Japanese mysterysnail (Cipangopaludina japonica)

Synonyms: Bellamya japonica, Heterogen japonica, Viviparus japonicus, Cipangopaludina japonicus

Common Names: Oriental mystery snail, Japanese black snail, Japanese trapdoor snail

Description:

Japanese mysterysnail (Cipangopaludina japonica) is a large freshwater snail from eastern Asia, one of several non-native "mystery snails" now living in North American waters [1][3]. It arrived through the aquarium and pond trade and Asian food markets, and people spread it by dumping aquariums and moving boats and bait [1]. MISIN tracks it because it builds up to very high numbers, competes with native snails, and can carry parasites that affect people and wildlife [1][2].

Identification:

The Japanese mysterysnail is one of the largest freshwater snails you are likely to see ? its rounded, globe-shaped shell can reach about 2.5 inches (6 cm) tall, with six to eight whorls [1]. The shell is light brown to olive-green in young snails and darkens to brown with age, and there is only a very narrow navel-like opening (umbilicus) on the underside [1]. A key feature is the "trapdoor": a hard, round plate (the operculum) that the snail pulls shut to seal itself inside its shell, which also lets it survive out of water for a time [1]. The name "mystery" comes from the way females seem to produce young snails with no eggs in sight ? they give birth to live crawling young [1].

These snails are easiest to find in the warmer months in shallow, slow or still water, crawling on mud bottoms or on plants and rocks; their large empty shells also wash up along shorelines [1]. Because several mystery snails look alike, the large size and the trapdoor operculum are the quickest clues that you are looking at one of this group [1].

At-a-glance cues:

1. Large, rounded (globe-shaped) shell up to about 2.5 inches tall [1]
2. Light brown to olive-green shell, darkening to brown with age, with 6?8 whorls [1]
3. A hard, round "trapdoor" plate (operculum) that seals the shell opening [1]
4. Often on mud bottoms in slow, shallow water; big empty shells on shorelines [1]

Habitat:

Japanese mysterysnails live in calm fresh water ? lakes, ponds, slow rivers, ditches, and rice paddies ? usually on soft mud or silt bottoms [1]. They feed on detritus (decaying matter), algae, and sludge, and they tolerate low oxygen and poor water quality, which lets them thrive in waters that stress native snails [1]. Their trapdoor lets them seal up and wait out dry or harsh conditions [1].

The snail is native to Japan, Korea, and Taiwan in eastern Asia [1]. In North America it is an introduced species, brought in through the aquarium and pond trade and the food trade, and now found in scattered lakes and rivers including parts of the Great Lakes region [1]. It continues to turn up in new waters, almost always where people have moved it [1][3].

Reproduction:

The way Japanese mysterysnails reach new waters is almost always people. They are dumped from aquariums and backyard water gardens, released from food markets, and carried between lakes on boats, in bilge or bait water, and on gear [1]. Because the trapdoor operculum lets a sealed snail survive for a while out of water, it can hitch a ride and still be alive when it arrives [1].

Once in the water, they multiply fast. Unlike many snails, females do not lay eggs ? they give birth to live, fully formed young, carrying anywhere from about 10 to 120 developing young at a time, and they can live up to about eight years [1]. This means a single released female can start a population on her own, and numbers can build into the thousands [1]. The practical lesson for control is prevention: never release or move these snails, since once they are established they are very hard to remove (see Management) [1].

Impact and Damage:

Japanese mysterysnails are not poisonous to touch, but there is a real health note: they can carry parasites, including a rat lungworm (Angiostrongylus cantonensis) that can cause a form of meningitis in people, so the snails should not be eaten raw and hands should be washed after handling them [1]. On a property, they can build up in a pond, and their large empty shells wash up and litter shorelines and beaches [1].

In natural waters, the bigger concern is sheer numbers. Japanese mysterysnails can reach very high densities ? historic seine hauls in Lake Erie reportedly pulled up as much as two tons of these snails at once ? where they compete with native snails for food and space [1]. They can also pass parasites to fish, birds, and other wildlife, and large die-offs can foul water and shorelines [1][2]. Because they are hardy, breed quickly, and spread through the pet and food trades, keeping them from reaching new waters is the main line of defense [1][2].

Similar species:

Several large, round-shelled "mystery snails" look alike and are easily confused.

Chinese mystery snail (Cipangopaludina chinensis) is the closest match ? a non-native snail in the same genus, nearly the same size and shape. The two are told apart only by subtle shell features and often need an expert to separate, and both are invasive and managed the same way.

Banded mystery snail (Viviparus georgianus) is another mystery snail, but it is usually smaller and its shell carries several reddish-brown spiral bands, which the Japanese mysterysnail lacks.

Native pond and river snails (other members of the family Viviparidae) share the general rounded shape and trapdoor operculum but are generally smaller, and an expert can separate them by shell shape and details. A very large, globe-shaped snail with a trapdoor is far more likely to be one of the introduced mystery snails.

Monitoring and rapid response:

Signs to watch for:

1. Large, rounded brown-to-olive snails (up to ~2.5 inches) with a hard trapdoor plate, on mud bottoms in slow, shallow water, mainly in the warmer months [1]
2. Big empty snail shells washed up along a shoreline or beach [1]
3. Snails appearing in a pond or lake fed by aquarium or water-garden dumping [1]

What you can do:

1. Never dump aquarium or water-garden snails, plants, or water ? or food-market snails ? into any pond, lake, stream, or storm drain; rehome them or seal them in a bag for the trash [1].
2. Do not move snails, water, or bait between waterbodies, and clean, drain, and dry boats, trailers, and gear before moving between waters [1].
3. On your own property, you can hand-collect snails and dispose of them in the trash; wash your hands afterward and never eat them raw, because they can carry parasites [1].
4. Report snails found in a new lake or river (below), since early reports help track and slow the spread [1].
5. Lake associations and crews should follow clean-drain-dry practices, monitor access points, and coordinate with the state aquatic-invasive-species program, since there is no easy way to remove an established population [1][2].

When to call a professional:

1. A suspected new infestation in a lake or river, or snails fouling a pond or water system ? contact your state aquatic-invasive-species program or local extension office for current guidance [1][2].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Japanese Mysterysnail (Cipangopaludina japonica) ? Species Profile. nas.er.usgs.gov
2. U.S. Fish & Wildlife Service. Ecological Risk Screening Summary: Japanese Mystery Snail (Cipangopaludina japonica). fws.gov
3. Center for Invasive Species and Ecosystem Health (Invasive.org). Japanese Mystery Snail (Cipangopaludina japonica). invasive.org

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