Larry Slawson received his Master's Degree from UNC Charlotte in 2018. He has a keen interest in biology.
Throughout South America, there lives one of the most feared predators of the Amazon. Known as the Red-Bellied Piranha or “Red Piranha,” this extraordinary species of fish is one of the most fascinating animals in the world due to its fierce reputation and insatiable appetite. This article provides an analysis of the Red-Bellied Piranha through an examination of the animal’s behavioral patterns and general characteristics. It is the author’s hope that a deeper understanding (and appreciation) of this remarkable animal will accompany readers following their completion of this work.
- Common Name: Red-Bellied Piranha
- Binomial Name: Pygocentrus nattereri
- Kingdom: Animalia
- Phylum: Chordata
- Order: Characiformes
- Family: Serrasalmidae
- Genus: Pygocentrus
- Species: P. nattereri
- Synonyms: Serrasalmus nattereri (Gunther, 1864)
- Conservation Status: Unknown (Not Evaluated)
Behavioral Traits and Characteristics
The Red-Bellied Piranha, also known as the “Red Piranha,” is a fish species found throughout South America. These fish are currently abundant in their local habitats, and are known to travel in shoals as a defense against larger aquatic creatures. The Red-Bellied Piranha belongs to the Serrasalmidae family, which describes a group of medium-sized characids, and includes fish such as the Pacus.
Despite being characterized as a vicious and ferocious fish, piranhas are actually relatively calm, and are a popular choice for many aquariums. Known for their grouping instincts, the piranha is also well-known for its nocturnal behavior; hunting for food between dusk and dawn. Despite its calm demeanor, however, piranhas are highly-efficient predators and pose considerable danger to fish, wildlife, and humans in times of hunger.
The piranha is known to possess long dorsal fins that line its compressed body. Reaching upwards of 8.6-pounds (3.9-kilograms), and lengths of nearly 20-inches (50-centimeters), the piranha is a relatively large fish with the ability to overpower smaller animals with ease. The animal also possesses a long jaw bone that contains a large array of razor-sharp teeth. These triangular-shaped teeth are similar to sharks, in that they neatly interlock with one another on the top and bottom of their mouth. Unlike sharks, however, the piranha’s teeth are usually not visible to observers as their thick lips often obscure them from view.
Giving power to these sharp teeth is a series of powerful muscles that are attached to the piranha’s jawbones. The positioning of these muscles affords the piranha remarkable bite force, allowing the animal to tear into prey with ease. Some specimens have recorded bite strengths of approximately 70+ pounds of force (approximately three times their own body weight)!
Rounding off their remarkable body is a rounded, snub-like nose that aids in the discovery of food. Similar to sharks, the piranha’s nose is capable of smelling blood from tremendous distances; alerting it to potential prey and easy meals. Recent studies on the Red-Bellied Piranha’s nose have indicated that the animal is capable of smelling one drop of blood within 200 liters of water (smithsonianmag.com).
Just as their name implies, the Red-Bellied Piranha has a reddish-colored belly, along with a grey and silver spotted upper-body. Females can be easily identified from the males due to their bellies being a darker shade of red. Likewise, juvenile piranhas are more likely to be silver in coloration before obtaining their reddish tint in adulthood.
Aside from their body, the piranha’s scales often take on a grey or silver coloration, with black spots forming around the gills and anal fin. The animal’s pelvic and pectoral fins, in contrast, usually follow a red or orange coloration that varies dramatically with age.
Although the Red-Bellied Piranha often feeds in a solitary manner, the fish is well-known for its natural tendency to travel in large groups (shoals). On average, the piranha tends to associate itself with groups of at least 20+ piranhas. As with all shoaling species, this instinctive behavior serves a variety of purposes. Large groups afford the animal greater protection from larger predators, while simultaneously allowing the piranha (and the shoal, collectively) to take down massive prey with ease.
Threat to Humans
Despite their fearsome reputation (a result of movies and television), the piranha poses a relatively low risk to humans. Although the animal has been observed consuming human flesh throughout history, nearly all cases of human contact occurred when the victim was already deceased (i.e. drowning victims). In fact, it is estimated that nearly 500 piranhas would be needed to devour an average-sized human (weighing 180-pounds) in 5 minutes (smithsonianmag.com). Given that the average shoal consists of 20 piranhas, the danger posed to humans is remarkably low. Nevertheless, experts continue to warn that extreme care should be taken when approaching the habitat of the Red Piranha. Deliberate provocation or haphazardly stepping (or swimming) near a shoal could lead to serious injury or harm.
Natural Habitat and Distribution of the Piranha
The Red-Bellied Piranha is found throughout much of South America due to the region’s tropical weather, warmer temperatures, and abundance of freshwater rivers and streams. The piranha thrives in the rivers of Argentina, Bolivia, Brazil, Ecuador, Colombia, Guyana, Peru, Paraguay, Uruguay, and Venezuela, and has particularly large populations in the Amazon River. The piranha prefers whitewater areas (areas that maintain a neutral PH level), as well as warmer water conditions between 59 and 95-degrees (Fahrenheit). Despite these preferences, some piranha have been spotted in blackwater regions (highly acidic zones of rivers and streams), and have been known to live in water temperatures of less than 50-degrees Fahrenheit.
Apart from major river systems and streams, the piranha is also common in various lakes, floodplains, and flooded forest regions of South America. To date, however, it has been observed that the majority of piranhas inhabit low-elevation regions of the Amazon River, with a clear preference for fast-moving segments of water.
Prey and Natural Predators
Although the Red-Bellied Piranha lives in shoals, they typically do not hunt in groups; preferring individual meals over sharing. For this reason, feeding frenzies are relatively rare occurrences, as the piranha tends to eat alone. In times of starvation or inadequate food, however, the Red-Bellied Piranha has been known to collectively attack large prey all at once; devouring their victim within minutes. Rather than being classified as a carnivore though, the piranha is actually considered an omnivore, and is capable of surviving on both plant and animal life.
As foragers, the piranha’s primary diet consists of insects, other fish, local plant life, as well as organic debris. They are also known to eat a variety of worms and crustaceans when opportunities arise. During times of starvation, piranha groups have been known to take down large animals, including the Egret, as well as the Capybara. Particular favorites of the piranha include small fruits (such as figs), shrimp, and earthworms. A large part of the piranha’s diet, however, consists of small bits of fins that the animal nips from larger fish as they pass close by (nationalzoo.si.edu). It remains unclear why the piranha actively feeds on fish fins. It has been hypothesized that the translucent appearance and motion of fish fins may serve as a natural attractant to the piranha species, in general.
Although the piranha is a natural predator, in its own right, the fish also faces an abundance of predators in the Amazon, including larger fish, anacondas, dolphins, caimans, and various aquatic birds. Humans also pose a significant risk to piranhas, as the animal is considered a delicacy in some cultures of the Amazon region.
Little is known about the breeding habits of the piranha, as they are difficult to observe in their natural habitats. Researchers believe, however, that a single female is capable of laying several thousand eggs at a time; hiding them near local vegetation, or near rocks. Mating begins after a male and female initiate a “courtship display” that involves swimming around one another in circles (nationalzoo.si.edu). Following reproduction, the male initiates construction of a bowl-shaped nest within sediment found around rocks or various crevices, allowing the female to lay her eggs with relative safety (nationalzoo.si.edu).
After only two to three days, the piranha’s eggs hatch, leaving the babies quite vulnerable to attack from larger predators due to their small size. To guard their young, adult piranhas periodically swim in small circles around the nest until the babies are old enough to swim on their own. Typically, the presence of two adult piranhas is enough to deter other fish from entering the nest. Larger characids, however, have been known to actively hunt baby piranhas in their early stages of development; particularly when the nest is left unguarded for brief periods of time.
Researchers currently believe that piranhas have two separate reproductive seasons that are tied to overall water levels and temperatures. Sexually active piranhas are also believed to lose their red coloration (a signal of their desire to spawn). It is during this period that males and females, alike, often migrate towards habitats that possess a variety of grasses or vegetation that is conducive to reproduction. More empirical observation is needed, however, to support these claims.
"I'm so used to swimming with the piranhas. And they're really not that bad."
— Brie Larson
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Communication and Signalling
In addition to travelling in shoals, researchers believe that the Red-Bellied Piranha is capable of communicating with its fellow piranhas through aggressive actions, as well as low-frequency signaling that is emitted from their muscles and swim-bladder. Three types of signaling have been observed by scientists, and are believed to be connected to the animal’s frontal display behavior, circling/fighting, as well as chasing. These various signals have often been compared to drum-like emissions that follow a harmonic pattern (similar to a bark). As a result, researchers believe that all sounds produced by the piranha are part of an intricate system of social communication.
Type One Sounds
Type one sounds are often harmonic, and last around 140 milliseconds at a reading of 120 Hz. Research currently associates “Type One” communication with the piranha’s frontal display behavior that typically occurs between two fish (particularly during the reproductive/mating process).
Type Two Sounds
Type Two forms of communication are far shorter, in comparison. On average, they last approximately 36 milliseconds at a reading of 40 Hz. These sounds are often associated with both fighting and circling behaviors, and are common when piranhas are competing with one another for food.
Type Three Sounds
Type Three signals are the shortest, and consist of a single pulse of sound lasting three milliseconds at a reading of 1,740 Hz. These louder signals are often associated with attacks, hunting, or when chasing specific fish or animals.
Piranhas in Popular Culture
There are numerous myths concerning the Red-Bellied Piranha that have been promulgated by Hollywood in recent decades. The 1978 film, Piranha, along with each of its sequels and remakes depict large shoals of piranhas attacking nearby humans, devouring them within seconds. However, such depictions are erroneous, as the piranha is generally quite timid. In fact, Red-Bellied Piranhas actually do well as aquarium fish, despite the need for extensive care and a large array of dietary options that include live and frozen fish on a regular basis.
Upkeep of piranhas is generally quite difficult due to the importance of maintaining good water quality, and proper lighting conditions for the animal. Inadequate care can prove disastrous for a pet piranha, as they are prone to disease and infection from dirty tanks, and are known to eat one another if proper diets are not followed thoroughly.
Because the piranha requires a variety of meat, feedings (particularly live feedings) can result in the spread of disease within your tank. This, in turn, can cause serious harm (or injury) to not only the pet piranha, but other fish living within the aquarium as well. As a result, regular cleanings are crucial for maintaining tanks suitable for the animal. It is also important to note that several species of fish, particularly the Goldfish, are known to contain growth-inhibiting hormones that have adverse effects on the piranha. Therefore, it is important for potential owners to research the type of fish they wish to introduce into their aquariums alongside the piranha to prevent problems down the road.
As of 2019, the Red-Bellied Piranha’s population figures appear to be both stable and abundant. In certain areas of the Amazon, the piranha is even considered one of the most common species of fish in the region. Due to their ability to rapidly reproduce, experts fear that the piranha could potentially cause issues in the Amazon with regard to other fish populations (destroying many of the local fish populations in the years to come).
Experts are also concerned about the piranha’s growing popularity abroad, as individuals continue to buy these animals as pets for their aquariums. This is problematic, as many individuals choose to release their piranhas into the wild, allowing them to spread beyond their natural habitats (worldwide). When this occurs, the piranha then becomes invasive to its new habitat as local animals are consumed by the animal in large numbers.
In closing, the Red-Bellied Piranha is one of the most fascinating animals in the world due to its natural behavior and unique characteristics that set it apart from other fish species. Because the piranha is both abundant and plentiful throughout South America (often outnumbering the majority of fish in the Amazon region), no conservation efforts have been established to protect this fish as of 2019. As more and more research teams are sent to South America to study these extraordinary creatures in the years and decades ahead, it will be interesting to see what new forms of information can be learned about their behavioral patterns, their signaling abilities, as well as their reproductive capabilities.
“Red-Bellied Piranha.” Smithsonian's National Zoo, July 12, 2018.
Thompson, Helen. “14 Fun Facts About Piranhas.” Smithsonian.com. Smithsonian Institution, July 8, 2014.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Larry Slawson
Larry Slawson (author) from North Carolina on January 27, 2020:
Thank you, Lorna! So glad you enjoyed :) I was actually wondering the same thing. They definitely show similarities to whales in a lot of ways. Would be an interesting theory to test, for sure.
Lorna Lamon on January 26, 2020:
Your love of biology shines through in these pieces Larry. I always thought piranha to be extremely fearsome fish, however, they do have their place in the ocean. Do they communicate in a similar way to whales? Fascinating article and a great read as always.
Larry Slawson (author) from North Carolina on January 25, 2020:
Haha thank you Eric. I really appreciate it. I totally agree.. won’t be adding them to my fish tank anytime soon either haha.
Eric Dierker from Spring Valley, CA. U.S.A. on January 25, 2020:
Cool your animal articles are always enlightening. The way you write them makes them so easy to learn from. I do not think I will be adding them to my fish tanks.
Larry Slawson (author) from North Carolina on January 24, 2020:
Thank you Pamela! All of this information was pretty new to me as well. Had always heard about the piranha, but had never read anything about them until this past week haha.
Pamela Oglesby from Sunny Florida on January 24, 2020:
All this information is new to me. I have heard of a red-bellied piranha but I had no idea that any fish could smell just a drop of blood. This is an interesting species for sure. I found this article to be very well-written and informative.