Pond snails can reproduce asexually. They do not require a mate to do so. Other snails, like Diffusa, formerly called Bridgsii (probably spelled wrong), do need a mate to produce viable fertile eggs. I consider pond snails a pest because they do not need a mate and will fill a tank quickly. The mystery snails, Diffusa, are somewhat difficult to breed with success for most of us, although there are people who breed them as easily as I breed Heterandria formosa.