Famed as a rapacious predator capable of rapidly stripping flesh from bone and a danger to any animal entering its native waters, P. nattereri is among the most notorious freshwater fishes in the world. As a result, displays featuring this “bloodthirsty” creature are found in most public aquaria, grisly Hollywood movies have been released, and the species has become popular in the aquarium trade.
According to Herbert Axelrod (1976), the myth began when American president Theodore Roosevelt made a visit to Amazonian Brazil in 1913. He was accompanied by many journalists and the Brazilians organised a number of publicity stunts, one of which was that the president would ‘discover’ a river which would then be named after him. A tributary of the Río Aripuanã previously known as the Río da Dúvida (River of Doubt) was chosen, and it is still referred to as the Río Roosevelt or Río Teodoro today.
When Roosevelt arrived at the river the Brazilians had a surprise prepared; a stretch of several hundred yards had been netted off and for a number of weeks fishermen had been catching hundreds of adult piranhas and isolating them there. They told the president that he and his men should refrain from entering the water as they would be eaten alive by vicious fish. Naturally this news was met with scepticism, so a cow, variously quoted as being “sick”, “old” and “in season” was introduced. Some even say that its udders had been sliced open. This caused a spectacular feeding frenzy among the trapped, starving piranhas, and newspapers were filled with stories of terrifying, flesh-eating fishes, yet there remains not a single record of wild piranhas killing a human being.
Informed captive maintenance of piranhas and their relatives has undergone a relative boom over the last decades, with a number of websites and forums now dedicated to the subject. Many different species are available, but the majority are wild caught, costly, and beyond the means of most hobbyists. P. nattereri is farmed in commercial quantities, however, with coin-sized juveniles sold rather cheaply for a fish requiring specialised and ultimately expensive care. For the enthusiast it makes an excellent aquarium subject, but considerable thought and research are essential prior to purchase.
In terms of the genus Pygocentrus, most modern workers continue to follow the nomenclatural system proposed by Fink (1993) in which there exist three species; Pygocentrus piraya (Cuvier, 1819), P. cariba (Humboldt, 1821), and P. nattereri. While the identities of the first two remain in little doubt, Fink was unable to accurately diagnose P. nattereri, and it remains uncertain whether the species represents a single evolutionary lineage or not. P. palometa Valenciennes 1850 was described from the Río Orinoco, is still listed as valid by some sources, and occasionally appears on trade lists, but is generally regarded as a nomen dubium.
Following Fink (1993), P. nattereri can be distinguished from congeners by the following combination of characters: adipose fin always without rays (vs. large specimens with adipose rays in P. piraya); humeral spot always small or entirely absent (vs. large in specimens > 100 mm SL in P. cariba); body often with numerous spots (vs. few in P. cariba specimens > 100 mm SL); branched dorsal-fin rays 14-18, usually 15 (vs. 15-18, usually 16, in P. piraya); usually 6 neural spines anterior to first pterygiophore (vs. usually 7 in P. piraya); vertebrae 35-38, usually 36 (vs. 36-39, usually 38 in P. piraya); usually 5 supraneurals (vs. usually 4 in P. cariba).
P. nattereri has proven difficult to identify for a number of reasons. For example, P. piraya and P. cariba are endemic to single river basins (the Río São Francisco in Brazil and Río Orinoco in Venezuela/Colombia, respectively) and possess distinct morphology. P. nattereri, however, has an incredibly wide distribution, and colour pattern can vary considerably even between individuals of a single population. Colour pattern is also known to vary with habitat-type; fish inhabiting blackwater environments tend to be darker with less red/orange than those from clear or white waters.
Morphology and patterning in adults can vary in terms of head and body shape, presence or absence of dark spots, or reticulated markings on the flanks and fins.
Such differences have led to some populations being described as distinct species, most notably P. altus Gill, 1870 from the Upper Amazon and P. ternetzi Steindachner 1908 from the Río Paraguay. Although piranhas continue to be traded under both names, Fink (1993) studied over 100 specimens from different parts of the Amazon and Río Paraguay drainages and was unable to find any consistent character (shape, colour pattern, meristic counts) that could be used to define them according to origin. Having found all three species non-diagnosable he decided to synonymise P. altus and P. ternetzi, with P. nattereri remaining valid since it is the oldest available name.
Though he admitted himself it was an unsatisfactory option, Fink’s classification is still in place today. In a later study (Fink & Zelditch, 1997)[5] differences in body form between ‘northern’ and ‘southern’ populations of P. nattereri were observed, but evidence that they represent different species was still considered insufficient.
The family Serrasalmidae contains 16 genera comprising the piranhas, pacus and relatives. Their characteristic features include a compressed body shape, long dorsal fin with 16 or more rays, and a variable number of sharp serrae formed by modified abdominal scales on the ventral surface. They are found in numerous habitat-types, from lowland floodplains and flooded forests to upstream headwaters, and occur in all major South American river systems east of the Andes. Some species perform unique ecological functions such as seed dispersal, or sustain important inland fisheries and aquaculture projects.
Members display three main feeding habits: carnivory (flesh-eating), frugivory (fruit and seed-eating), and lepidophagy (eating the scales and fins of other fishes). Carnivorous species normally possess a single row of tricuspid teeth on each jaw, frugivores tend to have two series of incisor or molariform teeth on the premaxilla, one row of teeth on the dentaries and often a pair of symphyseal teeth, while in lepidophages the teeth are tuberculate and located on the outer edge of the premaxilla.
The evolutionary history of serrasalmids has been studied by various authors, with the most recent analyses (Thompson et al., 2014)[8] supporting the existence of three major clades within the family. The "pacu" clade contains the genera Colossoma, Mylossoma and Piaractus, the "true piranha" clade includes Metynnis, Pygopristis, Pygocentrus, Pristobrycon, Catoprion, and Serrasalmus, and the Myleus clade comprises the Myleus-like pacus.
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