Impact on Streams' Ecosystem

NZMS is a nocturnal grazer, classified as a scraper/grazer, feeding on plant and animal detritus, epiphytic and periphytic algae ( Broekhuizen et al. 2001; James et al. 2000; Kelly and Hawes 2005; Parkyn et al. 2005; Zaranko et al. 1997, Winterbourn 1970; Haynes & Taylor 1984; Winterbourn & Fegley 1989). Because algae are the primary producers for the entire food chain, the New Zealand mud Snail’s interactions with them can create repercussions that will affect many organisms.  

It was noted that algal based ecosystems have the potential for higher production rates of NZMS then detritus based ecosystems (Hall et al. In Preparation).

NZMSs have also been associated with an increase in algal biomass. Possibly the snails fertilizing the periphytic algae by excreting ammonia (Riley 2002).

While many algae is generally  being depressed  by NZMS, particularly when NZMs is present in high densities, our studies showed that some filamentous  macro algae  like Cladophora sp. can directly benefit from such grazing activity. This macro alga is usually covered by diatoms (Cocconeis pediculus, Gomphonema sp, Rhoicosphenia curvata) that growth on its branches. But NZMS by selectivly grazing on the diatoms, releases Cladophora from direct competition for light and nutrients with other epiphytic algae that   growth  on  their branches (Bennett et al unpublished data) 

In contrast to our findings, in the Hagerman Valley - Snake/Salmon River area the replacement of a dominant keystone algae (Cladophora glomerata), with a less nutrient dependent algae, has been correlated with the introduction of NZMS (Shinn 2002).


Native invertebrates

 Schreiber et al. (2002) found that low densities of P. antipodarum (2000–4500 ind./m2) facilitated colonization of native stream fauna in short-term experiments in Australia. However, densities of P. antipodarum are often higher than the densities in the Schreiber et al. (2002) study. At high densities, P. antipodarum may compete with native macroinvertebrates for food or space.
Potamopyrgus antipodarum is a generalist feeder (i.e., both grazing herbivore and detritivore; Haynes and Taylor 1984) that uses the same food as many other macroinvertebrates (mostly periphyton and detritus), (Winterbourn and Fegley 1989, Death 1991). P. antipodarum is likely to compete with other grazers through exploitation. Given the remarkably high densities that P. antipodarum can reach; it also may compete through interference with macroinvertebrates that attach to rock surfaces (e.g., net-spinning caddisflies). Competition among P. antipodarum and native species could reduce the abundances and change the distributions of native species.
Presence of NZMS is known to reduced food resources and densities of native macroinvertebrates, particularly mayflies, caddisflies, and chironomids when densities of NZMS are very high (e.g. 28,000/m2 Chelsea Cada (pers. com.). Other studies are also showing negative correlation between NZMS and mayfly, stonefly,and caddisfly taxa (Kerans pers. com.).

There is concern that several native, threatened and endangered freshwater snails may suffer as a consequence of NZMS competition (Bowler 1991, Strayer 1999, Richards et al. 2001). As example, NZMS was found to out compete the threatened Bliss Rapid snail (Taylorconcha serpenticola) in the Snake River (Richards 2003). Over 75% of gastropods listed as threatened or endangered in North America are hydrobiids, the same family as NZMS, so their ecological similarity amplifies the concern that competitive displacement is a serious risk for federally protected species. 



Generalist predators such as fish, crayfish, and even ducks, have been considered as agents to regulate these and other snail populations (Roberts and Kuris 1990, Hofkin et al. 1992, Kuris 1994, Ndelea and Chimbari 2000, Ben-Ami and Heller 2001, Slootweg et al. 2004).
Very little information exists concerning the use of mud snails as a food resource by fish.  NZMS being operculate can pass through digestive systems intact (Ryan 1982, McCarter 1986, Vinson, unpub. data). They are generally avoided by most fish, based on gut analyses of several cyprinids and other benthic feeding species in Montana (Kerans, unpub. data). Bondeson and Kaiser (1949) and Haynes (1985) reported that mudsnails can pass through trout digestive tracts. NZMS have been found in the alimentary canal of mountain whitefish, Prosopium williamsoni, in the Madison River, but the amount of nutrition gained is unknown (Dwyer pers. com.).

Recent research indicates that for example starved crayfish (Pacifasticus sp.) have been observed to consume NZMS in a laboratory setting (Lysne 2003). The crayfish were more likely to consume another native snail (Valvata utahensis), but the difference was not statistically significant (Lysne 2003).

Our lab is working on determining whether any invertebrates or fish might play the role of natural predators in waters of the Western US, and whether they are capable of digesting the snail itself.



Potamopyrgus antipodarum also appears to be a poor food resource for secondary consumers such as fish, because it provides little energy and may pass through the gut undigested (Ryan 1982, Haynes et al. 1985, McCarter 1986). Usually fish avoid eating NZMS and prefer other macroinvertebrates instead, but when NZMS is present in high densities it may outcompate other grazers. Thus, effects of its presence may propagate through food webs and change community structure and function (Carpenter et al. 1985). Decreased growth of fish (brown trout, sculpin, white sucker and longnose dace) in high NZMS density areas was reported, particularly for sculpins (Cada 2003). NZMS have also been found in the stomachs of brown trout and suckers, demonstrating that the fish can eat snails (Dwyer 2001). Dwyer (2001) also has found NZMS while abundant in whitefish stomachs, is less often found in trout. In addition, close to 85% of mudsnails can survive in the stomachs of trout for 2.5 hours and some may survive as long as 5 hours in trout stomachs (Dwyer 2001).

It hase been speculated NZMS are potentially hosts for fish parasites such as the eye fluke (Diplostomum spathaceum), which is a parasite that has snail, fish and bird hosts (Staton 2003). NZMS populations on the Oregon coast inhabit important anadromous fish habitats and they may compete with critical prey species for juvenile salmon (Chapman 2003).