MnO4- is not the thermodynamically stable form of manganese in water; thus, permanganate tends to oxidize very slowly in water with the evolution of oxygen:
4 MnO4- + 4 H+ ® 4 MnO2 + 2 H2O + O2
Potassium permanganate has oxidizing activity. It has been observed that adult mussels retract their siphons while potassium permanganate is passing through water.
Zebra Mussel Chemical Control Guide