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  • Robert Morgan, Aquatic Invasive Species Ecologist

New Zealand Mudsnail in Spring Creek

Pennsylvania is facing increasing problems with incursions of aquatic invasive species (AIS). One of the most recent invaders is New Zealand Mudsnail (Potamopyrgus antipodarum) (NZM), which was discovered in Spring Creek in 2013. This was the first known occurrence of the species on the U.S. Atlantic slope. NZM have the potential to create a negative competitive impact on benthic communities of infested waterways, thus disrupting the base of the food web upon which higher organisms, such as fish, depend.

New Zealand Mudsnails are very small, with adults ranging from 3 to12 mm (0.1-0.5 in) in their native range, and only 3 to 6 mm (0.1-0.24 in) in invasive populations (Figure 1). The shell is elongate and coiled to the right in 7 to 8 whorls separated by deep grooves (Figure 2). Native to New Zealand, the species has spread to Europe, Asia, Australia, and North America.

Figure 1 (left). NZM infested rock from Spring Creek (PA Fish and Boat Commission)

Figure 2 (top). Adult and juvenile NZM (Jeffrey Dimick, Aquatic Biomonitoring Laboratory, Univ. of WI – Stevens Point)

At least three genetically distinct clonal populations exist in North America and are designated US1, US2, and US3. Clone US1 is widely dispersed in the American west and British Columbia, US2 is primarily found in the Great Lakes where it was discovered in 1991, and US3 is found in only one location in the Snake River in Idaho and was discovered in 1987. The US1 clone has been spreading east and it is the clone found in Spring Creek. Where in the western states the Spring Creek NZM originated is uncertain, but it is likely the Spring Creek population was introduced by recreational water users, probably anglers who unknowingly brought the invasive in on their gear.

New Zealand Mudsnails can be spread to new locations via attachment to boating and fishing and other recreational equipment, through the commercial movement of aquaculture products and aquatic ornamental plants, in mud attached to the bills or legs of birds, or even within the gut of birds or fish (PA Sea Grant 2013). Once introduced, NZM can reproduce very rapidly with each female producing an average of 230 juveniles per season. In the U.S., all populations are ovoviviparous (eggs develop and hatch in the female’s brood pouch) and parthenogenetic (asexually reproducing) consisting almost exclusively of clonal females, which are born with developing eggs and embryos and can produce daughters at about twice the rate of sexual females. These traits give the invasive snail the ability to reproduce and create colonies from a single triploid female (USGS 2012).

History of the Spring Creek NZM Discovery

Numerous gastropods were collected during the Pennsylvania Department of Environmental Protection (PA DEP) 2013 benthic macroinvertebrate sampling in Spring Creek. In their samples, PA DEP biologists discovered a previously unknown species. The specimens were sent to and identified as NZM by Dr. Robert Dillon, a malacologist at South Carolina’s College of Charleston.

When DEP received confirmation from Dr. Dillon in early October 2013 of NZM in Spring Creek, the Pennsylvania Fish and Boat Commission (PFBC) was first made aware of the situation. PFBC requested that samples be sent to another gastropod specialist for a second opinion. Consequently, PA DEP sent samples to Dr. Robert Hershler, research zoologist and curator of Molluscs at the Smithsonian Institution National Museum of Natural History. Dr. Hershler also confirmed the identity of the specimens as NZM.

PFBC and Penn State NZM Surveys

In late October 2013, PFBC Division of Environmental Services biologists conducted cursory NZM surveys of Spring Creek to quickly and informally ascertain the geographical extent of the NZM infestation. The species was found in a stretch of the creek roughly extending from the Shiloh Road bridge northward to about the West Water Street (PA Route 550) bridge in Bellefonte with Fisherman’s Paradise having by far the highest densities of the snails. Since that early survey, both PFBC and Penn State biologists have surveyed most of the Spring Creek watershed looking for NZM. Unfortunately, the invader was found in most samples including those from all major Spring Creek tributaries.

In the initial 2013 PFBC survey, dependent on the sampling location, NZM densities were estimated in the tens to low hundreds per square meter (m^2). In 2016 (the latest data available), a PFBC Spring Creek benthic macroinvertebrate survey indicated Fisherman’s Paradise densities had reached approximately 24,000/m^2. While this may seem high, and certainly represents a worrisome increase in snail density, NZM populations in several western states’ streams have reached densities of up to about 500,000/ m2. At that density, NZMs virtually cover the entire bottom of the stream and do major damage to the resource. While the effects of NZM on eastern waterways are still uncertain, based on the known effects of NZM in western U.S. streams, the outcome of the Spring Creek infestation is not likely to be favorable to the health of the watershed.

New Atlantic slope NZM populations

As of the summer of 2018, NZM have been found at three new Atlantic slope locations. Most recently, the snails were found in Little Lehigh Creek, Lehigh County and earlier were found in the Gunpowder River in Maryland and in the Musconetcong River in New Jersey (near Riegelsville, PA), which is a tributary of the Delaware River. The genetic investigation of specimens from these waterways is just beginning, so the origins of these populations is currently unknown.

Preventing the Spread of NZM and other Aquatic Invasive Species

Both plant and animal AIS are having major negative impacts on waterways across the Commonwealth. There are approximately twenty AIS in the state that are considered major problems and there are about two hundred total AIS found in Pennsylvania’s waters or that pose near term threats along the state’s borders.

Spring Creek and the other NZM infested streams above have at least one thing in common – they are all heavily fished streams, with anglers travelling to them from all over. While vectors spreading NZM may include animals and birds, by far the greatest likelihood is that the snails are being spread by human activities. This stems in large part from NZM being so small that they are hard to detect visually, particularly on wading gear (Figures 3 and 4).

Figure 3 (left) and Figure 4 (right): NZM can be very hard to detect on wading (and other) gear to which the mollusks readily cling (Photos: Ashley Wilmont, Pennsylvania Council of Trout Unlimited)

To prevent or at least slow the spread of AIS by humans, the following biosecurity disinfection measures are recommended for any gear, boats, etc. exposed to Pennsylvania waterways:

For all AIS except NZM:

  1. Visually inspect gear and remove and dispose of any clinging matter in the trash. Do not move mud, organic matter with you

  2. Completely dry the gear – continue drying 48 hours after it is dry to the touch

  3. Use chlorine bleach solution to disinfect the gear – liquid sodium hypochlorite (e.g. household bleach) in the following concentrations for the specified exposure times: 5% by volume for 30 minutes or 10% by volume for 10 minutes. Please follow appropriate safety precautions when using sodium hypochlorite and be aware that it can be corrosive to some plastics, rubber, some metals, and possibly other materials.

  4. Freeze gear for a minimum of 8 hours

  5. Steam clean or soak gear in hot water (120°F to 140°F) for 5 to 10 minutes. It is preferable to use liquid detergent in the soak water. This method is not recommended for GOR-TEX®

Specific for NZM: Because NZM can close-off their shells to avoid chemical disinfectants such as chlorine and they can survive out of water for several weeks, only thermal methods (4 and 5 above) are effective against the snails. In addition, NZM disinfection can be achieved by soaking gear for 5 minutes in a 1 to 1 solution of Formula 409® Cleaner Degreaser Disinfectant (a commercial cleaning product) and water. Simply spraying gear with the disinfectant or the mixture does not work. After soaking, thoroughly rinse the gear with plain water. Also note general cleaners (including other 409 products) have not been shown to be effective against NZM.

In summary, because of the resistant nature of NZM, only the thermal disinfection methods (4 and 5 above) will work for all known AIS in Pennsylvania.

Pennsylvania AIS Occurrence Locations

For the most up-to-date location data for AIS in Pennsylvania, go to: and

For more information about New Zealand Mudsnail, go to:



Alonso, A. and Castro-Díez , P. 2008, What explains the invading success of the aquatic mud snail Potamopyrgus antipodarum (Hydrobiidae, Mollusca)?, Hydrobiologia (2008) 614:107–116

Pa Sea Grant, New Zealand Mudsnail Potamopyrgus antipodarum (factsheet),

USGS, Potamopyrgus antipodarum (factsheet), , updated June 2012

Vinson, Mark A. and Baker, Michelle A. 2008, Poor Growth of Rainbow Trout Fed New Zealand Mud Snails Potamopyrgus antipodarum, North American Journal of Fisheries Management, 28:701-709


Robert Morgan serves as both the Conservation Planning Biologist and Lead Aquatic Invasive Species Ecologist for the Pennsylvania Fish and Boat Commission.

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