Piranha : The most dangerous amazonian fish

General description

Piranhas are fresh water, tropical fishes belonging to the family characidae, which also include pacus and tetras. There are many species of piranha, some are Serrasalmus marginatus, Serrasalmus spilopleura, Pygocentrus nattereri (red-bellied piranha)

Piranhas are agressive predators and opportunistic feeders. They have fearsome reputation as the most dangerous amazonian fish. But human regularly swim, bathe and wash clothes in piranha infested water without incident. The reports that piranha going after humans are extremely rare.

Appearance

Piranhas has deep, laterally compressed bodies. They vary in color by location, population and age. But many are silvery gray or golden and some have bright colors such as red or orange on their head, belly or fin.

Figure 1 show the variation of the piranha body forms. piranhas have a thin shape when viewed head on. From the side they appeared broad and round. Ranging from sharp- snouted to blunt-headed, with various degrees of body compression :

General body form (lateral and frontal view) of three species of piranhas from pantanal region, mato grosso. Brazil. (a. Serrasalmus marginatus (ZOEC 1603), b. Serrasalmus spilopleura (ZOEC 1602), c. Pygocentrus nattereri (ZOEC 1580))

Piranhas have short, powerful jaws, and lower jaws that protrude beyond their upperjaws. Both jaws are lined with sharp triangular teeth, which allows piranhas to bite into and tear the flesh of their prey.

Piranhas commonly grow to 12 inches in lenght and weight less that 2 (two) pounds, but have been reported to grow up to 16 inches and weight over 7 pounds.

Habitat preference

Piranhas are native to the central and southern river system of south america (most notably the amazon river), where they inhabit tropical rivers and stream and ofter found in murky water. Usually found in fast moving portion of rivers, piranhas will move to flooded riverside regions to feed and reproduce.

Distribution

South american rivers of colombia, venezuela, guyana, paraguay, brazil and central argentina.

Primary diet

Piranha feed on fish, insect, crustacean, amphibian, mollusk, worm, carrion, alge, and water plant. But they often feed (piranha attack) in large group (20 -30 individual) to kill prey like capybaras. (Omnivore)

Predator

Human, crocodile, caiman and larger fish species (including vampire tetra).

Life span

Piranha can live up to 8 years but red bellied piranha has been seen to live almost 10 years in aquarium setting.

Mating behavior

During mating season, red bellied piranhas will move to regions of slow moving waters. Female will lay up to 5,000 eggs at a time and ready to lay another batch in just a few weeks. Once laid, the male will swim by and fertilize the egg which are sticking to plant and sediment. Hatching periode vary 10 to 15 days, according to water temperature.

During the mating season, the male will guard the eggs and are extremely territorial toward all nearby fish and potential predators (including human). Young will hatch in a few weeks and spend the first few month feeding on insect during the day.

Predatory tactics

There are four main categories of hunting method toward fish prey were observed : lurk or ambush, stealthy approach or stack, approach under disguise, and chase (see figure 4)

Stalking was a frequent mode of hunting employed by both Serrasalmus marginatus and Serrasalmus spilopleura. The slow, stealthy approch within striking distance was usually done from behind or below the prey, which frequently was engaged in feeding (figure 4a)

Lurking was employed by Serrasalmus marginatus and by Pygocentrus nattereri during day time. Both species ambushed from within the abundant aquatic vegetation and dashed after passing prey mainly from behind. Most of these lunges by Serrasalmus marginatus were aimed at the flank or the rear of the prey and the predator obtained a mouthfull of scales, or more rarely a piece of fin.

Approach under feint was observed for Serrasalmus spilopleura, which lingered  close to the prey without any signal of stealth, behaving as of not interested in it. From time to time they join nearby shoals of small characin and repeatedly bit off the caudal fin of the shoaling fish. Apparently the characin did not recognize the piranhas as predators, as they showed no signals of being alarmed unless actually bitten.

Chasing after small fish was observed mainly for Pygocentrus nattereri preceeded by lying in wait within the vegetation. One individual of Pygocentrus nattereri was seen chasing after a characin or chiclid for about 4 meters before grasping the prey from behind (figure 4b) and cutting it in two. 

Watch here to see the piranhas feeding video :

Piranha facts.

• The bite force of piranha is 3 (three) times its body weight. 
• Piranha swallow their food whole.
• The red bellied piranha will bite when underfed, overcrowded or threatened. They also bite anything that move where blood is present in the water.
• The piranhas teeth are also use as scissors or razor blades by the native tribes.
• Piranhas can replace lost teeth
• Young piranhas have been known to feed on the fin of the other fish, but these fin will heal in just a short time.
• Piranhas help to keep rivers clean by eating injured or dead animals.
• Young piranhas are known as “fry” and a group of piranhas is known as a “shoal”
• The red bellied piranhas is a commonly food fish for local communities and is also caught for use as bait for larger cat fish

References

Ivan Sazima and Fransisco A. Machado. 1990. Underwater observations of piranhas in western brazil. Environmental Biology of Fishes 28: 17-31, Kluwer academic Publishers. Netherland

California Department of Fish and Wildlife. Piranha, Family Characidae.