Different piranha species reach different sizes and probably have very different feeding strategies, which may also change as they grow. Small species and young are mostly schooling fish, sometimes in mixed-species schools, but some of the larger species especially become solitary as adults, and probably have a feeding strategy similar to pike or perch. Most forms grow 20-30cm long, but large S.rhombeus approach 50cm.
Piranhas are not strict carnivores, and several forms feed heavily on seeds or plants at some seasons, but on the whole they feed on animal prey, ranging from aquatic crustaceans when small to larger animals, especially injured ones. At least one species, the Wimple Piranha Catoprion, is a specialist feeder on fins and scales, and lurks in weeds by itself waiting for larger fish to swim by. For the other species, favourite prey are fish that have been caught on hook and line, even if they are other piranhas, and when a large shoal attacks they could be dangerous. Despite that, actual fatal attacks on humans are vanishingly rare (although I suspect more than one murder has been passed off as a piranha attack) and in fact they are considered good eating throughout their range. It is not just humans who eat piranha however, giant otters, river dolphins, larger fish, and caiman all target piranhas as prey. The large schools they go around in when young are actually for self-defence, not hunger. Even so, they have formidable jaws, although ecept when feeding the teeth are not especially obvious. In prepared or dried specimens, the soft tissue cover is removed and their armoury is revealed.
The fearsome reputation of piranha is mostly down to the late Theodore Roosevelt, not a man to allow the facts to get in the way of a good story. In his book, Through the Brazilian Wilderness (1914) he gave a graphic description:
“The piranhas habitually attack things much larger than themselves. They will snap a finger off a hand incautiously trailed in the water; they mutilate swimmers—in every river town in Paraguay there are men who have been thus mutilated; they will rend and devour alive any wounded man or beast; for blood in the water excites them to madness. They will tear wounded wild fowl to pieces; and bite off the tails of big fish as they grow exhausted when fighting after being hooked. But the piranha is a short, deep-bodied fish, with a blunt face and a heavily undershot or projecting lower jaw which gapes widely. The razor-edged teeth are wedge-shaped like a shark’s, and the jaw muscles possess great power. The rabid, furious snaps drive the teeth through flesh and bone. The head with its short muzzle, staring malignant eyes, and gaping, cruelly armed jaws, is the embodiment of evil ferocity; and the actions of the fish exactly match its looks. I never witnessed an exhibition of such impotent, savage fury as was shown by the piranhas as they flapped on deck.”
Keeping a fish like that attracts a cetain kind of aquarist. Unfortunately, in the confines of an aquarium piranha have a tendency to attack and kill each other, and without a large aquarium for a school big enough to disperse aggression you tend to end up keeping only a single survivor. Worse, in warm climates it is not unknown for them to be dumped, with obvious risks if they started to breed in the wild. For this reason in many US states keeping them is illegal. As far as I am aware, so far at least this has worked and there are no introduced populations anywhere in the world (so far).
Next week, freshwater stingrays.
(Images from wikipedia)