Great Pond Snail - Lymnaea stagnalis

Great Pond snail (Lymnaea stagnalis ) 19 June 2019

There are about 40 different kinds of water snails in Britain, varying in size when fully grown from the tiny Nautilus Ram’s-horn – just 2 or 3 mm across – to the Great Pond Snail which grows up to 4 cm.

We all know snails can’t fly but the common water snails are good at getting around from place to place. Most are probably carried accidentally by birds or perhaps amphibians, and they are often introduced to ponds when people bring in the sticky eggs attached to bits of plants.

It’s sometimes said that snails are important for keeping a pond clean. It would be nice if this was true: collect a few snails, drop them into your mucky polluted pond and, hey presto, out comes a sparkling shiny wildlife habitat.

Unfortunately, it doesn’t work like this. Snails will thrive in polluted ponds, grazing on a superabundant growth of algae stimulated by excess nutrients. But the chances of the snails cleaning up your pond are nil. To get a clean pond you need to take away the pollutant and the snails don’t do that – they just recycle them around the pond.

Snails naturally need a bit of calcium in the water to grow their shells so in naturally acid ponds – which have very little calcium – they are either absent or a small part of the fauna. Where there is calcium for them, they will arrive in due course.

Some of our water snails are amongst the most sensitive and endangered of freshwater animals. The Glutinous Snail, for example, which looks pretty similar to the very common and tolerant Wandering Snail, is one of the most endangered animals in Europe. We found some of the last ones in England in a pond near Oxford in the 1990s but sadly it’s gone now. A once quite widespread animal has now become extinct in here – one population is hanging on in Wales. (Freshwater Habitats Trust)

30 June 2020