Tiny but Powerful: the Suberabundant, Invasive, New Zealand Mud Snail.

A New Zealand mud snail
Source: NBC

As a research assistant for the Center for Watershed Sciences at UC Davis, I see firsthand what invertebrates fill Northern California waterways. Freshwater invertebrates are important indicators of water quality because they are extremely sensitive to conditions such as water temperature, oxygen content, salinity, and nutrients. The presence or absence of a certain species could have varying implications for the true concern of conservationists: fish. Salmon and trout rely on these waterways for their survival, and changes in the water quality can devastate their populations, both directly and indirectly through the abundance of invertebrates: their major food source. 

A mayfly larva (Ephemeroptera), a common group in
rivers and streams. Source: Landcare Research

Fishermen likely already realize the importance of these freshwater invertebrates, as many fly fishing baits are designed to mimic the appearance of common prey for fish. Below is a "scud" used in fly fishing, along with the gammaridean amphipod, or "freshwater shrimp" it is designed to imitate. 

The diversity and abundance of freshwater invertebrates are vital to the health of waterways everywhere, and to our understanding of them. But a major biological invasion began in Idaho's snake river in 1987 that then spread throughout the United States during the past few decades. 

Close-up view of New Zealand Mud Snails. The bottom ruler shows
millimeter measurements. Source: wisconsinrivers.org

Potamopyrgus antipodarum, the New Zealand Mud Snail (NZMS), is now superabundant in California watersheds. I was unaware of their existence until I began sorting them out of a sediment sample, pulling thousands and thousands of tiny snails out of just a few quarts of sand. Scientists report over 500,000 snails per square meter in productive streams, easily becoming the dominant species in many areas of the western U.S. More recently, NZMS were noted in Wisconsin in 2012. 

Because most of the important invertebrate species rely on aquatic algae for food, fishermen and scientists alike are concerned that herbivorous mud snails will compete with them. Additionally, freshwater fish gain less nutritional benefit from eating NZMS than their native prey. The snails even emerge from trout guts whole and alive and tolerate wide ranges of salinities, making them seem near-indestructible. 

New Zealand Mud Snails encrusting a rock
Source: Patch.com

A Ph.D. student at UC Davis tested the impacts of invasive NZMS on native communities in several streams in Northern California, as well as an estuary in Oregon. NZMS were actually most abundant in the estuary, indicating that high salinity benefits these snails. In the freshwater systems, she found a negative correlation between between mud snail abundance and native diversity. This could be because healthy native communities have a better ability to resist invaders or a causative result of the snails diminishing habitat quality for native species. 

She also found that in the estuary, NZMS did not directly compete with species that eat the same types of algae, meaning that they are able to utilize resources in different ways. These results are optimistic for the future of these waterways, indicating that native species can coexist with this invader.

The researcher that I work for, Robert Lusardi, also experimented with NZMS, examining their environmental tolerances. He found that specific conductivity (a measure of dissolved ions in the water, including calcium) limits the snails' survival. The snails require a certain level of dissolved calcium to incorporate into their shells for growth and reproduction. 

Figure 5 of Herbst et al. 2008, linking specific conductivity to survival and growth rate of mud snails. 

This minuscule snail is impacting waterways and important species in large ways, so make sure the next time you enter a stream you take the necessary precautions to ensure you don't help the spread of this, and other, invasive species. 

Thanks for reading!

Sources

Herbst, D.B., Bogan, M.T., and Lusardi, R.A. 2008. Low specific conductivity limits growth and survival of the New Zealand mud snail from the Upper Owens River, California. Western North American Naturalist 68(3): 324-333.

Brenneis, V.E.F., Sih, A., and De Rivera, C.E. 2010. Coexistence in the intertidal: interactions between the non-indigenous New Zealand mud snail Potamopyrgus antipodarum and the native estuarine isopod Gnorimosphaeroma insulare. Oikos 119(11): 1755-1764.

Brenneis, V.E.F. 2009. Impacts of the invasive New Zealand mud snail in aquatic food webs: Community patterns, competitive effects, and trophic interactions. Dissertation.