Pond Snails in Your Pond

Few issues divide the ranks of pond-keepers quite so readily as the topic of pond snails. To some they are nothing more than an aquatic version of an all too familiar garden pest, ready to wreak havoc on prized water plants; to others they are a fascinating and welcome addition to the pond, performing a useful job by helping to keep it clean. The truth is probably somewhere in between and one thing is certain – love them or loathe them – if you have a pond, you’re not likely to avoid them for long!

For and Against

Most of the common species of snails in British ponds are scavengers, feeding on plant material and detritus, scraping away at their food with a muscular, rasp-like tongue called their radula. In the days before the widespread use of UV clarifiers, this used to make them very popular with pond-keepers since their natural menu included prodigious quantities of algae. However since technology took over their role, fewer pond-keepers are prepared to turn a blind eye to their tendency to munch on water lilies and other specimen plants.

Snail fans point to their continuing usefulness in helping reduce the build up of decaying organic matter on the pond bottom. On the other hand, set against that is the contribution their waste makes to increasing nitrate levels in the water and the fact that they can act as intermediate hosts for a range of parasites – neither of which endear them to fish-keepers.

In the end, for most people, you either like them or you don’t – and that’s an end to it!

A Little Natural History

There are more than 30 kinds of freshwater snail in Britain, ranging from the likes of the tiny Dwarf Pond Snail (Lymnaea truncatula) to giants such as the Great Pond Snail (Lymnaea stagnalis) or Great Ramshorn (Planorbarius corneus). Although some snail-lovers will buy snails from aquarists suppliers or beg a few from friends’ ponds, freshwater snails have a habit of turning up, invited or not, usually attached as eggs on water plants. The eggs themselves are usually quite easy to see if you look for them, contained in a mound or strip of thick, jelly-like material on the surface of leaves and stems. Since snails are prolific breeders, once they are established in a pond, their numbers can increase very rapidly – so if you don’t want them, it pays to be vigilant.

There are two main groups of pond snails – the “pulmonates”, which breathe air using lungs and the “operculates” which get their oxygen from the water, using gills. Many of the species which are most familiar to water gardeners, including the Great Pond Snail and the various varieties of Ramshorns are pulmonates. The ability to breathe air gives them an advantage over their gilled relatives because they can colonise most types of pond, even when the dissolved oxygen level is low. By contrast, the operculates – snails such as Jenkin’s Spire Shell ( Potamopyrgus antipodarum) and Bithynia tentaculata are restricted to clean, well-oxygenated water.

The Sex Lives of Snails

Another interesting difference between pulmonates and operculates lies in their approach to reproduction. While most of the gill-breathing operculates have separate males and females, pulmonates are hermaphrodite, each individual being able to both fertilise and be fertilised by another snail – which goes some way to explain their ability to multiply quite so quickly.

However, the pulmonates aren’t the only ones with a clever breeding trick; Jenkin’s Spire Shell – an operculate – goes in for a spot of virgin-birth. Technically known as “parthenogenesis”, this allows the snail to reproduce without the need for a mate, which is a useful thing to be able to do if she find herself all alone in a suitable pond – although confusingly all of her “daughters” will actually be her sisters!

No matter how you feel about these animals themselves, it is impossible to deny that they are really very good at what they do. Even if they are never going to be your favourite kind of pond-life, their unique biology and remarkable ability to find their way to just about everywhere makes them an interesting group – however grudgingly you choose to admit it!