Piranha - biology.

Piranha - biology. I INTRODUCTION Piranha, common name for South American fishes often depicted as forming schools that strip the flesh from living prey. Tales of such attacks are largely myths. Piranhas, also called caribes, are found in river systems in tropical regions of most South American countries east of the Andes. Scientists recognize more than 30 species. Most types of piranhas eat plant material, including fruits and nuts that fall into water. Some species also eat small fish and scavenge dead animals. There are no documented human deaths from piranha attacks. II DESCRIPTION AND BEHAVIOR Piranhas have a flattened oval shape and are from 25 to 60 cm (10 to 24 in) long. Powerful jaws with sharp, wedge-shaped teeth that mesh like cutting shears enable the fishes to cut the flesh from prey or open tough seed cases. The rows of sharp teeth are constantly replaced. Large muscles can snap the jaws open and shut extremely rapidly. Mainly carnivorous (meat-eating) species such as the red-bellied piranha (Pygocentrus nattereri) have blunt heads and thickened skulls. Studies of stomach contents from different species of piranhas show that they eat a varied diet that includes plant material (fruits, nuts, seeds, flowers, leaves, roots), algae, insects, worms, and fish. A common feeding behavior is to nip off parts of the fins or scales from other types of fish. This cropping tactic allows the victim to survive and regrow the injured parts, providing a kind of renewable food resource for piranhas. Juvenile piranhas feed on tiny crustaceans and insects. Species that are more carnivorous, such as the red-bellied piranha, may take bites of flesh from larger fish. In addition, they also eat small fish and occasionally frogs, lizards, snakes, and even their own kind. They also scavenge. Some piranhas may hunt in schools. Feeding frenzies by schools of red-bellied piranha are possible over a carcass or a nearly dead animal. Such behavior is more likely in the dry season when water levels are low and food is scarce. The meal may be left as a skeleton. Violent attacks on active large animals or people are extremely unlikely. Piranhas usually spawn in the rainy season when large areas may be flooded, providing food for offspring. Species such as the red-bellied piranha become more territorial and defend a spot chosen as a nest, usually rocks or vegetation. The male guards the nest where the female lays thousands of eggs, fertilized by the male. The eggs hatch in about 10 days depending on the water temperature, and parents may continue to protect their brood for a time. III PEOPLE AND PIRANHAS In the popular imagination, piranhas are deadly man-eaters that lurk in jungle waters, ready to attack unwary humans. Scientists dismiss this idea as a wild fantasy. The piranha's terrifying image is traced in large part to Teddy Roosevelt, the former American president who explored part of the Amazon in 1913. Roosevelt recounted his adventures in the widely read book Through the Brazilian Wilderness (1914). He described piranhas as "the most ferocious fish in the world," known to "rend and devour alive any wounded man or beast; for blood in the water excites them to madness." The myth of fearsome man-eating piranhas entered popular adventure fiction, and later, motion pictures and television. There are no authenticated human deaths from piranha attacks. Stories of the fish devouring humans appear to be based on dead bodies that fell into water and were later scavenged by the fish. In regions where they are found, piranhas rank as a minor risk of injury to humans compared to stingrays, electric eels, and caimans. Nevertheless, fishermen are sometimes bitten and can lose fingers while handling piranhas. When humans enter the water, piranhas may bite feet and fingers, especially if a breeding male is defending territory. More serious bites have also been documented, none fatal. Local peoples eat piranhas, though most species are very bony. The fish is a staple for poor people in some regions. Indigenous peoples have also used the fish's razorsharp teeth to make tools and weapons. Piranhas are not listed as threatened or endangered, although deforestation and dam construction may have reduced their numbers in some regions. Piranhas are also caught for the pet trade as an aquarium fish. Some fish hobbyists have let piranhas loose in the wild. Released piranhas have been reported in rivers and lakes in different parts of the United States, but the fish likely cannot reproduce without a tropical climate. Aggressive piranhas could pose a threat to native fish species. At least 20 states have laws against keeping piranhas. Some states even require scientific organizations to obtain special permits to import the fish for study. China, India, Malaysia, Vietnam, and other countries have bans or restrictions on piranhas, which might survive in local waterways in warmer regions. Scientific classification: Piranhas belong to the family Characidae of the order Characiformes. Species in the genera Pygopristis and Pristobrycon mainly eat plant material; species belonging to the genera Pygocentrus and Serrasalmus are more carnivorous. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.