Sherlock's face fell as he saw dark shapes in odd places all over John - it only took a couple of seconds to recognize what they were, though he'd never seen them in action before. Piranhas. But what little he knew about them assured him that they never acted like this.

He took a sharp intake of breath as he grabbed the knife from John's hand and pulled him down to the ground, immediately using the blade to try to knock the vicious creatures free.