To explain: The reason for the pre-domination of sexually reproducing snails in the unpolluted water.
The snails are mollusks that are classified under the large and diverse of class Gastropoda. The snails have many different species in different habitats showing different behaviors and anatomies suitable for those particular habitats. They can reproduce either sexually or asexually. However, some species like Potamopyrgus antipodarum can reproduce both in asexual and sexual mode of reproduction.