fins
Muscles.
mouth or gills
Legs
each part is used to protect a fish from danger in its own specific way
Its muscles - its legs.
They use their feet to waddle and their flippers to keep balance.
EAR
The Legs
Fish use density in order to move in the water since they do not have lungs. When the density of water keeps changing, fish use it to move and thereby using less energy.
Legs just like all mammals.
That would be there fins, as this would help them move quickly away. Most fish are defenseless so there response to an attach is not fight but flight. There are of course those fish the have built in defenses: Piranha - Sharp Teeth & Jaws and Pufferfish - Long External Spines & Poisons.
The DNA is homogenous through-out the entire body of the fish (or all creatures for that matter). What you should be asking is 'which part of the fish's genome do you mix with your own DNA to become a mermaid?'