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Environment

Motherly bites

Piranhas are not as aggressive as had been imagined, and in general attack in defense of their offspring

ALFRED R. WALLACEAfter having seen “Piranha”, the American horror film released in 1978, it is easy to let yourself be dominated by fear when swimming even in a slow flowing river like the Tietê, the largest river in the state of Sao Paulo, which, less than 200 km from the capital, is not polluted anymore  and has fish. In the film, the voracious piranhas, with their triangular and sharp pointed teeth, devour imprudent bathers in minutes and the water turns turbid with blood. It is impossible not to think on the risk of being the next poor victim if you were to wet your feet in the river.

But recent research shows that this image of the bloodthirsty attacker is really unfounded. Vidal Haddad Junior, a medical dermatologist from the School of Medicine of the  São Paulo State University (Unesp) at Botucatu, has studied the attacks by piranhas on bathers in the interior of the state of São Paulo and, in partnership with the zoologist Ivan Sazima, from the State University of Campinas (Unicamp), they have reached conclusions that undo the prejudice towards these carnivorous fish, which have an oval shaped body, found only in South America. Although located throughout the rivers of the state, they generally attack to defend their offspring (eggs or larvae) and their bites do not cause large wounds. The cases studied up until now consist of only a single bite on the leg or foot – the wound bleeds, but the member remains intact. So far, there has not been any trustworthy registration of the death of a single person from a piranha attack . Even so, it is prudent to stay far from them.

Undue fame
The interest of Haddad for the accidents caused by aquatic animals came about some ten years ago when he realized that little was known about how to treat the wounds that they cause. Only after the publication, in 2000, of the Atlas de animais aquáticos perigosos do brasil: Guia médico de identificação e tratamento [Atlas of dangerous aquatic animals in Brazil: a medical guide for identification and treatment] (160 pages, editor: Editora Roca), which dealt principally with maritime species such as the sea urchin and jelly fish, he began to dedicate himself to fresh water fish and managed to demonstrate that the attacks by piranha were a lot less serious than had been supposed.

In March and April of 2002, Haddad went to Santa Cruz da Conceição, a town of 3,500 inhabitants and 200 km from the city of São Paulo, to investigate piranha attacks on bathers who went out on Sundays to enjoy themselves on the beach of the calm and shallow waters on the banks of the river Mogi-Guaçu. During five Sundays, at a first aid clinic close to the beach, he attended to thirty-eight people bitten by these fish while swimming.

In general, the people who arrived at the first aid clinic had a single wound, around 2 centimeters in diameter – the same as the jaw of the piranha –, in the form of a crater. The wound bled freely and the bathers were impressed, which most certainly contributed to feed the myth of the ferocity of these fish. But there was no fatal case. Half of the people wounded had been bitten on the leg and another 40% on the foot, close to the heel. Only one bather had been wounded on the arm and three others on the hand. Of the 38 people wounded, five had be taken to the neighboring town of Leme as they had intensive bleeding, and one suffered the amputation of a finger. The greatest number of accidents (16) occurred at the weekend when more bathers entered the water. In a general manner, the treatment indicated is very simple and includes the cleaning of the wound for around ten minutes with water and soap to eliminate the possibility of contamination by bacteria. In the case of deeper bites, taking an anti-tetanus vaccine is recommended.

Sign of alert
“The wounds caused by the piranhas are much less serious than those caused by the sting of fish such as the yellow catfish or the ray”, Haddad explains. Also a graduate in biology, he identified the species that swim in the waters of Santa Cruz da Conceição: Gold piranha, dark banded piranha or pirambeba (Serrasalmus spilopleura or Serrasalmus maculatus). Common throughout the country, this species, when young, has a yellow tail with a black stripe, a silver or gold dorsal fin and a yellow underbelly, covered with dark spots. When adult, the body becomes dark gray, and it grows up to 26 centimeters in length.  But the reason why the Serrasalmus spilopleura – today common on the beaches that are formed along the banks of the river Tietê after its damming – had been attacking people was unknown.

The answer came shortly afterwards when Haddad met zoologist Ivan Sazima, a specialist in the behavior of fish who, since the decade of 80s, has been studying the habits of piranha as part of a thematic project coordinated by Professor Marcio Roberto Costa Martins, from the Biosciences Institute of the University of Sao Paulo (USP). Zoologist Sazima has already investigated the fame of “people devourers” attributed to these fish by way of the reports of three cases in which human bodies had been found with bites and even the entire removal of the flesh from the bone by piranhas. However, in all of these cases the person had died from drowning or a heart attack before being attacked by the fish.

After a year of visits to Santa Cruz da Conceição the researchers managed to associate the characteristics of the wounds to the environment where they had occurred. At only 300 meters in length, the municipality’s beach is in a calm stretch of the river Mogi-Guaçu, where there is a proliferation of water orchids (Eichhornia crassipes), floating aquatic plants of rounded dark green leaves and blue flowers. It was not difficult to establish the relationship. In 1985, zoologist Sazima had discovered that the Serrasalmus spilopleura laid its eggs close to or in the middle of the roots of the water orchids, which would later on provide shelter and food for the hatchlings. Other studies also demonstrated that, in order to protect its offspring, the male or the female of this species attacks any possible predators of the hatchlings with a warning bite. “It’s only an alert”, says zoologist Sazima, co-author with Haddad of the paper on the attacks at Santa Cruz da Conceição published at the end of last year in the magazine Wilderness and Environmental Medicine. “The piranhas are sending a message: get away from my nest”, says the zoologist from Unicamp.

Not very aggressive, the S. spilopleura is in general a solitary animal that feeds itself on small portions of fins and muscles of other fish. As well as eating insects, crustaceans and pieces of birds, frogs and snakes, the pirambebas are accustomed to stealing fishermen’s bait, sometimes taking the hook, line and sinker in one single bite. According to zoologist Sazima, only in extreme cases would a person run the risk of being devoured – for example, if he entered the water with a bleeding wound in a stretch of the river isolated by drought and with shoals of piranhas or in a portion of a river into which skeletons from slaughterhouses were being discarded. Even then, there is greater probability that any fatal attack would be brought about by another species, bigger and more robust, the Pygocentrus nattereri or red bellied piranha, whose reddened belly gave it the local name of piranha-cashew, found in much larger shoals.

Before uncommon in the state of Sao Paulo, the attacks by the pirambebas on bathers has become frequent over the last five years. Fishermen attribute these episodes to a series of factors. One of the most important is the damming of the São Paulo rivers for navigation, the production of electrical energy and water supplies for towns, that originate in the backwaters – places favorable to the procreation of fish and the proliferation of water orchids, where the piranhas lay their eggs –, and the beaches. As well as this, the cycle of reproduction of the piranhas coincided with summer when there is an increase in the number of bathers on the beaches of the river.

Rays in the river
After the attacks at Santa Cruz da Conceição, Haddad personally verified close to one hundred other accidents from bites of the pirambeba in the municipalities of Iacanga and Itapuí, close to the town of Bauru, in the northwestern part of the state of Sao Paulo. In each of these towns on the banks of the river Tietê, the researcher from Unesp observed fifty attacks on bathers during only two weekends. Haddad and Sazima alert to the risk of another type of accident much more serious: those brought about by the stings of the fresh water rays of the genre Potamotrygon. Cartilaginous fish related to sharks, the rays have a body in the form of a disc of up to 50 centimeters in diameter. It has a long serrated bony spiked tail, surrounded by glands that produce a potent poison that brings about the death of tissue.

The rays migrated some 20 million years ago from the Amazon region to the rivers of the interior of the country. The researchers believe that the construction of the dams, such as Itaipu, has favored the proliferation and the migration of rays, allowing them to reach the river Paraná.  Haddad, who last year detailed out the treatment of wounds from rays in the book Animais peçonhentos no Brasil: Biologia, clínica e terapêutica dos acidentes [Poisonous Animals in Brazil: Biology, clinic and therapy of accidents] (468 pages, Editor: Editora Sarvier), has already heard fishermen telling of the capture of rays in Ilha Solteira, close to the region in which the river Paraná receives the waters of the river Tietê. “In a few years”, he stated, “rays could become common in the São Paulo rivers.”

The Project
Natural history, ecology, and evolution of Brazilian vertebrates (nº 00/12339-2); Modality Research Projects – Thematic Grants; Coordinator Marcio Roberto Costa Martins – Universidade de São Paulo; Investiment R$ 851.027,74

 

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