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Pacu Fish: The Piranha Cousin With Human-like Teeth

Human thumb holding down a fish's lower lip to reveal human-like teeth
We apologize in advance for the nightmarish qualities inherent to the pacu fish, an exotic species that appears to have human teeth. jean-claude soboul / Getty Images

In 2023, 11-year-old Charlie Clinton from Oklahoma made headlines by catching a pacu — a South American fish known for its human-like teeth — in a pond behind his house, far from its native Amazon habitat.

The pacu fish was released back into the pond under the local catch-and-release guidelines before the Clintons realized its potential impact on the ecosystem. The Oklahoma Department of Wildlife Conservation has since highlighted the ecological risks of releasing non-native species into local habitats. They advise anyone who catches a pacu to keep it to prevent disruption to local wildlife.

This rare occurrence of a South American species in Oklahoma waters has drawn attention to the broader issue of invasive species and their effects on ecosystems. But what exactly do we know about this fish, and just how dangerous is it to non-native habitats?

Scientific Classification

The term "pacu" refers to several species of freshwater fish that are native to South America, known for their distinctive square, flat teeth that resemble human molars.

Pacu fish belong to the family Serrasalmidae, which is part of the order Characiformes, a large and diverse group of freshwater fish found predominantly in the Americas, especially in South America.

Within the Serrasalmidae family, pacus are closely related to piranhas but are distinguished by significant differences in morphology, diet and behavior.

The taxonomy of pacu fish has evolved with ongoing research and genetic studies, leading to a more nuanced understanding of their relationships within the family Serrasalmidae. Pacus were once classified under the family Characidae, but later revisions placed them in Serrasalmidae due to their close relation to piranhas and other similar fish.

Pacus encompass several genera, including Piaractus and Colossoma, among others. Species within these genera, such as Ossubtus and the black pacu (Colossoma macropomum), are known for their distinct characteristics, such as large size, unique dental structure and primarily herbivorous diet.

Types of Pacu Fish

Here's a rundown of some pacu species within the Serrasalmidae family, distinguished by their size, color and specific habitat preferences.

  1. Red-bellied pacu (Piaractus brachypomus): One of the most recognized pacus, the red-bellied pacu is similar in appearance to the red-bellied piranha but grows much larger and has a more peaceful temperament. They are known for their distinctive red belly, especially when young.

  2. Black pacu (Colossoma macropomum): One of the largest pacu species, black pacus are known for their dark coloration and rounded bodies. It's a popular species in aquaculture, due to its rapid growth rate and adaptability to different environments.

  3. Pacu piranha (Piaractus mesopotamicus): Also known as the small-scaled pacu, this species is native to the Paraguay-Paraná and Uruguay river basins. It's popular in sport fishing due to its size and strength.

  4. Xingu river pacu (Ossubtus xinguense): Also known as the parrot pacu, this freshwater pacu is native to the Xingu River in Brazil, a major tributary of the Amazon River. The species is characterized by its silvery body with a slight greenish hue. It can grow to a considerable size, making it noteworthy among the pacu species.

Native Range in South America

Pacus are native to South America and inhabit a wide range of freshwater environments across the continent. Their natural range includes the Amazon basin, which spans multiple countries such as Brazil, Peru, Colombia and others.

This vast region, with its extensive river systems, floodplains and forested waterways, provides an ideal environment for pacus, offering abundant vegetation and fruit to support their herbivorous diet.

Another significant habitat for pacu fish is the Orinoco basin, primarily in Venezuela and Colombia. The Orinoco River and its tributaries form a complex network that serves as a home for various pacu species. These waterways offer a mix of fast-flowing rivers and quieter backwaters, both suitable for the different life stages of pacus.

In addition to these major basins, pacus can also be found in the Paraguay-Paraná and Uruguay river basins. These diverse ecosystems — from clearwater to blackwater rivers to deep channels and shallow flood zones — provide pacus with various habitats that cater to their needs for food, shelter and breeding grounds.

Outcompeting Native Species

Originating from South American freshwater rivers, these piranha relatives are integral to their native ecosystems, aiding in seed dispersal due to their herbivorous diet. However, when introduced to non-native environments, they can become invasive and threaten local ecosystems.

They will compete with native species for food and habitat, prey on smaller fish and invertebrates and introduce new diseases and parasites to which native species have no immunity. These issues are particularly pronounced in regions where pacus have been introduced without natural predators, allowing their populations to grow unchecked.

Responsible pet ownership and effective management/control strategies are crucial in protecting biodiversity and maintaining the balance of aquatic ecosystems where these fish are not naturally found.

Unusual Features of the Pacu

Pacus have several distinct physical features that differentiate them from other fish.

Teeth

Their most notable characteristic is their human-like teeth, which are square and flat and resemble human molars (quite different from their piranha cousins' sharp, pointed teeth). This dental structure is unusual among fish and specially adapted for their natural feeding habits.

Size

Aside from their human chompers, they're also pretty massive in size. Pacu fish can grow much larger than many other freshwater species. For example, the black pacu, a particularly large pacu species, can reach 3 feet (almost 1 meter) in length and weighs around 65 pounds (a little over 29 kilograms).

Fins and Body Shape

Their plate-shaped bodies are robust and somewhat rounded, distinguishing them from the more streamlined bodies of predatory fish.

Pacus are equipped with several fins that each serve specific functions, contributing to their adept maneuverability and stability in the aquatic environments of South America.

The dorsal fin, situated on the fish's back, plays a pivotal role in maintaining upright stability, aiding the pacu in navigating through slow-moving rivers and avoiding potential threats with ease.

This fin is particularly important for preventing the fish from rolling. It assists in making sudden turns and stops, which is crucial for a species that often dwells in environments with complex vegetation and varied topography.

They also have a well-developed caudal (tail) fin, which provides the propulsion necessary for swimming, chasing down food and evading predators. The anal and pelvic fins, located on the underside of the fish, further aid in stability and steering, allowing the pacu to make precise movements through the water.

The pectoral fins on its sides function like rudders, helping the fish steer and maintain balance.

Plant Diet and Feeding Behavior

These subtropical fish are primarily herbivores, meaning their diet consists mostly of plant material. They consume various plant-based foods, including decaying plant matter, fruit, nuts and seeds, and aquatic vegetation in the wild.

While their diet is mainly plant-based, some pacus might occasionally eat insects, small fish or other aquatic invertebrates, making them omnivorous. This behavior is more common when their preferred food sources are scarce.

You may be wondering how fruit could be a part of a freshwater fish's diet, but there's an explanation for that!

Pacus are often surface feeders, coming to the top of the water to feed on fruit and seeds that fall into the water from overhanging trees. This behavior is more commonly observed in their natural habitats, like the Amazon basin, where these abundant food sources exist.

In addition to surface feeding, pacus also forage for food along riverbeds and among aquatic plants, using their specialized teeth to grind down tougher plant materials.

By consuming fruits and defecating the seeds elsewhere, pacu fish play a vital role as seed dispersers in their ecosystems. This behavior helps in the propagation of various plant species, contributing to the health and diversity of their habitats.

Are Pacus Safe for Humans?

Pacu fish are generally not considered a danger to humans. Unlike their carnivorous relatives, the piranhas, pacu are primarily herbivorous, and their interactions with humans are typically non-aggressive.

There have been occasional reports of pacu nibbling at swimmers, likely due to curiosity or mistaking appendages for food, but such incidents are rare.

As for consumption, pacus are indeed safe to eat and are a popular food source in their native regions of South America. The fish is known for its white, mild-flavored flesh, which is often compared to that of tilapia or other freshwater fish.

In regions where pacu is farmed or caught in the wild, they are prepared in various ways, including grilling, baking or frying.

However, as with consuming any fish, it's important to be aware of local advisories regarding water quality and potential contamination. In areas where the water may be polluted or where pacu are not a native species, you should take caution to ensure the fish is safe for consumption, based on local environmental conditions.

Can You Own a Pet Pacu?

Young Charlie Clinton's surprising pacu discovery in Oklahoma raises the question: Could he have kept it? Pacu ownership is regulated by local laws, which vary based on potential ecological concerns. In regions where keeping pacu as pets is permitted, they are sometimes sold in aquarium shops and pet stores.

However, the reality of keeping a pacu involves significant considerations. These fish can grow very large, often necessitating a spacious tank or pond, far beyond the capacity of standard home aquariums.

This size aspect means a considerable commitment to space and maintenance. Moreover, their care, particularly their mostly herbivorous diet, requires attention to ensure it aligns with their natural feeding habits.

For anyone contemplating pacu ownership, it's crucial to check local regulations to ensure compliance with ownership laws. Understanding the full scope of care for a pacu, from their dietary needs to their substantial space requirements, is essential before taking on such a responsibility.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Now That's Interesting

Before 2013, the red-bellied pacu was not widely known globally. However, this changed when a sensationalized news report issued warnings to Scandinavian men about the risks of swimming nude in the Oresund Strait, the waters between Denmark and Sweden, citing the potential danger of having their testicles bitten by the fish. The report featured a quote from biologist Henrik Carl of the Natural History Museum of Denmark, which confirmed the danger. (More on that shortly.) Despite the media frenzy that followed, the sensational story is largely unfounded. Carl, a fish expert at the museum, clarified that while the pacu has a strong bite and there have been rare incidents in places like Papua New Guinea, the museum's initial warning, which labeled the pacu as the "ball cutter," was intended more as a humorous caution than a serious threat.

Original article: Pacu Fish: The Piranha Cousin With Human-like Teeth

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