The secret life of freshwater snails

Freshwater snails are little-known and rather mysterious creatures, but there are 69 native species in New Zealand and they play an important ecological role.

On Te Ahumairangi Hill in Wellington’s Town Belt there’s a rare creature that, as far as we know, you won’t find anywhere else. You also probably won’t see it. The animal in question is the tiny freshwater snail Potamopyrgus oppidanus. Its tower-shaped shell grows to a maximum of just 3 mm.

The places this snail lives – on the edges of streams in damp gullies – make it even more difficult for the non-attentive eye to spot. So much so that it eluded recognition for a long time. Potamopyrgus oppidanus was discovered in 2003 by Martin Haase in a single stream in the suburb of Wadestown, Wellington. It was formally recognised as a new species in 2008 and given a Latin name which refers to its home in the capital city of Aotearoa – oppidanus means ‘urban’.

The snail was assigned a threat status of ‘Threatened – Nationally Critical’ in 2013 and holds that status today. The species has already disappeared from certain areas of its habitat and while greater survey effort might find it in other locations, current information suggests it is highly threatened. This little snail might be found in an isolated area, but that doesn’t mean it’s alone in its circumstances.

“There are 69 native freshwater snails currently listed in New Zealand, all of which are endemic to New Zealand. We also have non-native snails,” says Tom Drinan, a Technical Advisor Freshwater at the Department of Conservation.

Fourteen of our native freshwater snails are listed as ‘Threatened – Nationally Critical’, the highest threat category. Thirty-seven of them are listed as ‘Data Deficient’, which means there just isn’t enough information to be able to classify their risk of extinction.

“Many of the rarer snails are only found in a handful of locations, several from only a single location, whereas the more common species like P. antipodarum are found in fresh waters pretty much all over the country,” Tom says.

The largest native freshwater snail in New Zealand is Zemelanopsis trifasciata, with a shell up to 38 mm high. On the other end of the scale, the majority of Hydrobiidae (mud snails) are very small, with shell sizes rarely exceeding 3 mm.  Another tiny example is Meridiopyrgus inanga, known to grow a shell of up to just 1.7 mm high and 0.75 mm wide.

“Many native snails have only been formally described in the past decade, mainly for the same reasons as Potamopyrgus oppidanus – lack of dedicated research,” says Tom. “Furthermore, they’re small creatures, often found in seepages, springs, and cave streams – the sort of freshwater habitats that are easily overlooked.”

Identifying freshwater snails takes almost as much skill as pronouncing their Latin names.

For example, P. oppidanus looks very similar to the more common P. antipodarum. Slight differences in the shell size and shape give the first indication to the naked eye. However, Tom says, there are a number of other features that an experienced malacologist (that’s a mollusc specialist) would need the help of a microscope to identify.

Freshwater snails may generally be small but they play an important ecological role in freshwater environments. Snails are grazers, feeding on algae and detritus. They’re also an important food source for other native animals higher in the food chain, such as crayfish/kōura, fish, and waterfowl.

When species – including rare ones – are taken from the ecosystem, it can upset the food chain and have ongoing effects to the ecosystem itself. Protecting the habitat of known populations of freshwater snails is vital for species’ survival.

Any degradation, including sediment runoff and the clearing of vegetation, or loss of habitat could further fragment the living spaces available to these little creatures and lead to further species declines.

One way people can help Potamopyrgus oppidanus and other native freshwater snails, Tom suggests, is to be conscious of these often sidelined freshwater habitats. Treat these places with respect and remember that even if you can’t see animals, it doesn’t mean they’re not living there.