They’re good parents—at least initially. A mom may lay 600 eggs at once, dad promptly fertilizes them, and both parents guard the brood once it hatches. (Later, they might eat some of their young. But let’s not focus on the negative.)
Despite their scary looks, they’re actually cowards. Okay, that’s a bit unfair, but studies have shown that rather than congregating to hunt cooperatively, as was always believed, they join forces because they’re afraid of being eaten. They’re especially likely to band together—in schools as large as 1,000 fish— at times of year when predators such as caimans and dolphins are regularly present. Apparently, they’ve gotten the message (evolutionarily speaking) that there’s safety in numbers even if you yourself have really sharp teeth.
That's just a sample of the things you might not know about piranhas. Link -Thanks, Marilyn!