Cool game.
When you add more graphics, I hope you go for some extra-gore :D
I think the piranha-behaviour is interesting, but it can get annoying to order them in place every second, so maybe you could tune down their free will a little?
I also found some piranhas spontanously disappearing, I dunno if this is a bug, on purpose, or if I just missed some police-guy taking them out. Just wanted to say it in case it's a bug.
Thanks for uploading, I enjoyed playing your game & hope it will grow further. Might get the "Maneater" of 2d-games :D