The Eater of Plankton is a quest item that is needed for the Angler. It can be found by fishing underground in the Corruption after he has requested it.
I bet you're not brave enough to find the Eater of Plankton. A corrupt fish that was mutated from a severed piece of the Eater of Worlds itself! Capture it and bring it to me, and prove to me you're not a wuss!
- Added to the game.