Piaractus brachypomus

Geographic Range

The natural range of red-bellied pacu extends from 23°N to 11°S latitude, in the Amazon and Orinoco river basins/flood plains. Common names for this species vary by region. It is known as pirapitinga in Brazil, paco in Peru and cachama blanca in Colombia. Introductions of red-bellied pacu populations have been reported in many regions around the globe, in places as unlikely as Vancouver, British Columbia. Occurrences such as these are most likely a result of fish outgrowing hobbyists’ aquariums and the owners releasing fish into local waters. Red-bellied pacu were introduced to India sometime between 2003 and 2004 from Bangladesh and have become the focus of several aquaculture projects.

• Biogeographic Regions
• oriental oriental
  • introduced introduced
• neotropical neotropical
  • native native

Habitat

As fry and juveniles, red-bellied pacu can be found in and around floodplains of nutrient rich tributaries, or in headwaters when nutrients are poor. Pacu move further out into main waterways as they mature. Optimal water pH is 6.8, with an optimal temperature of 26°C. Considered a mid level swimmer, this species is found at depths of up to 8 meters.

• Habitat Regions
• tropical tropical
• freshwater freshwater

• Aquatic Biomes
• rivers and streams

• Wetlands
• marsh marsh
• swamp swamp

Physical Description

Red-bellied pacu are often confused with other pacu ( Piaractus mesopotamicus , Colossoma macropomum ) or piranha ( Serrasalmus nattereri ) species, due to their similar appearance. The body is deep and laterally compressed, with silvery sides (becoming darker approaching the dorsum) and red coloration on the belly, chin, pectoral fins, and occasionally the leading rays of the anal fin. The remaining rayed fins are uniformly dark-colored. As in other characin species, a small, unrayed adipose fin is present approximately midway between the dorsal and caudal fins. The dorsal fin contains 15-18 rays, the pectoral fins 16-19, the anal fin 24-28, and the pelvic fins 8. The first few rays of the dorsal and anal fins are longer than the remaining elements. A row of sharp serrae formed by modified scales is found on the abdomen. Although not as well-developed and sharp as in their piranha cousins, red-bellied pacu have two rows of hard, flattened teeth used for crushing seeds and nuts. This dentition is comprised of 2 series of molariform incisors located on the premaxilla and 1 row of dentary teeth. The largest individuals can weigh up to 25 kg and measure 88 cm, though these numbers are usually lower in captivity. Smaller captive sizes are most likely due to insufficient nutrition; the notion that ‘they will grow to fit their environment’ is a widespread myth.

As juveniles, red-bellied pacu mimic piranha by displaying dark grey to black spots on the body, a standard characteristic of piranha. This wards off attacks by predators, including piranha themselves, when pacu are at a vulnerable age. As pacu get older and surpass the size of an average piranha, the spots disappear.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

Eggs generally hatch between 12 and 20 hours after fertilization. At birth, fry are approximately 2 mm long and weigh .16 grams. By the time that they hatch, pacu fry have a fully developed nervous system with a heightened number of neurons in the fore- and hindbrain. This is believed to correspond to the main motor cortex and allows quicker reaction by tail muscles in prey acquisition or predator escape. After a month, juveniles more than quadruple in weight and almost double in size. Pacu reach sexual maturity around 3 years of age. Except for the body spots displayed by juveniles, young fish look similar to adults.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

During spawning, females scatter adhesive eggs; fertilization then occurs externally. Although parents abandon their eggs, pacu are brood hiders, minimizing the chances of the clutch being discovered by predators and scavengers.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Breeding occurs in annual cycles with the onset of the wet season. Spawning begins as early as November, when the waters first begin to rise, and can last through February. Prime spawning conditions are achieved at 4.0 mg/L of oxygen and 27°C. Spawning is preceded by the release of just a few eggs by females and a ‘knocking’ sound produced by the males. On average, eggs measure 1.2 mm across and weigh just above 1.6 mg. In large pacu species, mature females lay an average of 150,000 eggs, though this number can occasionally reach 1,000,000. After laying a clutch, females can be ready to spawn again in 10 weeks. Although males produce sperm year-round, one study found that females were sensitive to seasonal cycles and sometimes would not ovulate even after artificial induction.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external
• broadcast (group) spawning
• oviparous oviparous

Besides attempts to hide egg clutches, pacu eggs and juveniles receive no parental care.

• Parental Investment
• pre-hatching/birth
  • protecting
    • female

Lifespan/Longevity

Red-bellied pacu are long-lived fish, with popular aquarium sites quoting life spans of 25 years and beyond. One account by MSNBC reported on a 43 year old black pacu Colossoma macropomum , a similar species that is often confused with its red-bellied relative.

Behavior

In native environments, red-bellied pacu are shoaling fish that may become more independent with age. A pacu’s entire life is characterized by annual cycles corresponding to the rainy and dry seasons. They migrate upstream during the months of June through October and then, in response to the first rains of the wet season, start to spawn from November to February. All pacu species are known for being able to adjust to unfavorable environmental conditions, such as depleted oxygen levels. Behavioral responses to these conditions include spending greater amounts of time near the surface, increased movement, and protrusion of the lower lip, all of which facilitate the use of surface water in respiration.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• motile motile
• migratory migratory
• territorial territorial
• social social

Home Range

Information regarding the home range size of this and other pacu species is currently lacking. Though young individuals form shoals and are peaceful to conspecifics, observations of aquarium specimens have indicated that aggression and territoriality increase as the fish age.

Communication and Perception

Pacu, like other characins, can be very sensitive to environmental disturbances. In addition to the normal range of sounds detectable by other fishes, they can detect higher-pitched sounds through the use of a modified osteological complex (composed of several anterior vertebra and other bones and known, collectively, as the Weberian apparatus) that bridges the swim bladder with their inner ear.

Pacu are capable of intraspecific communication using an alarm chemical known as "Schrekstoff". If one fish is injured, this substance is discharged from the wound into the water, alerting nearby conspecifics and related species of possible danger.

Pacu species also use their lateral line system when identifying movement in the water, which aids in maintaining proper shoaling behavior.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

• Other Communication Modes
• mimicry mimicry
• pheromones pheromones
• vibrations vibrations

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• vibrations vibrations
• chemical chemical

Food Habits

The dietary composition of red-bellied pacu (and a number of closely related species) shifts depending on the season. During the wet season, they rely heavily on seed predation from the newly dropped fruit of riparian trees and plants. Although red-bellied pacu are widely considered to be frugivores, they are actually omnivorous, also eating crustaceans and smaller fishes, especially in the dry season. As some of the largest fish in the Amazon, pacu require large amounts of food. They feed in multiple “bite events”, with each event containing a number of individual bites, which is similar to the feeding behavior observed in true piranhas.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • insectivore insectivore
  • molluscivore molluscivore
• herbivore herbivore
  • frugivore frugivore
  • granivore granivore
• omnivore omnivore

• Animal Foods
• fish
• insects
• mollusks
• aquatic crustaceans

• Plant Foods
• leaves
• seeds, grains, and nuts
• fruit

• Other Foods
• detritus detritus

Predation

Information regarding specific predators of red-bellied pacu is not available. Larger fishes and wading birds likely represent the greatest predatory threat to juveniles. The large size attained by adult pacu would protect them from all but the largest predatory fish species, such as pirarucu ( Arapaima gigas ). Black caiman ( Melanosuchus niger ) are another likely predator of adult red-bellied pacu. Pacu are popular aquaculture species and native populations are subject to predation by humans.

Ecosystem Roles

The role of pacu species is integral to the growth and development of the Amazonian habitat. With the rise of seasonal flood waters, their diet shifts almost exclusively to seeds and nuts from the trees and plants that line the river, making these fish an important vehicle of seed dispersal. A study of the contents of pacu stomachs recorded seeds from 27 tree and 26 non-woody plant species. This relationship has been negatively impacted by climate change, ranching, irresponsible farming practices and logging.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

As robust, rapidly growing species prized for their meat in many South American countries, pacu are increasingly attractive candidates for sustainable aquaculture projects in many regions. A recent increase in research on red-bellied pacu shows clear links to these commercial interests, with many studies focusing on optimal feeding strategies. Not all pacu farming is large scale, as pacu are cultured in many rural areas for use as a direct food source or income supplement. Hybridization of red-bellied pacu and other closely related species is being explored to combine the most favorable aquacultural attributes of these fishes.

• Positive Impacts
• pet trade pet trade
• food food

Economic Importance for Humans: Negative

Concerns have been raised about the potential impact of red-bellied pacu as an invasive species. Due to their large size and food requirements, it may easy for pacu to outcompete native species. Hybridizations of red-bellied pacu with common carp Cyprinus carpio and other exotic species are also currently being tested. Although these cross-species strains are intended to establish favorable traits for aquaculture, the intelligence of developing hybrid fish with a voracious appetite and the ability to survive in local waterways, should they somehow be released, has been questioned on a number of occasions.

• Negative Impacts
• injures humans
  • bites or stings

Conservation Status

Although this species has not been evaluated by IUCN Red List, native populations are facing increased stress due to commercial fishing demands, with both legal harvest and poaching leading to population declines. Furthermore, with growing demands for food production, agriculture is now threatening populations of red-bellied pacu. A study examining the effects and lethal concentrations of chemicals in the herbicide Roundup, widely used on coca and poppy crops in the region, found that pacu exposed to the herbicide, even at low levels, developed lesions and showed possible nervous system damage that could negatively affect future reproduction.

Additional Links

Encyclopedia of Life

Contributors

Blake Hintz (author), Radford University, Karen Powers (editor), Radford University, Christine Small (editor), Radford University, Jeremy Wright (editor), University of Michigan-Ann Arbor.

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