness. They will tear wounded wild fowl to pieces; and bite off the tails of big fish as they grow exhausted when fighting after being hooked.” What Roosevelt was unaware of was that these piranhas had been cut off from the rest of the river and were starved as part of a planned show by the fishermen to amaze the president. When the cow was brought in the water, it comes as no surprise that the fish went crazy. This account was likely among the first that started many rumors about these fish.

However, there also have been other stories that have added to the piranha’s fearsome reputation. On the night of September 19, 1981, the Sobral Santos II, a Brazilian ferry, was travelling between Santarém and Manaus as part of its weekly run. The boat was overcrowded with more than 500 people and their cargo and ended up capsizing in the port of Óbidos. More than 300 people perished in the disaster. Many survivors who ended up in the water reported other swimmers being yanked under. Most of the remains were found all chewed up and dismembered, and as such, many blamed piranha attacks for most of the death and carnage—similar to the shark attacks following the sinking of the USS Indianapolis in 1945. However, even the largest schools of piranhas do not have the ability to pull people underwater. According to Jeremy Wade, British fisherman, zoologist and host of Animal Planet’s River Monsters, the victims most likely were drowned by large catfish who accidentally mistook them for prey. Other bigger predators, like caimans, soon followed in the disturbance. It is more than likely that when the piranhas finally arrived, it was to scavenge the remains of those who were already dead.

The fact of the matter is that people have been swimming in areas where piranhas have been present for many generations, and there have been very few recorded attacks. Even when attacks do occur, they rarely result in fatalities. Many scuba divers exploring the Amazon River have surrounded themselves with piranhas, filming them in their habitat without any incident. Piranhas, much like sharks or any other animal for that matter, are rather wary of humans and prefer to stick with their natural prey, as well as any carrion or vegetation they come across. If you happen to be swimming in an area where piranhas are present, there are a couple of steps that can be taken to avoid attacks. Don't go swimming in the water if you are bleeding or have open wounds, and don't splash around excessively. Much like sharks, piranhas are attracted to the smell of blood and thrashing around. Unlike sharks, which are more active in areas where people swim at night, piranhas are diurnal, hunting by day and sleeping at night, so it's best to wait until nighttime if you have to get into the water, though there definitely are other predators to worry about, like caimans. Also, avoid going into the water during the dry seasons, as food isn't as plentiful, and piranhas will be more opportunistic. These are animals that deserve to be treated with the same respect as any other. As long as you leave them alone and don't harass or provoke them, they will generally leave you alone.

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