it really depends on the species, but many are hermaphroditic. they have both organs and can reproduce asexually. but they do exchange DNA, and can store that DNA for further reproduction and evolutionary means.
so both is the best answer.
edit: that is a really quick and broad answer. sry
The hermaphroditic species of snails still do require sexual reproduction, but they can store sperm or fertilized ova for long periods. Similar to the situation with certain common live-bearing fish. MTS are the exception, they appear to be all parthenogenic females.
Okay. Well I had a hitchhiker when I bought a plant and I hope it reproduces since I have an algae problem but then again I'm getting an otto so that should solve it.