Pacu are primarily plant-eaters that feed heavily on fruits, seeds, and nuts, but they are true omnivores capable of shifting their diet dramatically depending on what their environment offers. In the wild, their meals change with the seasons, the local plant community, and whether the fish happens to be swimming in its native South American waters or in a river system halfway around the world. That flexibility is central to understanding pacu, and it makes a simple food list less useful than grasping why and how their diet changes.
Fruits, Seeds, and Nuts Form the Core Wild Diet
In their native range across South American river basins, pacu spend much of the year eating fruit that drops from overhanging trees into the water. This is not a casual snack. Gut-content studies of herbivorous serrasalmids consistently show that terrestrial plant material and fruits dominate the diet, with terrestrial plants reaching over 70 percent of dietary importance in some dry-season samples and fruits contributing around 20 to 30 percent depending on the season and location.1Brazilian Journal of Biology. Integrated study of indigenous people’ local ecological knowledge and diet analysis to understand the feeding ecology of the herbivorous serrasalmids in a dammed Amazonian River Pacu will consume whole fruits, crack open hard seeds, and strip plant parts that other fish cannot manage. Their relationship with fruit is so well developed that researchers classify certain pacu species, particularly Colossoma macropomum (the tambaqui or black pacu), as specialized frugivores.
A 2025 study investigating tooth mechanics in fruit-eating fish found that Colossoma teeth are tuned for exactly this kind of work. Their dental performance matched a pattern of low force but high work, meaning the teeth sink gradually into tough fruit flesh and seed coats rather than shearing through them quickly. This is the signature of a fish that evolved to process resilient plant foods, not one that merely tolerates them as filler.2PubMed Central. Something to sink your teeth into: the mechanics of tooth indentation in frugivorous fishes
How Flooding Reshapes the Menu
If you picture a pacu swimming through the same grocery store year-round, you are missing the biggest factor shaping its diet. South American rivers have dramatic wet and dry seasons. When floodwaters rise, they inundate riparian forests and suddenly give fish access to an enormous buffet of falling fruits and submerged vegetation. When the waters recede, that buffet disappears.
Research on pacu in different catchment areas shows that fruit consumption spikes during the flood season in upstream reaches, where riparian forest is more intact. In those same upstream areas, the fish eat more non-fruit plant parts during the dry season, when fruits are scarce. The pattern in downstream areas is different: fruit consumption stays relatively stable between seasons, and the fish eat more animal prey during the dry season instead.3Frontiers in Environmental Science. Spatiotemporal diet shifting in a neotropical fish species: The role of riparian vegetation and seasonal inundation The takeaway is that pacu do not follow one fixed diet. The surrounding landscape and the hydrological cycle dictate what ends up in their stomachs.
The consequences of disrupting that cycle are severe. When a dam blocked floodplain forests along one Amazonian river, a pacu relative (Mylossoma duriventre) that had been essentially vegan, living on fruits and seeds, switched to a carnivorous diet dominated by insects. The fish maintained its body condition in the short term, but researchers flagged the loss of its plant-based food supply as a direct and immediate ecological impact of the dam.4Forest Ecology and Management. Flipped reducetarianism: A vegan fish subordinated to carnivory by suppression of the flooded forest in the Amazon This kind of forced diet flip illustrates just how tightly pacu feeding is linked to the health of surrounding forests.
The Animal Side of an Omnivore’s Diet
Despite their vegetarian reputation, pacu regularly eat animal material. Insects, snails, crustaceans, and other invertebrates all turn up in gut analyses. The proportion varies by species, size, and habitat. In some settings, particularly where plant food is limited, animal prey becomes a significant part of the diet rather than a minor supplement.
Perhaps the most striking example comes from red-bellied pacu (Piaractus brachypomus) introduced to Papua New Guinea. In their native South American rivers, these fish feed mainly on terrestrial plants and invertebrates. In the Sepik-Ramu river system, however, researchers found that their diet was dominated by fish remains and aquatic plants. The fish remains included scales, bones, fin rays, and occasionally internal organs from other fish. Crucially, many of the scales were decaying and appeared to have been picked up off the river bottom, suggesting scavenging rather than active predation.5Neotropical Ichthyology. Diet shift of Red Belly Pacu Piaractus brachypomus (Cuvier, 1818) (Characiformes: Serrasalmidae), a Neotropical fish, in the Sepik-Ramu River Basin, Papua New Guinea So while pacu are not predators in the way piranhas are, they are perfectly willing to eat dead fish, scavenge carcasses, and exploit whatever protein source is available.
Teeth Built for Crushing
Pacu teeth look eerily like human molars, a resemblance that has fueled many a startled headline. But the similarity is superficial. Pacu teeth are engineering marvels tuned for a specific job: crushing hard seeds, nuts, and tough fruit.
Structural analysis of pacu (Colossoma macropomum) teeth reveals several features that support this crushing lifestyle. The teeth are broad and cusp-shaped. Their enameloid layer, the hard outer coating, is thicker and more irregular than that of piranhas, and the junction between enameloid and the softer dentin beneath has an interlocking pattern that resists cracking under compressive loads. On top of that, the outer cuticle layer of pacu teeth contains elevated concentrations of iron, which increases surface hardness.6PubMed. Tooth structure, mechanical properties, and diet specialization of Piranha and Pacu (Serrasalmidae): A comparative study Compare that to piranha teeth, which are conical, stiff at the tip, and designed for piercing flesh. The contrast in tooth architecture maps directly onto their feeding strategies.
These dental differences are not just academic curiosities. Genomic work on the entire piranha and pacu family (Serrasalmidae) has shown that traditional classification of these fish by tooth shape can be misleading, because similar diets can evolve independently in distantly related species. Herbivorous and omnivorous lineages within the family have converged on similar tooth forms multiple times, making teeth a surprisingly unreliable guide to evolutionary relationships even though they are an excellent guide to what the fish eats.7PubMed. Phylogenomics of Piranhas and Pacus (Serrasalmidae) Uncovers How Dietary Convergence and Parallelism Obfuscate Traditional Morphological Taxonomy
Pacu as Seed Dispersers
One of the more remarkable aspects of pacu feeding is what happens after they eat. Many seeds pass through the gut intact and are deposited elsewhere, making pacu important seed dispersers for riparian trees. This process, sometimes called ichthyochory (seed dispersal by fish), links forest health directly to fish populations in a way that most people do not expect.
A study of Piaractus mesopotamicus in Brazil’s Pantanal found a clear positive relationship between fish size and the number of intact seeds in the gut. Larger pacu had bigger gapes, ate more fruit, and dispersed more seeds of the palm Bactris glaucescens. The researchers raised a pointed concern: because commercial fisheries preferentially target the largest individuals, overfishing could undermine the dispersal system that certain tree species depend on for regeneration.8Biotropica. Big Fish are the Best: Seed Dispersal of Bactris glaucescens by the Pacu Fish (Piaractus mesopotamicus) in the Pantanal, Brazil In other words, the diet of a pacu is not just a question of fish biology. It is a question of forest ecology.
Not all seeds survive the trip, though. The same tooth adaptations that let pacu crush hard nuts mean they also destroy many seeds in the process. Whether a pacu acts as a seed predator or a seed disperser depends on the size and hardness of the seed relative to the fish’s bite force, and on how quickly gut passage occurs. For some plant species, pacu are indispensable dispersal agents. For others, they are primarily seed predators that grind up the reproductive package entirely.
What Happens When Pacu End Up Outside South America
Pacu have been introduced to waterways on multiple continents, sometimes through aquaculture escapes and sometimes through pet owners releasing fish that outgrew their tanks. When a pacu lands in a river system that lacks the fruiting trees it evolved alongside, its dietary flexibility becomes an ecological wildcard.
The Papua New Guinea case described earlier is the best-studied example. Red-bellied pacu introduced to the Sepik-Ramu basin shifted heavily toward fish remains and aquatic plants, items that played a smaller role in their native diet. Juveniles in the system ate 28 different food types spanning 11 broad categories, from aquatic vegetation to terrestrial fruits to carrion. The researchers concluded that this dietary plasticity makes pacu more likely to establish breeding populations after introduction, but also makes it harder to predict what damage they might do to native species.5Neotropical Ichthyology. Diet shift of Red Belly Pacu Piaractus brachypomus (Cuvier, 1818) (Characiformes: Serrasalmidae), a Neotropical fish, in the Sepik-Ramu River Basin, Papua New Guinea
This matters for anyone hearing about pacu caught in temperate rivers in the United States or Europe. These fish are unlikely to find the tropical fruits that form their ideal diet, but they are capable of surviving on whatever is available, including aquatic vegetation, invertebrates, and scavenged material. A hungry pacu is a resourceful pacu.
Feeding Pacu in Aquaculture
Pacu are one of the most commercially important freshwater fish in South America, and a growing body of research has worked out their nutritional needs in farm settings. The results paint a picture of a fish that is relatively easy to feed compared to carnivorous species, because it handles plant-based diets well.
Feeding trials on Piaractus mesopotamicus juveniles found that the best balance of protein, fat, and carbohydrate for growth fell at roughly 23 percent digestible protein, 4 percent lipid, and 46 percent carbohydrate. Higher fat levels did not improve growth and actually increased body and liver fat content. The fish used carbohydrates as effectively as lipids for energy, a trait that sets them apart from many carnivorous fish species that struggle with starchy diets.9Aquaculture Research. Growth and metabolism of pacu (Piaractus mesopotamicus Holmberg 1887) juveniles fed diets containing different protein, lipid and carbohydrate levels For adult pacu, optimum digestible protein levels for maximum body weight were modeled at around 23 to 25 percent, with economically optimized levels slightly lower, around 21 to 22 percent.10Aquaculture. Modeling optimum levels of balanced digestible protein for adult pacu (Piaractus mesopotamicus) and the development of an economic optimization procedure for different marketing strategies of the final yield
One of the most practical findings for the aquaculture industry is that fishmeal can be partially or totally replaced with soybean meal in pacu feeds without significantly affecting weight gain, feed conversion, or growth rate.11Global Aquaculture Advocate. Sources and levels of protein in pacu fingerling diets in Brazil Corn gluten meal can also substitute for a portion of soybean protein, improving growth performance and body yield at moderate inclusion levels without affecting blood parameters or fillet quality.12Aquaculture International. Corn gluten meal in pacu Piaractus mesopotamicus diets: effects on growth, haematology, and meat quality This makes pacu farming considerably more sustainable than raising fish that require heavy fishmeal inputs.
Plant protein-based diets supplemented with amino acids like methionine have also shown promising results, producing satisfactory growth while reducing nitrogen emissions into the water compared to higher-protein formulations.13Aquaculture. Dietary supplementation of lysine and/or methionine on performance, nitrogen retention and excretion in pacu Piaractus mesopotamicus reared in cages Even novel ingredients like spirulina (Arthrospira platensis) have been tested: adding spirulina to pacu diets improved growth, lowered blood cholesterol and triglycerides, and helped the fish cope with ammonia stress, likely due to antioxidant compounds in the algae.14Aquaculture. Diets containing Arthrospira platensis increase growth, modulate lipid metabolism, and reduce oxidative stress in pacu (Piaractus mesopotamicus) exposed to ammonia
Feeding a Pet Pacu
If you keep pacu in a home aquarium, the research on wild and farm-raised fish translates into a few practical principles. First, the foundation of the diet should be plant-based. Chopped fruits and vegetables like grapes, bananas, peas, squash, and leafy greens mimic the natural fruit-and-vegetation diet. Commercial herbivore pellets or omnivore pellets designed for large cichlids or catfish can serve as a staple, with fresh produce offered regularly for variety and enrichment.
Second, protein matters but does not need to dominate. The aquaculture research consistently points to digestible protein levels in the low-to-mid 20-percent range as optimal.15Aquaculture Nutrition. Digestibility and performance of pacu (Piaractus mesopotamicus) juveniles — fed diets containing different protein, lipid and carbohydrate levels Occasional protein-rich treats like earthworms, shrimp, or insects are appropriate and reflect the invertebrate component of the wild diet, but a diet heavy in animal protein is neither necessary nor ideal. Overfeeding fatty foods can lead to excess liver and body fat, the same problem seen in aquaculture trials with high lipid levels.
Third, you need to account for growth. Pacu grow fast and can reach sizes over 60 centimeters in captivity. A fish that fits in a 200-liter tank today may need something resembling a small swimming pool within a couple of years. Diet alone will not solve the space problem, but underfeeding to limit growth is not a humane strategy either. Anyone keeping pet pacu should plan for a very large tank or pond from the start.
Gut Anatomy Adapted for Plant Processing
Pacu intestines are long relative to their body size, a hallmark of herbivorous and omnivorous fish. A longer gut provides more surface area and transit time for breaking down tough plant material, which is harder to digest than animal tissue. The intestinal lining itself appears responsive to dietary changes. When maize distillers dried grains with solubles (a corn processing byproduct) were added to pacu diets, the fish showed dose-dependent improvements in intestinal health markers, including reduced inflammation-related changes in the intestinal lining and lower overall gut damage scores.16British Journal of Nutrition. Maize distillers dried grains with solubles alter dietary digestibility and improve intestine health of pacu, Piaractus mesopotamicus juveniles This kind of gut plasticity fits with a fish whose wild diet can swing from pure fruit to insect-heavy depending on the season.
An Ancient Lineage with Carnivorous Roots
For all their fruit-eating habits, pacu may descend from meat-eating ancestors. Analysis of fossilized teeth from a Plio-Pleistocene serrasalmid in Brazil found that the enameloid structure shared features with carnivorous fish lineages, despite the teeth belonging to the broader pacu/piranha family. The pattern of calcified fibers along the tooth periphery, with a random distribution internally, matched the arrangement seen in predatory species. Researchers interpreted this as evidence that the herbivorous and omnivorous members of the family may have evolved from carnivorous stock.17Zoologica Scripta. Dental Histology of a Characoid Fish from the Plio‐Pleistocene of Acre, Brazil
This evolutionary trajectory, from predator to plant-eater, runs counter to the assumption many people hold that herbivory is somehow a “primitive” default and carnivory is the sophisticated specialization. In the serrasalmid family, the shift toward crushing fruits and seeds was the derived innovation. The molar-like teeth, the reinforced enameloid, the iron-enriched cuticle layer, and the long intestine all represent evolutionary investments in a plant-based lifestyle that has been enormously successful across South American river basins for millions of years.
Why the “Vegetarian Piranha” Label Misleads
Pacu are often described in popular media as “vegetarian piranhas,” a label that gets two things wrong at once. It overstates their commitment to plants (they eat animals regularly) and it frames them as a variant of piranhas rather than a distinct group in their own right. While piranhas and pacu do belong to the same family, the dietary distance between a specialized frugivore like Colossoma macropomum and a predatory piranha like Pygocentrus nattereri is enormous. The microstructure of their teeth alone tells the story: pacu teeth are built to withstand compressive forces from hard seeds, while piranha teeth are optimized for piercing and slicing.18PubMed. Microstructural and compositional variation in pacu and piranha teeth related to diet specialization (Teleostei: Serrasalmidae)
The more accurate picture is that pacu sit along a spectrum. Some species are heavily frugivorous. Others are broad omnivores that eat whatever is available. And all of them are capable of adjusting their diet when circumstances force a change, whether that change comes from seasonal flooding, dam construction, or introduction to an entirely new continent. If there is one consistent rule about what pacu eat, it is that the answer depends on where they are, what time of year it is, and what options the river gives them.