Project Objectives + Experiments


Fresh water systems that are supplied mostly from snowmelt runoff, or have a high water velocity usually do not have a high density population of P. antipodarum. Often vegetated areas with slower water velocity seem to provide refuge for small NZMS and may act as nurseries (Richards et al. 2001).

Water velocities are frequently related to possible passive distribution of this snail. Stream velocities over 0.6 m/s can be expected to facilitate the passive distribution of NZMS since at this velocity all NZMS were observed to detach from its substrate (Lysne 2003). In addition NZMS appear to have a greater attachment velocity then native snails (Lysne 2003).


The optimal salinity is probably near or below 5 ppt, but P. antipodarum is capable of feeding, growing, and reproducing at salinities of 0–15 ppt and can tolerate 30–35 ppt for short periods of time (Jacobsen and Forbes 1997; Zaranko et al. 1997; Leppakoski and Olenin 2000; Costil et al. 2001; Gerard et al. 2003). Populations in saline conditions produce fewer offspring, grow more slowly, and undergo longer gestation periods


Capable of tolerating a wide range of temperatures with upper thermal limits of 34°C and lower thermal limits near freezing (Zaranko et al. 1997; Cox and Rutherford 2000). NZMS reproduction is known to fail almost entirely at 24-27°C; and reproduction is being delayed and fecundity was reduced at cooler temperatures of 12°C relative to the optimum at 18°C (Dybdahl and Kane 2005, Bennett et al in press).  NZMS will probably not reach high densities in cold, headwater streams. Mudsnails will proliferate in cool springs and spring creeks, as well as in waters with moderate winter temperatures. Population of NZMS decreases during winter Gustafson (pers. comm.), Shinn (pers. com.), Kerans (pers. com.), but average water temperatures as low as 7º C (45º F) did not prevent survivorship, growth or reproduction of NZMS in the greater Yellowstone Area (Dybdahl, 2003).


Surveys and  experimental studies, indicates that the snail may be excluded from waters where dissolved solute content is low, and the survival and growth could be  a function of varied specific conductivity (SC) and calcium availability (Herbst et al 2008). Significant reductions in survival and growth were observed among treatments diluting river water from 300 to 50 μS cm–1. No growth was found at or below 25 μS cm–1. Growth was also inhibited in calcium-free artificial water compared to natural river water with the same SC, showing that lack of this mineral impedes development. These results suggest that many streams in the range of 25–200μS  cm–1 cannot support productive NZMS populations and that nuisance invasions may be most prevalent in waters above 200 μS cm–1 where sufficient dissolved mineral content is present for growth (Herbst et al 2008).

Substrate preferences:

NZMS occupies a wide variety of substrates including silt, sand, mud, concrete, vegetation, cobble, and gravel.  Shinn (pers. com.) even reported them in high abundance at depths up to 60 feet in the Snake River. They do however appear to be limited by unstable substrates associated with spring runoff. New Zealand mud snail has been shown to display a preference for sediment-contaminated cobbles and the presence of filamentous green algae (Suren 2005).

NZMS frequently burrow into sand substrate, where they probably consume epipsammic algae (Holomuzki 2003).

Where we are not finding NZMS

While the New Zealand mud snail seems to be invading nearly all types of fresh water bodies, there are some that seem more resistant to invasion.  Fresh water systems that are supplied mostly from snowmelt runoff, or have a high water velocity do not have a high density population of NZMS.  The fact that the NZMS is not present in waters that are supplied with fresh snowmelt runoff, (Statzner & Holm, 1989), as opposed to those that are feed by groundwater, may be due the low dissolved ions concentrations in these waters (Herbst, 2008).  Low ion concentrations, below an SC of 25µS per cm, may reflect the consequence of deficiencies in Na+, K+, and Mg++ for ionic and osmotic regulation.  The dependence on higher ion concentrations can be further supported by the snail's strong presence in brackish water.


Biological Control Development 

Biological control or ‘biocontrol’ is the introduction of predators or parasites that regulate populations of pest organisms in the region where these pests, whether animal or plant, originated so that the same predator-prey or host-parasite relationships can be utilized for invasive species management in infested ecosystems. The overarching objective of our research is to evaluate the use of host-specific parasitic organisms, with an initial focus on the castrating trematode (Microphallus sp.), as a safe, sustainable and cost effective means to suppress NZMS abundance in North America. The introduction of a biocontrol agent depends on establishing that the organism causes substantial pest impact (efficacy) and poses no significant risk to non-target species (safety), including species from the same spring snail family (Hydrobiidae), many of which are sensitive or highly restricted in their geographic ranges.

Specific objectives of our research are to:

  • Establish procedures for inducing infection of P. antipodarum by Microphallus
  • Conduct host-range tests with non-target taxa, both within the Hydrobiidae family and subsequently with less closely related snails
  • Determine effects of modulating environmental parameters on Microphallus infectivity to NZMS haplotypes present in North American
  • Determine if and how infection affects susceptibility to predation by final hosts (ducks), and if parasites have any effects on these hosts
  • Examine NZMS invasion in Australia to determine whether the parasite regulates populations there [where it was fortuitously introduced along with the parasite] and whether Microphallusinfects any Australian snails
  • Initiate a public outreach campaign if biocontrol appears justifiable
  • Conduct a Cost/Benefit assessment using research results and other available information to evaluate ecological and economic effectiveness of biocontrol program implemention

Ecological Assesments 

In support of the NZMS biocontrol development effort, we  are also investigating the ecology of NZMS and its impacts to North American stream ecosystems with a series of field and laboratory studies. Resulting information will be used to evaluate whether the costs and potential risks posed by introducing biocontrol agents into the ecosystem can be justified by the anticipated ecological and economic benefits from NZMS biocontrol. In addition, pre-release population information is necessary in order to document the effectiveness of biocontrol agents in actually controlling the invasive snails, when and if the program is implemented.

Ecological Objectives:

  • Document the status of P. antipodarum invasion in the Pacific Southwest (California/Nevada/Arizona) by regular monitoring of existing population patterns, and assessment of new establishment by NZMS across the region
  • Conduct experiments to measure effects of temperature, light and resource conditions on P. antipodarum growth and survival rates, and reproduction
  • Characterize effects of snail grazing on algal resource quantity and composition (as the resource base for other aquatic organisms)
  • Test presence and strength of competition between NZMS and native Hybrobiidae, other stream invertebrates, especially aquatic insects (the primary resource base for stream fish), and vertebrate herbivores, particularly amphibian larvae or tadpoles [e.g. western toad (Bufo boreas) as a surrogate for the endangered arroyo toad- (Anaxyrus californicus; formerly Bufo canorus) or introduced bullfrog (Rana catesbiana) larvae as a proxy for  relict leopard frog (Rana onca) and red-legged frog (Rana draytonii)
  • Evaluate NZMS as a food resource for invertebrate and vertebrate stream predators, both as a resource for native consumer feeding and growth (being operculate, they often pass through the guts undigested) and for the potential of native predators to provide some level of NZMS populations control
  • Predation studies include feeding and growth trials with common stream fishes, using them as closely related surrogates of endangered taxa (e.g. unarmored threespine stickleback (Gasterosteus aculeatus williamsoni), Santa Ana sucker (Catostomus santaanae), tidewater goby (Eucyclogobius newberryi), Virgin River chub (Gila seminuda), Woundfin - Plagopterus argentissimus) and others.

As many as 14 tremotode parasites in NZ and several native fish species frequently eat them (Ryan 1982, Sagar and Glova 1995, Cadwallader 1975, Lively 1996). Research on potential biological control methods includes the use of a trematode (a fluke), which shows some promise (Emblidge and Dybdahl 2004). 

Our Biocontrol page has more information about the NZMS Biocontrol research taking place in the RiVRLab.

NZMS  Population Stream Monitoring

Piru Creek is a large stream in Northern Los Angeles County and Eastern Ventura County, California. It is a tributary of the Santa Clara River, the largest stream system in Southern California that is still relatively natural. It drains an area of about 497 square miles (1,290 km2) and is about 50 miles (80 km) long. When the water flows, the reach between Pyramid dam and Lake Piru becomes one of the most unique, isolated, and scenic one-day, road accessible, whitewater runs in the Western U.S.  More than 10 miles of Piru creek is totally wild and cannot be accessed by road or maintained trail. Such wilderness is rare in Southern California.

In March 30, 2009 President Barack Obama signed the Omnibus Public Lands Act, extending federal protection to more than 2 million acres and 86 rivers in nine states. Included in the Act are provisions to create new wilderness areas and expand existing wilderness areas in California, and to extend Wild & Scenic protection to eight rivers in California, including 7.25 miles of Piru Creek. Piru Creek is the first stream in Los Angeles County to be protected by inclusion in the National Wild & Scenic Rivers System. Approximately 4.5 miles of this reach were protected.

Unfortunately this unique ecosystem has problem with aquatic invasive species New Zealand Mud Snail. His tiny invaders are posing serious threat to natural ecosystem in California’s rivers, lakes and streams however long-term effects of this exotic species on the indigenous invertebrate fauna are still in search for answers. The lack of extensive historical information about the population changes in Southern California region brings many questions about how the snails behave in Mediterranean type climate. It is critical to accurately define the scope of the current snail infestation to asses if future methods of eradications will have positive effect.

Our team is conducting monthly monitoring of densities of New Zealand Mud Snail to determinate the status and trends in snails populations over time and  to assess seasonal changes in the size class distribution. Each month, we visit the stream and record snail’s densities on heavily impacted site at Piru Creek (Frenchman’s Flat). This site is located at the eastern end of the Santa Ynez Mountain Range (of the Transverse Ranges of Southern California), below Pyramid lake.

Snails collected each month at Piru, are being counted and measured in the lab. Measurements are done using microscope camera. The height and width in µm is measured and recorded for at least 50 randomly chosen snails from each sample. The proportion of juvenile and adult snails is also estimated in each sample. 
These types of studies of abundance will be used to demonstrate:

- the different densities of particular species at different substrate types over time
- the relationship between environmental variables and the density of the snail
- the proportions of different age classes within a population
- give a baseline for future effect of  biocontrol methods implemented to control NZMS


NZMS vs. Abiotic Factors

Since temperature typically influences reproduction and population dynamics and specifically Potamopyrgus (Winterbourn 1970, Okland 1979, 1983, Hylleberg and Siegismund 1987, Quinn et al. 1994 (read more), we reasoned that invasion dynamics might depend on the range of thermal performance.

We are interested how reproduction and growth rates of New Zealand Mud Snail, are being influenced by temperature and light. This project was collaborative effort between RIVRLab(UCSB) and Dr. Sean Anderson and his student Heather Martin from California State University Channel Islands (CSUCI).

NZMS and Primary Producers

We are studying the effect of Potamopyrgus antipodarum on primary produces (periphytion) in streams ecosystems. NZMS is known to consume up to 75% of the gross primary production (Cada 2001, Hall 2001,Hall et al. 2003), Previous research has also shown that the productivity of invertebrate grazers, and hence the productivity of the rest of the food web including the fisheries, is often limited by the quality of the algal food. Thus the impact of NZMS on periphyton communities could be important factor that can influence the whole ecosystem. Major goals of this research are to examine if NZMS and other herbivory grazers vary in ability to choose preferable type of algal food. We are also interested if NZMS could affect the resource availability and particularly resource quality for other herbivory grazers.

    The effect of NZMS on filamentous algae: Cladophora sp.: 
While most of algae is generally being depressed by NZMS grazing, particularly when NZMS is present in high densities, our studies showed that some filamentous  macro algae  likeCladophora sp. can directly benefit from such grazing activity. This macro alga is usually covered by diatoms(Cocconeis pediculus,Gomphonema spRhoicosphenia curvata). But NZMS by selectively grazing on the diatoms, releasesCladophora from direct competition for light and nutrients with other epiphytic algae that growth on their branches(Bennett et al unpublished data).

NZMS Competition with Native Invertebrates

New Zealand Mud Snail, by achieving very high population densities has negative impacts on native fauna. Is known to reduce or displace native invertebrates (e.g., decrease in densities of herbivorous invertebrates, decrease in attached filter-feeding organisms). Abundant populations of introduced P. antipodarum may outcompete other grazers and inhibit colonization by other macroinvertebrates (Kerans et al. 2005). Negative correlation between populations of mayflies, stoneflies,
caddisflies, and chironomids and New Zealand Mud Snail densities of <28,000/m2  was recorded in a spring creek in south western Montana. In Europe, P. antipodarum causes declines in species richness and abundance of native snails in constructed ponds (Strzelec 2005).
Predation Experiments

Predation often has major impacts on prey communities and populations. Understanding these interactions is thus of great importance for invasive ecology.
New Zealand Mud Snail doesn’t have too many natural enemies in introduced habitats. Common predators for NZMS in New Zealand are common bullies (Eleotridae: Gobiomorphus cotidianus)   and short finned eels (Anguilla australis). 
Recently new studies suggested that this role in United States could be could be played by trout, that can ingest the snails, however but the nutrition value is being know to be minimal. Previous feeding studies showed that rainbow trout fed exclusively on NZMS diet, with unlimited availability of these snails, lost from 0.14-0.48% of their initial body weight per day. Collection of Rainbow trout feces showed that  only 8,5% of the shells were empty and these snails were assumed to be digested, 53.2% of NZMS passed through the digestive system alive ( Vinson & Baker 2008)
The overall goals of our project are to study predator and prey behaviors that underlie the impacts of potential predators like invertebrates (dragonfly larvae, damselfly larvae), crayfish and native fish on New Zealand Mud Snail population.


  • We intend to use sub-adult fish (likely to experience growth if fed adequate food) that are acclimated to living in aquaria and withholding food for one day to ensure they are likely to feed without being physiologically stressed. A fish is placed into the test arena (small aquarium) after wet weighing (by volume displacement in graduated cylinder) and allowed to rest for one hour. Snails are then introduced into the arena and fish behavior is observed over the course of one day. Feeding effort and  response to presence of snails are documented, and the number remaining after a 24-hour period is recorded. If feeding on snails occurs, we would continue to provision the test fish with adequate snail numbers and feces collected over the course of 4 days to verify that digestion likely occurred (shell fragments in fecal material). If this is the case, the trial would be continued for a total period of 4 weeks, after which the test fish would again be wet-weighed to determine if it lost, or stayed the same weight (poor digestion), or gained biomass (adequate food source). A minimum of 3 replicate fish would be used for each species.
  • We will be varying the size of the fish, comparing juvenile and adult individuals, along with the size of Potamopyrgus antipodarum, from small (1-3mm) to large (4-7mm). We are particularly interested in rainbow trout, arroyo chub, speckled dace, three-spined stickleback, prickly sculpin, and green sunfish.

Other Predators

  • We are determining which local invertebrates can act as predators to the invasive New Zealand Mud Snail.  To test this, we will collect various sizes of damselfly and dragonfly larvae, as well as crayfish.  We will feed them both juvenile and adult snails and record how many of each was consumed.  
  • Preliminary results suggest that each of these invertebrates does eat the snails and that size of the predator is a factor effecting predation rate.  Further experiments will determine whether these predators prefer to eat the invasive or local species of snails.