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| Sunday,13 November 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Are piranhas man-eaters? We have seen movies about killer anacondas and heard stories about ferocious sharks. But the truth is, these are man-made imaginary tales. Another common fiction is about merciless piranhas that sink their sharp teeth into their victim's flesh and eat it greedily as soon as they can. And again, the question remains, are they as dangerous as they are portrayed to be? Piranhas, sometimes called cannibal fish, are a group of carnivorous freshwater fish living mainly in South American rivers. They are normally about 15 to 25cm (6 to 10 inches) long. They are known for their sharp teeth that have the capability to bite through a steel fishing hook, and an aggressive appetite for meat. They are normally only found in the Amazonian, Guyanas' and Paraguayan river systems. The piranhas are members of the genus Characidae, Characins, a large family of more than 1200 species. They belong to a sub-family called the Serrasalmidae, a name based on the fact that all members have a sharp, serrated (notched, sword-like edged) keel running over the belly to make the body more streamlined (for faster swimming). The name piranha may come from a hybrid language composed of Tupi-Guarani languages; it is believed to be a compound word made of the components 'pir ', meaning 'fish', and 'sanha' or 'ranha', meaning 'tooth'. There are about 12 different species of piranhas, and the one that we are most familiar with is the red-bellied piranha or the red piranha. The Red-bellied Piranha (Pygocentrus nattereri) is the Amazon's most notorious animal. Its powerful bite and sharp teeth can sever a finger or a toe. The White Piranha (Serrasalmus rhombeus) grows much larger than the Red-bellied Piranha. Although bigger in size and possessing similar dentition, this species specialises on hunting other fish and does not "swarm" in large schools, so it is less dangerous than the Red-bellied Piranha. Body characteristics Piranhas can be split into a number of different genera(groups), with different visual treats. Members from the genus Pygocentrus are all recognisable by the convex shape of their head and massive bulldog-like lower jaw (more powerful and muscular than most Serrasalmus species). This reflects their diet: all 'Pygocentrus'- species are fullblown predators, that often actively chase their prey. 'Serrasalmus piranhas have a more concave head shape, and less powerful lower jaws. A number of species feed themselves mainly on the fins and scales of other fish, and even nuts and fruits, and therefore they do not need the same muscle packed lower jaw to rip through skin, muscle and bone. All piranha species have a powerful, high, thick but laterally compressed body shape, with keel-like edges running over the upper part of the body from head to dorsal fin, and on the lower body running over the belly. Together with a large and powerful tail and a body covered with very small scales, their streamlined bodies make them very fast and agile swimmers. Unlike many fish species, piranhas have a small adipose fin between tail and dorsal fin. This feature is characteristic of the Characin family, although members from some other families, like catfish, have an adipose fin as well. Their predatory lifestyle is reflected by large eyes and a large nose with big nostrils to maximise the water inflow. They have a very acute sense of smell: in their natural habitat, murky rivers in South America, even more darkened by overhanging vegetation, scent is their main way of tracking down their prey. Social behaviour In the wild, Piranhas live in large shoals, roaming the South American rivers. Most shoals will have one or more dominant animal, depending on the size of the shoal: the leader(s) of the pack. Even though the fish will often hang out together, in crucial moments, the dominant fish will show its might. A group of Pygocentrus piranhas live in a state of constant fear and mutual mistrust; even when all seem calm, the animals are capable of severely wounding or even killing each other. To survive, the fish must always know where the others are, in what states of mind they are in and how they might act the next moment. Letting their guard down may turn out to be fatal. This behaviour may be a partial explanation as to why many captive piranhas are relatively skittish and nervous, despite their fearsome reputation. One moment of carelessness may mean the end of their life, even more in the wild, where piranhas are part of the diets of many predators, like jaguars, caimans, botos (freshwater dolphins) and other, larger predatory fish, which live there in abundance. And young piranhas are even preyed upon by adult relatives. The staple diet of the carnivorous Pygocentrus piranha species consists of fish. They catch their prey by active chasing, or by ambushing. Besides fish, they also eat insects, crustaceans, young birds that have fallen into the water, mammals, and reptiles, supplemented with fruits and seeds. Most larger animals that are attacked, are weakened, sick or injured: even a large shoal of piranhas will attack a healthy animal only on very rare occasions: during the dry season, when moving around freely is as good as impossible, and food is scarce. Cannibalism (eating the flesh of its own kind) amongst piranhas is not unusual during this period, and many others fall victim to predators looking for an easy meal. Piranhas are attracted by the splashing or the erratic (irregular) movement of an animal in distress. Once they start feasting on their prey, other piranhas rush to the scene, drawn by the splashing of the victim, the blood and the noises and disturbances caused by the frenzied piranhas that are already feasting. When a piranha pair is ready to produce eggs, they form a little territory from which other fish are aggressively chased away. Both fish become much darker in colour, and their behaviour becomes more territorial and aggressive. In their territory, the fish start to build a nest on the bottom, by clearing all plants and removing stones etc., with their tails and by blowing it away. When the nest is ready, the male tries to lure the female into the nest. Here she will lay her eggs, which will be fertilised quickly by the male. After the eggs are laid, the male guards the nest, and chases off every fish that comes too close. Sometimes the female will be expelled, but sometimes she helps with the guarding. The orange-coloured, fertilised eggs will hatch in 2-3 days. A few days later, they will start swimming around freely. Surprisingly, as ferocious as they are believed to be, not all types of piranhas are meat-eaters! Some of them simply rely on their razor sharp teeth and powerful jaws to crack fruits and seeds that fall into the water. The types that do eat meat like the red-bellied piranhas often attack animals smaller than themselves, such as other fish, frogs, insects, or birds. Piranhas don't usually attack a large animal unless they are very hungry, or the victim is weak or injured. In fact, the Indians of South America have been known to swim in rivers full of piranhas without getting bitten even once. Piranhas generally pose little threat to humans, and attacks on humans are extremely rare. Natives frequently swim in piranha infested water without attacks or scratches. It is not recommended to swim where piranhas live in the drought season because of increased aggressiveness caused by food scarcity. Six genera of piranhas * Pygocentrus (true piranhas) with three species: Pygocentrus cariba (Shoulder Spot Piranha), Pygocentrus nattereri (Red Bellied Piranha) and Pygocentrus piraya (Rio Sao Francisco Piranha). * Serrasalmus (Pirambebas) with more than 20 species, including Serrasalmus rhombeus (Black Piranha), Serrasalmus spilopleura (Gold Piranha) and Serrasalmus manueli (Green Tiger Piranha). * Pristobrycon with 5 species. * Pygopristis with Pygopristis denticulata (Five-Cusped Piranha). * Catoprion with Catoprion mento (Wimpel Piranha). * Citharinus with Citharinus citharus. |
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