The miniscule snail that has invaded rivers and streams across the West has been confirmed in Colorado for the SECOND time, raising concerns that the fast-spreading invertebrate could push out native species and compromise the long-term health of the region’s aquatic ecosystems. New Zealand mud snails (Potamopyrgus antipodarum), which are native to the Southern Hemisphere, were found recently in the South Platte River in Elevenmile Canyon, below Elevenmile Reservoir Dam.  In the Fall of 2005, they were first discovered in Boulder Creek, just northeast of Boulder. The discovery was somewhat unexpected, but n ot surprising because the nearest known population of mud snails is in the Green River in northeast Utah.

U.S. biologists first discovered the snails in Idaho’s Snake River some 20 years ago. Aside from Colorado, they have spread into Montana, California, Arizona, Oregon, Utah and Wyoming, including Yellowstone National Park. The only other known population in the United States is in Lake Ontario between New York and southeastern Canada.  Colorado biologists had focused their monitoring efforts in northwest Colorado because they were concerned the snails might spread downstream from Utah. So far, however, there have been no confirmed reports that the snails have spread into the Green River in Colorado.
 
Researchers believe New Zealand mud snails may have been introduced to North America by European ships sailing into the Great Lakes. The snails have invaded streams and rivers across Europe, and appear to be working their way through North America.  While the snails can cling to birds and other wildlife, human activities are another possible way these snails are spread. Biologists indicate that these organisms can stowaway on boats and other watercraft, and on boots, waders, nets and other fishing gear.

 New Zealand mud snails are nearly impossible to contain once they have invaded an aquatic ecosystem.  For instance they are so small (only up to 6 mm in length) that they cannot be skimmed from waters. Highly resilient, the snails can survive several days out of water and can withstand a wide range of temperatures. The tiny invertebrates can even pass unscathed through the digestive tracts of fish. Because they are self-reproducing “livebearers” that give birth to well-developed clones, it only takes one New Zealand mud snail to start a new colony in a stream or river. These asexual reproductive capabilities ensure their long-term survival.

The Colorado Division of Wildlife is trying to limit the spread of the snails to other streams in Colorado, through outreach efforts and through guidelines outlined in a New Zealand mud snail management plan.

PART I.   Biology and Ecology of the New Zealand mud snail 

A.  Life History

 New Zealand mud snails are small organisms, only reaching up to 6 mm in length in western U.S. populations.  They become reproductively active when they reach 3 mm. With growth rates ranging between 0.03-0.1 mm/d (depending on water temperatures), these snails can reach maturity after only 6-9 months. Amazingly, mud snails that have invaded the Western US are all females and do not require males to reproduce. In other words, they are a parthenogenic, clonal species. Females bear between 20-120 live offspring every three months during warmer season (between March and October) in the western US. Thus, it only takes one individual snail to start a new population in a stream! In the absence of co-evolved predators and trematode parasites, New Zealand mudsnails can multiply to astounding numbers if conditions are favorable. For instance, in under a decade, snail densities have gone from undetectable levels to 10,000 - 500,000 snails per square meter of streambed in rivers of Yellowstone National Park! 

 B.  Preferred Habitat

The New Zealand mud snail inhabits a wide range of aquatic habitats in the US, including rivers, streams, and reservoirs of the west, the Great Lakes in the mid-west, and estuaries in the west coast.  Thus far, they have not been identified in small pond or wetland habitats.  Although these snails can tolerate a wide range of temperatures (from near 0º C to 32º  C in laboratory tests), they prefer thermally stable streams with moderate flow variability in the western US. Regulated rivers, spring, or geothermal-driven streams, and low gradient, foothills streams better fit the habitat requirements of the mud snail than high elevation, cold and turbulent mountain streams or canyon-bound, flash-flooding creeks.  Because of desiccation resistance and salinity tolerance of the New Zealand mud snail, streams that temporarily dry out or have high salinity in some reaches cannot be excluded as potential habitats.  Within a stream, the omnivorous New Zealand mud snail can be found consuming diatoms, periphyton, or decaying plant material in cobble and gravel substrates or on aquatic vegetation. Like most of our native “grazing” invertebrates, they are less common in sandy or silty areas.  

C.  Transportation Vectors 

How the New Zealand mud snail first arrived in western US water bodies is still somewhat of a mystery, but the spread is clearlt tied to trout fly-fishing anglers.  Fish aquaculture operations and fish stocking have also been implicated as a potential invasion vectors for the snail in some streams and rivers of Idaho.  Biologists indicate that other human-related vectors have played a large role. Potential “human” modes of invasion include the snails “hitchhiking” on gear of aquatic recreationists, such as boats, rafts, fishing equipment, and waders/boots/sandals.  Even spreading via clinging to fire-fighting earth-moving equipment, or to feet of teachers, school teachers, hikers, and pets (dogs, pack animals) have been considered.  How does the New Zealand mud snail take advantage of these vectors? They are desiccation resistant and can survive for days up to weeks on damp to wet materials that are kept at high humidity between 9-25 ºC.  Recreational gear and earth-moving equipment that are not thoroughly dried before their next use may serve as a mass transit for these snails.  Once the New Zealand mud snail colonizes a stream, they can spread within that stream through several dispersal mechanisms. First, they are considered a “fast” snail, with movement across the streambed at rates of 1 meter per hour.  Second, unlike many native snails, they do not “attach” to substrate and can thus drift in the water column when they are small or when the streambed is disturbed. Finally, there is some evidence to suggest that they can survive for several hours in the digestive tract of fish, and can thus travel via fish movement.   

D.  Ecological Concerns 

Any new biotic component to an aquatic ecosystem, including invasive species such as the New Zealand mud snail, must carve a niche for itself.  In doing so, the structure (e.g. species diversity) and function (e.g. energy flow) of the native food web is disrupted.  The invasive snail competes with native invertebrates, including native mollusks, for space and food resources. Because of their prolific nature, mud snails can compromise up to 80% of invertebrate biomass and can consume more than 75% of the gross primary production in the stream. Thus, they control the energy dynamics and nutrient cycling in the aquatic ecosystem.  Adverse impacts to lower levels of the food web may have implications for organism at higher levels, such as fish, which rely on lower-level organisms as a food source. Mud snails may reduce the availability of native invertebrate prey for fish such as trout and sculpin, and at the same time, are not a viable food source themselves!  Their hard shell and resistance to digestion allow them to pass through a fish gut unscathed, thus lending no nutrition or calorie input to the fish.  Interestingly, negative impacts on fish populations have not yet been documented in areas of high mud snail densities. While fish can still swim to un-invaded reaches to seek food, many biologists feel that it is only a matter of time until the mud snail spreads far enough within invaded streams to begin having a negative impact to fish growth. In general, it often takes decades for the impacts of an invasive species on native biota to fully manifest. 

E.  Identification of the NZMS

 New Zealand mud snails, because of their small size and similar coloration to native Colorado mollusks, take some training to identify.  The following are the characteristics of the mud snails found in the western United States:

 Color:  dark to very dark brownish

Size:  to about 6 mm in length

Shell:  dextral (if held tip down and opening toward you, the opening is on the left), the shell has about 5.5 strongly convex whorls (North American strain lacks a ridge or processes on the whorls).   The shell opening is roundish to somewhat ovate as opposed to strongly ovate or lunate as in the Genus Lymnaea.  There is an attached operculum which can close off the opening.

Animal:  The animal tissue squashes under low power microscopy show pale to medium yellowish embryos contrasting with much darker parental tissues. Embryos possess eye spots.

6. Control

The only sure way to prevent spread by anglers, or researchers is to make sure that waders, boots, tackle, sampling gear, etc. are thoroughly washed, and soaked in a solution of 50% water and 50% Formula 409^®  for 5 to 10 minutes before the boots, waders, equipment are used in another stream. 

Freezing overnight will also kill the snails.