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They may not look it, but the Fathead Minnows in your aquarium are actually masters of chemical warfare and espionage and are even able to save their compatriots from beyond the grave.

 
Fathead minnows (Pimephakes promelas) have special cells in their skin called club cells. When these are damaged, they release a special pheromone into the water that acts as an alarm signal to other minnows in the area, warning them of trouble. When they detect the pheromone, surviving minnows go into a panic response designed to evade the predator, using cover more often, darting about more energetically and frequently, and shoaling together more closely for protection. Even more interestingly, minnows that have been warned in this way will exhibit this anti-predator response, even if they have never encountered that type of predator before.
 
However this chemical signal lasts even beyond death...when one unfortunate minnow “takes one for the team” and is eaten by a predator, the alarm pheromone is released even in the predator’s faeces. This means that when a pike, for example, enjoys a nice meal of Fathead Minnow and then later defecates, the faeces will release the pheromone and “label” the area as being an unsafe one, thus warning other minnows to keep away and helping them to avoid predation themselves.
 
But for every canny tactic in war, there is always a countermeasure. Predatory pike have learned to adapt their behaviour in order to minimise the pheromone’s effectiveness and counteract the effects of being labelled as a predator - by deliberately defecating well away from those areas in which they hunt.