New Zealand mudsnail (Potamopyrgus antipodarum)

Synonyms: Potamopyrgus jenkinsi, Hydrobia jenkinsi, Amnicola antipodarum

Common Names: New Zealand mud snail, Jenkins' spire snail

Description:

New Zealand mudsnail (Potamopyrgus antipodarum) is a tiny freshwater snail from New Zealand that has spread into rivers and lakes across much of the United States [1]. It is barely the size of a grain of rice, but a single snail can start a whole population, and it can reach staggering numbers on a streambed [1]. MISIN tracks it because it outcompetes the native bottom insects and snails that fish feed on, gives fish almost no nutrition in return, and is nearly impossible to remove once it arrives [1].

Identification:

New Zealand mudsnail is very small ? usually only about 4 to 6 millimeters long, or up to roughly a quarter inch ? so it is easy to miss [1]. It has a cone-shaped, tightly coiled shell, gray to brown in color, with 7 to 8 turns (whorls), and the coil winds to the right [1]. When the snail pulls back inside, it seals the shell opening with a hard trapdoor (an operculum) [1]. Because it is so tiny and looks like several native snails, it usually cannot be told apart in the field without a hand lens or expert [1].

The snails are present year-round and can coat rocks, plants, and mud in huge numbers ? a small patch of streambed can hold hundreds of thousands of them [1]. Look most carefully in clear, cool streams and along lake edges, where dense crusts of tiny snails on the bottom are a warning sign [1].

At-a-glance cues:

1. Very small snail, only about 4?6 mm (up to ? inch) long [1]
2. Cone-shaped, right-coiling shell with 7?8 whorls, gray to brown [1]
3. A hard trapdoor (operculum) that seals the shell opening [1]
4. Sometimes present in enormous numbers, coating rocks, plants, and mud [1]

Habitat:

New Zealand mudsnail lives on the bottom of fresh water ? streams, rivers, lakes, ponds, and canals ? clinging to rocks, plants, and mud [1]. It tolerates a wide range of conditions, including varied water quality and temperature, which helps it thrive in many kinds of water [1].

The snail is native to New Zealand [1]. It was introduced to North America and first found in the Snake River in Idaho in 1987; it is now established in more than two dozen states, including western trout rivers and the Great Lakes [1]. As a grazer, its "food" is the algae, tiny plant and animal bits, and sediment it scrapes from surfaces ? the same food native bottom invertebrates rely on [1].

Reproduction:

New Zealand mudsnail spreads almost entirely on wet gear and equipment. It rides along on anglers' and paddlers' waders, boots, nets, boats, and buckets ? and felt-soled wading boots are especially good at trapping it [1][2]. It can survive for days out of water if kept moist, can pass alive through a fish's gut, and can float on the water and on mats of algae, so one wet wader can move it from one river to the next [1].

What makes it so dangerous is how it breeds. Almost all the snails in North America are genetically identical females that reproduce without males ? a single snail carries developing young and can found an entire population by itself [1]. Each female can produce a couple hundred young in a year, and the snails can build to extreme densities ? over 300,000 in a single square meter in some rivers [1]. Because of this, the key to control is never moving the snail to a new water in the first place [1].

Impact and Damage:

New Zealand mudsnail is not harmful to people or pets, so the damage is to streams and fisheries. At its enormous densities, it takes over the streambed and outcompetes the native insects and snails that fish ? especially trout ? depend on for food [1]. The mudsnail itself is poor food: fish that eat it often pass it through their gut alive and gain almost no nutrition, so a stream can fill with snails while the fish go hungry [1].

For natural areas, these dense populations also change how algae are grazed and how nutrients cycle through the water, reshaping the bottom community of a river [1]. Once the snail is established, there is no proven way to remove it without harming the whole stream, so an infestation is essentially permanent ? which is why preventing its spread to new waters is the only real defense [1].

Similar species:

New Zealand mudsnail is very hard to tell apart from several small native and non-native snails without a hand lens, so any tiny operculate snail in a new water should be treated with care [1].

The faucet snail (Bithynia tentaculata) is another invasive snail with a trapdoor (operculum), but it is larger (often about ? inch) with a broader, rounder shell and fewer whorls [1]. Several native springsnails and pebblesnails (family Hydrobiidae) are the same tiny size and shape as the mudsnail and can only be separated by close, expert examination [1]. Native pond snails and bladder snails (such as Physa and Lymnaea) can look similar at a glance, but they have no operculum and many coil to the left [1].

Monitoring and rapid response:

Signs to watch for:

1. Very small (rice-grain or smaller) gray-to-brown snails with a pointed, coiled shell and a trapdoor [1]
2. Dense crusts of tiny snails coating rocks, plants, or mud in a stream or along a lake edge [1]
3. Tiny snails clinging to waders, boots, nets, or boats after being in the water [1][2]

What you can do:

1. Because the snails are tiny and a single one can start a population, prevention is the only real tool. After being in any water, clean, drain, and dry all your gear ? waders, boots, nets, boats, and buckets [2].
2. Scrub off all mud and plants and inspect boots, laces, and especially felt soles, where the snails hide [2].
3. Decontaminate gear before moving to a new water: let it dry completely for several days, freeze it overnight, or soak and scrub it in hot water (at least about 120 ?F / 49 ?C) [2].
4. Switch from felt-soled wading boots to rubber soles, which trap fewer snails [2].
5. Never move water, mud, plants, fish, or bait between waters, and never dump bait buckets.
6. Report what you find (below) ? early detection is the best chance to keep it out of a new water [2].
7. Natural area crews and agencies focus on monitoring (including eDNA testing) and on enforcing gear cleaning, because there is no way to remove an established population without harming the whole stream [1].

When to call a professional:

1. Any suspected new occurrence ? confirming the snail's identity and responding to it is a job for your aquatic invasive species or natural resources agency.
2. Any water used by many anglers or paddlers, where the risk of spreading it is high.
3. 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. Potamopyrgus antipodarum (New Zealand Mudsnail) ? Species Profile. nas.er.usgs.gov
2. Minnesota Department of Natural Resources. New Zealand Mudsnail (Potamopyrgus antipodarum). dnr.state.mn.us
3. U.S. Fish & Wildlife Service. New Zealand Mudsnail (Potamopyrgus antipodarum) Ecological Risk Screening Summary. fws.gov

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