Commonly kept in home aquariums, mystery snails -- also known as apple snails -- keep their gender a mystery to the uninformed observer. The snails may mysteriously change sex from male to female, or vice versa, but are not hermaphrodites like some snail species. Carefully observe your snails' behavior to figure out which are male and which are female.
If you're lucky enough to witness your snails engaging in mating behavior, there will by no mystery to who's who. The male mounts the female to deliver a sperm packet with his penis from the front of his shell to hers. The female will lay eggs above the waterline, often on the hood of the aquarium.
The shapes of snails' shells vary slightly between males and females. The males' shell openings are rounder, to accommodate extension of the penis during breeding. When the snail emerges from the shell, you'll see a noticeable penis sheath on the right side of the snail's body next to the gills. The female does not have this bulge.
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