There are three things that are important in the life of a fish (or any other organism
for that matter). One: to acquire something to eat. Two: to avoid being eaten. And three (if you succeed in getting enough nutrients and steer clear of your hungry neighbors): to reproduce.
One thing that can help a fish to achieve the first two objectives is to look like
something that it is not, like an algae encrusted rock or a toxic sponge. This
antipredation/feeding adaptation is referred to as camouflage or crypsis.
The leaf scorpionfish sways from side to side to enhance its crypsis as shown in
the animation right.