Plankton is the thirteenth fish encountered after the Shark and before the Manta Ray. Their movement is similar to the Crab, Sea Horse, etc. They are easy to slice and give a fairly good amount of coins. They can be found very deep in Tranquil Sea, along with the Pistol Shrimp. They are worth 32 coins and take one slicing power to kill. if using the Muramasa Blade. They are also very small, and have one eye, but they are not hard to see.
They used to be in every depth of the Tranquil Sea, but since one of the early updates, they are now restricted to over 590m in depth.
Even though the Plankton is classified as a fish, it is not really a fish, but is really based upon the copepod.