Potamopyrgus antipodarum

New Zealand mudsnails (Potamopyrgus antipodarum) are native to freshwater streams and lakes of New Zealand and small, neighboring islands. However, by several speculated means of human introduction, they have become an invasive species in Australia, Europe, and North America. Occurrences of the species in North America, most of which are in the western United States, have been carefully documented since it was first discovered in Idaho. The snails are known to be established in Oregon, California, northern Arizona, New York, and Canada, affecting major freshwater systems such as Lake Ontario, Lake Erie, and Lake Superior. They also have been recently introduced in Japan. (Alonso and Castro-Díez, 2008; Benson and Kipp, 2009)

New Zealand mudsnails prefer to live in streams and the littoral zones of lakes. They prefer shallow areas but can be found up to 60 m deep. They can be found in aquatic habitats of varying substrate types, including silt, sand, gravel, cobble, and macrophyte/vegetation. Densities are highest in macrophyte habitats and lowest in silt/sand habitats. Individuals of this species live in both eutrophic and clear waters, but they thrive in disturbed or degraded waters. New Zealand mudsnails can tolerate a wide range of temperatures, from near freezing to 34ºC. The optimal salinity of the water for the snails is near 5%, but they can tolerate brackish waters and even survive salinities as high as 30 to 35% for short periods of time. New Zealand mudsnails prefer low water velocities but can be found in high velocity areas buried in the sediment or underneath cobbles and boulders. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003; Benson and Kipp, 2009; Gustafson, et al., 2004; Richards, 2002)

New Zealand mudsnails are shelled organisms that are either gray in color or some shade of light to dark brown. Male and female New Zealand mudsnails are very similar in physical appearance, but females are distinguished from males by the presence of developing embryos in their reproductive systems. In the western United States, the average length of the shell of the New Zealand mudsnail is 4 to 5 mm, with a maximum length of 6 to 7 mm. In their native range, the maximum length of the shell is 12 mm. The surface of the shell is characterized by right-handed coiling of 5 to 6 whorls demarcated by sulci. The shells of some individuals have a keel in the middle of each whorl and/or spines for defense against predators. A terminal oval aperture covered by a thin operculum is also present. New Zealand mudsnails may resemble snails native to the United States, but they are distinguished by their longer, narrower shells that have a greater number of whorls. (Alonso and Castro-Díez, 2008; Benson and Kipp, 2009; Crosier and Molloy, 2010)

New Zealand mudsnails are ovoviviparous, meaning that the development of embryos in their eggs actually occurs within the female. After completing development, the eggs hatch within the female, and the female then gives birth to the young snails. New Zealand mudsnails have been observed to grow 0.1 mm/day at 21ºC under laboratory conditions, with growth rates depending on the size of the individual. Females reach maturity at 3 to 6 months of age. (Crosier and Molloy, 2010; Gustafson, et al., 2004)

New Zealand mudsnails are dioecious. Populations in New Zealand consist of sexual males and both sexual and asexual females, whereas introduced populations are comprised entirely of asexual females.

In their native range, individuals that reproduce sexually are promiscuous. During copulation, the male is found on top of the shell of a female, and the apertures of the two snails are in contact. Females can either maintain their position and proceed with mating or move in a manner that displaces the males. The duration of copulation is typically between 20 minutes and 1.5 hours. Males do not discriminate between sexual females and asexual or parasitically-castrated females, although their genes will not be passed to the offspring of the latter two types of females.

Populations found in the United States consist of triploid females that reproduce asexually by way of parthenogenesis. This type of asexual reproduction is also observed in their native range and leads to populations of genetically identical females or clones in both their native and introduced habitats. (Benson and Kipp, 2009; Crosier and Molloy, 2010; Neiman and Lively, 2004)

In New Zealand, reproduction typically occurs every three months. In the western United Staes, reproduction occurs throughout the year, with seasonal peaks during the months of March and October. Females reach sexual maturity at a shell length of 3 mm and produce approximately 230 young per year. Larger females produce more offspring than smaller females, and asexual females produce double the number of female offspring produced by sexual females. This species is ovoviviparous, carrying as many as 10 to 120 eggs at a time for development and giving birth to live snails. Developing embryos are sometimes present within the reproductive system of asexual females at the time of birth. (Benson and Kipp, 2009; Crosier and Molloy, 2010; Gustafson, et al., 2004)

This author found no published information on parental investment by New Zealand mudsnails.

Although females retain their eggs until they hatch, it's not known whether the embryos are nourished in anyway other than via the yolk created when each egg is produced.

Under laboratory conditions, marked individuals were observed to survive over one year. The lifespan of New Zealand mudsnails in natural conditions is unknown. (Gustafson, et al., 2004)

New Zealand mudsnails exhibit positive rheotactic behavior -- they tend to crawl against the current in flowing water. One authority estimated substrate cruising speed at greater than one meter per hour, pretty fast for a snail. They also float, alone and in mats of algae such as Cladophora. During unfavorable environmental conditions, such as dry or cold periods, individuals of this species are observed to bury into the substrate. (Alonso and Castro-Díez, 2008; Benson and Kipp, 2009; Gustafson, et al., 2004)

New Zealand mudsnails are reported to be nocturnal grazers, although non-brooding females and juveniles foraging more during the day. It is believed that non-brooding females and juveniles behave in this way that risks predation in order to obtain the necessary energy required for reproduction and growth, respectively. Infection by trematode parasites of the genus Microphallus alters the foraging behavior of the snails, causing them to forage more during the morning hours when ducks, the predators of New Zealand mudsnails and the final hosts of Microphallus, are foraging. This change in foraging time increases the likelihood of parasite transmission. (Benson and Kipp, 2009; Levri and Lively, 1996)

Home range sizes for New Zealand mudnails are unknown.

Perception in New Zealand mudsnails is mainly via chemical cues. In their native range, the chemical odor of predatory fish causes the snails to hide under rocks in an attempt to evade predation. They are also able to sense light. (Benson and Kipp, 2009)

New Zealand mudsnails are considered scrapers/grazers. Their diet consists of diatoms, epiphytic and periphytic algae, and animal and plant detritus. Therefore, they can be considered planktivores, algivores, and detrivores. (Benson and Kipp, 2009)

Predation on New Zealand mudsnails in North America is unknown. They are known to survive passage through the digestive tracts of some birds and fish, including mountain whitefish and rainbow trout.

However, in their native range, the snails are consumed by several species of fish and waterfowl and are infected by as many as 14 parasitic trematodes of the genus Microphallus. Levri and Lively (1996) observed the foraging behaviors of grey ducks (Anas superciliosa), mallard ducks (Anas platyrhynchos), black swans (Cygnus atratus), Canada geese (Branta canadensis), and scaups (Aythya novaeseelandiae), and they reported that grey ducks and mallard ducks are the most likely predators of NZMS.

New Zealand mudsnails that experience predation may have spines on their shells for defense against predators and may forage less frequently in the presence of predators, especially during the morning when waterfowl predators are most active. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003; Benson and Kipp, 2009; Gustafson, et al., 2004; Levri and Lively, 1996)

In their freshwater ecosystems, New Zealand mudsnails occupy the role of scrapers/grazers and are considered to be a link between primary producers and fish. They also play a vital role in the transmission of Microphallus to ducks by serving as intermediate hosts. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003; Gustafson, et al., 2004; Levri and Lively, 1996)

New Zealand mudsnails are known to exist in extremely high densities and may comprise more than 90% of the macroinvertebrate biomass in introduced habitats. Due to their abundance, they may out-compete native mollusks and grazers for resources such as food. The decrease in the availability of resources is a likely explanation for the decline in species diversity in the presence of NZMS, the negative correlation between the NZMS population and that of mayflies, caddisflies, stoneflies, and chironomids, and the listing of five species of mollusks as endangered species. New Zealand mudsnails may also alter nutrient (carbon and nitrogen) cycling, interrupting energy flow and potentially threatening many other members of their ecosystems. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003; Benson and Kipp, 2009; Crosier and Molloy, 2010; Richards, 2002; Richards, et al., 2010)

Because New Zealand mudsnails thrive in disturbed and degraded waters, this species can be used as an indicator of ecosystem status. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003)

There are no direct, adverse effects of New Zealand mudsnails on humans, but control of the species is difficult and may be expensive in regions where it is not native. ("Monitoring the aquatic food base in the Colorado River, Arizona during June and October 2002: Annual report", 2003; Benson and Kipp, 2009; Crosier and Molloy, 2010; Richards, 2002; Richards, et al., 2010)

New Zealand mudsnails are not a protected species in their native range. Outside the native range, actions are being taken against the species to limit its spread as a pest. (Richards, et al., 2010)

Several aspects of the ecology of New Zealand mudsnails have contributed to their success as an invasive species. First, their tolerance of a wide range of abiotic conditions, such as temperature and salinity, aids them in transport from their native range via the ballast water of ships. Second, their escape from natural predators and parasites and their high competitive ability at the early stages of succession contribute to their establishment in introduced habitats. Third, their high fecundity, fast reproductive rate, and active and passive means of dispersal aid in their spread. Finally, their great abundance allows them to impact the ecosystem by consuming most of the primary production, dominating nutrient cycles and secondary production and decreasing populations of other mollusks and grazers. (Alonso and Castro-Díez, 2008)

The potential and realized negative impacts of New Zealand mudsnails have been recognized, and measures are being taken to control their spread based on what is known about their ecology. New Zealand mudsnails experience mortality when exposed to freezing or high temperatures with low humidity. It is recommended that all equipment that could be harboring the snails be frozen for several hours or be exposed to temperatures of 29 to 30ºC and low humidity for a minimum of 24 hours or temperatures greater than 40ºC and low humidity for a minimum of two hours. (Richards, et al., 2010)