At one time, I only had one. I tried to remove it manually( exactly what I was advised not to do)and it retracted into the rock. One week later I had three. After there were more than one they started having babies, little ones started popin up everywhere. I'm always moving stuff around to keep the detritus down, and that's when I noticed that they move. I have kept track of where they were and haven't seen any new baby's pop up. So either they haven't left any behind or they swim or crawl somewhere else. And I'm not giving a guaranteed method, it's more like another good trick to save money.
I would like it if people tried the method and posted their results. That would be awesome.