piranhas teeth are in rows, it's normal for them to lose them in pieces which are about 25% of their teeth at a time. They regrow.
OPEFE:
http://www.angelfire.com/biz/piranha038/piraparts.html
When a piranha loses a single tooth, it is replaced quickly as a whole set. This piranha is ready to lose its set of teeth. The new teeth are already formed in the gum area and do not drop down until the replacement is needed. Microscopic slides made at OPEFE do show the "ready teeth" on the standby position. Some of the individual species teeth are as variable as the species itself. Some are very broad while other are very narrow, and most are small for a species considered deadly. The largest tooth I have examined and measured based on a 23.5 cm SL specimen is only 8 mm (a bit larger than 1/4 inch long). The teeth are serrated which allows nice, clean slices. The piranha also have additional smaller teeth-like structures called "ectopterygoid teeth" (approximately 2-3 mm long). The ectopterygoid itself is a bone (palatine) is like an elongated plate that forms on the roof of the mouth with these small teeth.