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.