Attack of the clones

A New Zealand freshwater snail takes over the world

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At times you get the impression that New Zealand wildlife has been dealt a uniquely bad hand when it comes to the struggles that confront it. Only too familiar are those stories of introduced rats nibbling their way through the last few mohua, of foreign wasps beating native birds and insects to the honeydew of beech forests, of exotic algae choking the Waikato River. It is so unfair, those hordes of unwelcome interlopers destroying native organisms and overrunning whole habitats. Our indigenous species seem always to be on the back foot, caught unprepared by more worldly-wise opponents, outmanoeuvred at every turn. Indeed, biologists in the late 19th century saw New Zealand wildlife as being quite unable to compete with sharper species from overseas. They regarded the demise of native forms and their replacement by superior introduced stock as inevitable.

Yet problems with introduced competitors and predators aren’t restricted to New Zealand. Indeed, it has been said that invasion by exotic species is rapidly overtaking habitat destruction as the number one factor in biodiversity loss worldwide. Over the last century, 20 to 40 per cent of the world’s fish, reptile, bird and mammal species have been exterminated by invasive species. Freshwater ecosystems have been hardest hit. And an insignificant native New Zealand freshwater snail is increasingly being recognised as one of the most damaging invasive species in lakes and rivers around the temperate regions of the world.

In New Zealand, NZMS stands for New Zealand Map Series, but in the US it is short for New Zealand mud snail. Entire conferences in North America have been devoted to this little pest, and foreign biologists haunt New Zealand waterways in the hope of figuring out what makes it tick—and how it could be made to stop ticking.

Potamopyrgus antipodarum (pronounced “potamapergis”) is a smooth-shelled, highish-spired brown snail 4–10 mm long found in just about any sort of freshwater throughout the country. It can stand a lick of salt, so extends into the upper arms of estuaries as well. Even where other native organisms are knocked back by dirty runoff, effluent or silt, Potamopyrgus usually survives. Furthermore, when the animal retracts into its shell, it seals off the aperture with a tough operculum, or door. In this state it can survive out of water entirely for a time—certainly hours, probably days, and conceivably longer in cool conditions. Thus sealed, it has been reported to pass through the digestive system of a trout unharmed. However, water temperatures above 28º C do not find favour with this snail so it has not spread into Queensland, for example, or into other tropical area.

The snail is an unfussy feeder, using its rasplike chitinous ribbon of teeth to scrape diatoms off algae, stones, wood or any other surface on which they may be found. Despite its popular name in the US, it prefers something firm to crawl around on, although compacted sediment and clay are acceptable.

So far, then, it is a hardy but otherwise typical gastropod. But there is one activity at which it excels—reproduction. Snails can grow at 0.1 mm a day, and specimens longer than 3 mm are reproductively active. Snails of this size from the Yellowstone National Park area of the US usually contain between 10 and 90 embryos (the mean is 22). Live young, rather than eggs, are produced. They can be born year-round, although in the US, where winters are generally colder than in New Zealand, most young are born between March and November.

The reproductive potential of the species is vast. One Montana ecologist, David Richards, has claimed that six generations per year are possible and 50 embryos per brood commonplace, meaning that with no mortality one snail could theoretically give rise to 3.188 x 108 descendants in a year.

Six generations per year sounds optimistic but the snail is undoubtedly a very prolific breeder.

Although Potamopyrgus antipodarum was first discovered in the US in 1987, it has been in Australia and Europe since the 19th century. Indeed, until recently it was considered native in Europe, where it was known as Potamopyrgus jenkinsi. That species, though, is now recognised as New Zealand’s own P. antipodarum. It is the commonest freshwater snail in Britain, and has invaded France, Denmark, Belgium, Poland, Switzerland, Slovakia, Norway, Spain, Italy, Portugal, Germany, Corsica, Belarus, Japan and probably elsewhere. In 1991 it was found in Lake Ontario, from where it is expected to spread throughout the Great Lakes region. It is thought to have arrived there in ballast water from Europe.

But how did a New Zealand species travel so far so early? Nobody can be certain but the suggestion is that it was carried on plants taken from New Zealand as ornamentals or in ships’ water casks. Casks were washed out and refilled at many ports of call, often in clean streams. Nobody would notice or care about a few minute brown snails. It is thought that the snail reached eastern Australia in the late 1850s and travelled thence to Europe.

Biologist John Chapman has noted that the “massive invasion of North America by the tiny P. antipodarum has seemed to escape public attention [because the snail is not] as big as an alligator, as glamorous as a walking catfish, or as dangerous to human health as West Nile virus”.
Biologist John Chapman has noted that the “massive invasion of North America by the tiny P. antipodarum has seemed to escape public attention [because the snail is not] as big as an alligator, as glamorous as a walking catfish, or as dangerous to human health as West Nile virus”.
The recent infestation of the western US is thought to have originated with the importation of trout from New Zealand to an Idaho fish hatchery. However, recent genetic analyses indicate that the snails in America are more closely related to Australian than to New Zealand samples. Potamopyrgus has since spread to the Snake, Yellowstone, Madison, Columbia and Colorado River systems and the headwaters of the Missouri. Indeed, it is present in California, Arizona (in the Grand Canyon), Washington, Oregon, Idaho, Nevada, Wyoming, Utah, Montana and Colorado. Since the snail lets go of the substrate very readily when its surroundings are disturbed, and is even happier in slow-moving waters than fast, it is easy to envisage it moving down the giant Missouri–Mississippi river system towards the Gulf of Mexico.

There is a rather peculiar twist to the reproduction of Potamopyrgus that facilitates genetic analysis and the tracing of relationships between populations. In New Zealand, the commonest individuals are females with three sets of chromosomes (most organisms have two sets) and which give birth to live young without mating or any male involvement—a phenomenon called parthenogenesis. All these young are also females, and mother, daughters, granddaughters, etc. are all genetically identical, i.e. clones. However, there is also a smaller number of males and females with the usual two sets of chromosomes, and these reproduce sexually. In many New Zealand locations, populations are a mixture of all three types of individual, and there is genetic evidence that the asexual females have arisen from sexually reproducing animals nearby.

With the possible exception of some of the snails in southeastern Australia, all populations of Potamopyrgus outside New Zealand are clonally reproducing females. In Europe, three clones are present, each representing a separate arrival. One clone occupies Britain, a second the estuaries around Denmark, and a third everywhere else. In eastern Australia there are multiple clones, but possibly only single clones in Tasmania and Japan. The Lake Ontario infestation is the same clone as the widespread European variety. All individuals in the western US belong to a single clone.

In New Zealand, though not elsewhere in the world, Potamopyrgus antipodarum is very variable in appearance. While most shells are smooth and black-brown in colour, fawn shells with long black spines can be also found, along with intermediate forms.

In New Zealand, Potamopyrgus can reach high densities (100,000 per sq m) but seems to be contained by parasitism by small worms that spend part of their lives in snails and part in ducks (see sidebar). These parasites do not occur elsewhere in the world, although they are now being investigated in the US as possible biological control agents.

Without the pressure of parasites, Potamopyrgus seems to go mad. In Lake Zürich, it has reached densities of 800,000 per sq m. In Victoria, Australia, it has blocked water intakes in dams and even poured out of domestic taps. In Yellowstone, the world’s oldest national park, it is spreading quickly and numbers of 500,000 per sq m have been recorded.

In high densities, Potamopyrgus consumes most of the food generated by an ecosystem’s primary producers (algae), and the animal biomass comes to consist of little more than Potamopyrgus. Native snails and other invertebrates seem unable to compete with it. For instance, in three Yellowstone rivers, Potamopyrgus has been determined to be responsible for 65 to 92 per cent of total invertebrate production—among the highest secondary production rates ever measured for a river animal. In Polecat Stream, the Yellowstone waterway in which the invader has reached its wayward zenith, daily summer Potamopyrgus production in 2001 was higher than the annual production of all insects in a well-studied tropical stream. Seventy-five per cent of all primary production in the stream was being consumed by Potamopyrgus.

A newcomer can’t jump into an ecosystem and make this big a splash without creating tidal waves for the locals.

Where the snail is abundant in Yellowstone, it is thought that stream insects such as caddis flies, mayflies and stoneflies may be declining. And that is a potential problem because Yellowstone’s rivers are some of the most-valued trout-fishing waters in all of America, and trout feed on aquatic insects.

Introduced brown and rainbow trout have long flourished in Yellowstone, along with the native Snake River cut-throat trout. Tiny hard-shelled snails are not popular fare with trout. As noted earlier, it has been reported that Potamopyrgus can survive passage through the gut of a trout, and you can be sure that food which emerges alive from a fish’s intestine has not proven especially nutritious. In several places, including on the Yellowstone National Park website (, it is stated that the snails yield “as little as 2 per cent of their nutritional value when eaten by trout”. Perhaps this means that most ingested snails survive passage through the gut.

Overseas, where parasites are absent, the snail can reach plague numbers, as on this rock from the US on which there are scores of Potamopyrgus.
Overseas, where parasites are absent, the snail can reach plague numbers, as on this rock from the US on which there are scores of Potamopyrgus.

Trout fisheries in the western US generate over US$2 billion annually. But trout are not the only fish with the potential to be adversely affected by Potamopyrgus. A variety of other native species of fish from the western US have shown nothing but disdain for the snails in captive feeding tests, even when they are starving. Apart from threatening freshwater fishing, there is justifiable concern that Potamopyrgus might drive indigenous American snails and other invertebrates towards endangerment.

The patchy and unpredictable way in which the snail keeps appearing in new catchments or way upstream in an already-infested river has led to suspicion that humans are spreading it. Contaminated fishing equipment and, in accessible areas, boats and trailers seem the most likely culprits. Anglers are being urged to either heat or freeze all their gear after use, including waders, to kill any snails that may have become attached.

Once Potamopyrgus is established in a waterway, there is nothing that can be done to dislodge it. Chemical treatments that might deal to it would also put paid to endemic species. However, there is a glimmer of hope. In some areas of Europe where population densities of the snail were once very high, numbers have since inexplicably crashed. And there is interest in importing the New Zealand parasites as biocontrols.


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