The Japanese Black Trapdoor Snail (Viviparis malleatus), Viviparis meaning live bearer, are the preferred snail for water gardening. They are dark in color and grow up to 3 inches. They generally produce 3-5 young at a time.They scour the pond eating algae off the liner, planters and even cleaning up your plants without harming them. They also eat dying plant matter and clean up left over fish food. They are cold hardy even if your pond freezes over, unlike many snails because they do not breath air (many snails have a breathing apparatus that they send to the surface to suck in air with). Japanese trapdoors stay in the water at all times unless removed. If you do take them out they will seal their trap door and wait for you to return them to the water.
The biggest question is how many do you need. "The golden rule" published in books for many years is one per square foot of surface space, this is fine for small ponds, but can get overwhelming and quite expensive to buy if you have a large pond. I suggest one for every three square feet of pond surface on larger ponds, keep in mind that they do multiple. At any rate the more snails you have the more benefit you will see in algae control, buy what you feel comfortable with, 10 at a time seems to be a magic number this year for many pond keepers--After all you can always get more net time you visit.
Want to name your snails? You can sex your Japanese Trapdoor Snails if you can view them with their antenna out. Note in the picture above that one antenna is shorter, this is a male, and the reason for the shorter blunted antenna is that is also doubles as its sex organ. The females will have antenna of equal lengths. Also note its mouth, as tiny as it he moves it from side to side as he peruses your pond cleaning an path crystal clear.
