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!
A Little Natural History about Snails
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 Lives of Pond 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!
The habitat of pond snails.
Pond snails enjoy living in al kinds of freshwater ponds where they can breed freely and also feed on dead animals and plants. These species have benefits for the pond but they can also be pests. These water bodies such as ponds attract all kinds of animals, including the ones which, in some cases, can be harmful to the pond itself. These include pond snails which have their pros and cons also. On one hand pond snails eat algae and dead animals which is very helpful, but they can also eat the vegetation and harm your plants instead. Fortunately these aquatic snails are not attracted to fish eggs and prove to be friendly neighbours and thus safe to keep in a pond with fish. They can, however, breed in large numbers which is harmful to a pond. It is therefore important for one to learn and understand the life cycle of pond snails, which are likely to inhabit ponds with ample vegetation or make their way through neighbouring water bodies. It is important to be vigilant of snails in your pond but also remember that they are not always a threat.
Life Cycle of these Hermaphrodite Species
Entomologists recognise snails as “peppered shell and dark brown colour with dots on their bodies”. The hard shell protects the slimy body from any external damage. Snails can grow into large pond snails or dwarf pond snails, and sense food through their tentacles. Pond snails are also called aquatic snails as they cannot live outside of water bodies, leading to death on dry land. One reason that they live in ponds is to be able to hide from predators by burrowing. These molluscs can stay embedded underneath ponds substrates for months, and appear when conditions get favourable for reproduction. These snails have a greater tendency to damage pond habitat because they are Hermaphrodite which means that both males and females can lay eggs after fertilisation. A thousand eggs are laid at a time by both sexes, these eggs have the physiology of a mass of jelly attached to the plants in ponds followed by maturation to baby snails which occurs after four weeks. However, predators such as ducks and dragonflies can prevent these aquatic snails from completing their full life cycle.
These hermaphrodite species are said to take over ponds that are contaminated with faeces as stated by the WHO. This highlights the importance of keeping pond water clean. Pond snails feed on plants, algae, and leafy vegetation and can destroy them if overpopulated, but if a pond has thick vegetation these molluscs won’t cause a lot of damage before you can easily remove them. As they eat algae, they can be used in ponds that have a thick blanket of weed. Pond snails feed on different species of algae except for free-floating green algae which these snails can’t feed upon. Keeping this in mind, snails can be used to keep the natural habitat of a pond in balance, without them breeding out of proportion.
For and Against Pond Snails
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!