Pondkeepers in search of help in controlling algae growth on the bottom, sides and rocks in the pond will be glad to hear that the Japanese Trapdoor snail ( Viviparis malleatus) just might be the answer. This is a live-bearing snail that can survive the winter in northern areas of the country, given an area of the pond that does not completely freeze. As a live-bearer, it only slowly multiplies, having “kids” only a couple of times a year. They will not take over a pond like their egg laying cousins can under the right circumstances.
The Japanese Trapdoor snail is a black shelled, black bodied snail that can grow to approximately 3 inches in size. They will cruise the bottom and sides of the pond looking for algae and uneaten pond fish food. Japanese Trapdoor snails will also help clean live plants and their planting baskets. You may see them on your plants, but they are not eating the plant, only scraping off a thin film of algae growing on the plant. The Trapdoor will eat decaying leafy debris on the bottom of the pond and can hopefully help keep the buildup of sludge under control. Their coloration helps them blend in with the pond walls and pots, but they are still susceptible to predation from raccoons, if these masked bandits are in your area.
When Japanese Trapdoor snails breed, it is best to remove any baby snails from the pond (even the fish can eat these) and grow them out in a separate container until they reach approximately an inch in size. Once they reach this size, they can be added back to the pond. Normal stocking levels for the Trapdoor snail is one snail for every 3 square feet of surface area. If your pond is 15’ by 10’, your total surface area would be 150 sq ft; divided by 3 would be a total of 50 Trapdoor snails. Obviously, you can start with fewer and see how they keep up with the cleaning. Well shaded ponds or ponds with lots of floating plants (Hyacinths, Lilies, Parrot’s Feather, etc) might require fewer snails. For a natural way to help control algae, the Japanese Trapdoor Snail might just be the best option.