Piranha

Piranhas have 30 health and can be found in water in the Jungle, Underground, and Cavern layer. They have an increased spawn rate in the Jungle. They swim in a straight line left or right until hitting a wall, where they flip directions; upon breaking the surface of a body of water, they will flip upside down briefly, then right themselves when they re-submerge. They will swim towards the player if he/she is submerged in water; otherwise they pay no attention to the player.

Link to a video of a Piranha Farm: http://www.youtube.com/watch?v=tA41j1xUEtM

Piranha