I'm planning on playing as a merfolk in a future Pathfinder campaign. Since merfolk only have a land speed of 5 feet, can a merfolk take a 5-foot step so as to avoid attacks of opportunity even though that's a merfolk's maximum movement?
Looking at the 5-foot step entry in the PRD answers this:
You can only take a 5-foot-step if your movement isn't hampered by difficult terrain or darkness. Any creature with a speed of 5 feet or less can't take a 5-foot step, since moving even 5 feet requires a move action for such a slow creature.
So no, they cannot.