Piranhas are a common enemy in the Jungle. Up to four may be generated randomly in any body of water.
They are the smallest enemy in the game, but can be one of the deadliest if encountered in a group. They are a guaranteed spawn on a Flooded Cavern level, along with the shoal leader, Megamouth.
By themselves, Piranhas are not hard to deal with. They die in one hit from the whip or any projectile. Note that Piranhas (like any enemy) can not be damaged by jumping if the Spelunker jumps while underwater, but it is possible to jump from outside the water to stomp on and kill a Piranha swimming near the surface. They will swim horizontally across whatever body of water they are in, and will only move vertically to chase you once you've entered their pool. Usually, one Piranha can be dodged by waiting for it to swim to the ledge closest to you then jumping in and quickly exiting the pool. Multiple Piranhas are a different story, especially on the rushing water level (where there may be 20+ of them generated in the river). In groups, Piranhas become deadly if they are able to touch you, easily doing multiple hearts worth of damage instantly. In a normal pool, you can simply bomb or undermine the pool, which will drain the body of water, instantly killing any fish exposed to air (any Piranhas that die this way leave behind skeletons which can be picked up and thrown). In the rushing water level, you must deal with them either with a gun or extremely clever use of the whip.
There are also skeletal Piranhas on the Restless Dead level. Skeletal Piranhas are exactly the same as regular ones for all intents and purposes. The only difference is that skeletal piranhas do not drop blood when killed.
Because of this, "Flooded Cavern" level feelings are very dangerous if you are wanted, as the Shopkeeper guarding the exit will fall in the lake when he becomes aggro, and you will be blamed when he dies from the Piranhas.