Piranha and Pacu
While the majority of fish species concentrate their efforts on bite-sized invertebrates, small fish or plant matter, there are other options for the more adventurous piscine diners. This is exemplified by two very closely related South American characins, the red-bellied piranha (Pygocentrus nattereri) and the tambacqui, or pacu (Colossoma spp.), which pursue two unusual and highly specialized diet plans.
How deadly are piranhas?
The infamous piranha was first brought to popular attention in the early 20th century by the visit to Amazonia of then president Theodore Roosevelt. Journalists covering his visit reported stories of horses being stripped to the bone in mere seconds by these voracious fish. The pressmen created a lasting image of a relentless and deadly predator that lurked in the mysterious forest rivers of Brazil. The truth, as ever, is rather more prosaic.
For most of the year piranhas are relatively shy creatures, seldom attacking much larger animals. Indeed, the children of the forest peoples bathe, swim, and splash in piranha-infested waters without harm. But there is a central grain of truth to the early reports of the piranha's flesh-shredding activities. During the dry season the fish are constrained to live in small and shrinking pools. Food at these times is in extremely short supply and the fish are far more likely to attack, especially if their victim -- often a large aquatic rodent such as a coypu -- is injured. Once the smell of blood is in the water, the excited fish go into a feeding frenzy, tearing at their victim and, sometimes, at each other. Piranhas also often gather beneath the nests of storks and similar birds, waiting for the fledgling youngsters to make a mistake and fall into the water.
So what's so unusual about piranhas?
Plenty of fish eat flesh of one kind or another, but piranhas are exceptional in the way they take bites out of larger animals. Most fish only attack prey that they can swallow whole. Piranhas are equipped with a mouthful of large, razor-sharp teeth that enable them to slice almost surgically through skin, flesh, and cartilage and remove a chunk.
The second unusual aspect of piranha behavior is that unlike almost all major predators, they live in schools. This is likely to be as a result of the distribution of their food. It also means that during a feeding frenzy, any individual that is not quick enough to seize a mouthful and retire may be itself targeted in error by its schoolmates, resulting in serious injuries, even death.
Are there any vegetarian piranhas?
The pacu is a close relative of the piranha and shares its deep, round body, but unlike its cousin, it is a vegetarian. Even the young fish, which are carried along with the other river fish onto the forested floodplains during the rains, eat grass seeds. As they grow, they graduate to the fruits, nuts, and seeds shed by the trees and which float on the water surface. Pacu have immensely powerful jaws and molarlike teeth that enable them to crush tough seed casings and nut shells and to access food that no other fish can. Interestingly, this process often does not kill the seed but rather activates it and its passage through the fish's digestive system triggers germination. As a result, pacu are important distributors of plant seeds throughout the flooded forest in their native home.