One can definitely reproduce by itself. I started with one small hitchhiker on some plants from the LFS and I tossed it in my 5 gallon as opposed to "humanely euthanizing" it. Now about 3 months later, that one turned into around 100 and the biggest one is about the size of a quarter. If you don't want them, get rid of that one asap. Mine started laying eggs after about a week.