A new guide for identifying freshwater snails

Ben Rowson is Senior Curator of non-marine molluscs at the National Museum of Wales. He is the co-author of a new AIDGAP guide to the Freshwater Snails of Britain and Ireland.

No one in Britain and Ireland is ever far from freshwater snails. Almost any pond-dipping session, except in the most acidic and polluted waters, will produce at least one snail species. Snails may be small and superficially similar, but their biology and impacts are impressive. They play a vital role in water purification.  Some snails indicate unspoilt freshwater habitats, while other host unwelcome parasites, or are nuisance invaders.

Much of what is known about the 50 species in Britain and Ireland depends on accurate identification and recording. This new AIDGAP identification guide aims to make this easier for beginners and experts alike.

Reference collection of freshwater snails

What are freshwater snails?

Freshwater snails are gastropod molluscs that live most or all of their lives in fresh water (i.e. water with a salinity below 5 parts per thousand). The families can be lumped into two roughly equal-sized groups.

First are the operculates; snails which carry an operculum, a trapdoor-like plate used to close the shell. Operculates all breathe will a gill and have a snout or proboscis.

Second are the pulmonates, a set of related families which have lost the operculum and also evolved a lung. They breathe air from the surface or carried in a bubble within the shell, which tends to be thin, making them buoyant. Pulmonates include the ram's-horn snails, freshwater limpet and river limpet.

Getting started with identification

Freshwater snails front coverAn intact, adult shell is all that is needed to identify most of our species. The guide provides three different routes to identification

  • A simple key to the shells of freshwater snails.
    Aimed at beginners, this key directs attention to the most important features one at a time. It divides into two clear sections: flat snails (ram’s-horn snails) and raised snails (plus limpets). We have deliberately minimised the use of technical terms in this particular key.
  • Comprehensive species accounts and images. This provides essential support for identification, particularly for juveniles and variable species.
  • Pictorial index showing typical examples.

These three routes can be used separately or together. There are also sections on dissection of snails, plus the identification of juveniles, eggs and spawn. All scientific names and distribution maps are fully up to date.

Testing the draft key

This guide is part of the FSC’s AIDGAP series (Aids to Identification in Difficult Groups of Animals and Plants). As with all guides in the series, the Freshwater snails AIDGAP underwent extensive testing before publication. Several hundred copies of a preliminary draft (the test version) were sent free of charge to potential users in 2019. The draft was also trialled in identification workshops around Wales. Subsequently we amended the key in the light of feedback from users.

Field survey techniques

Finding snails is often easy, though obtaining a full species list for a water body can be more difficult. Remember that most species are small (under 10mm long). In each water body there will be different microhabitats, each supporting different species. Here are four places to check, but there are many others.

  • Sample different depths - at least the surface, mid-water (if water weeds are present) and bottom sediment.
  • Larger stones and cobbles - should be carefully lifted so that both surfaces can be examined.
  • Leaves and stems of aquatic plants - can produce species such as limpets that may not appear in netted samples.
  • Flood debris - is often rich in dead shells, a good source of specimens for a reference collection.

A full description of when, where and how to sample different habitats is given in the guide, including the use of nets, biological dredges and sieves.

Fieldwork at Cardiff Bay

Habitats and water quality

Sites with the highest diversity of snails (over 10 species) include large lowland ponds, ditches, disused canals and slow rivers. Often these sites have a high diversity of aquatic plants, insects and other invertebrates. So a high diversity of snails is likely to indicate a rich freshwater habitat.

Salinity, pH, hardness and nutrient pollution are all important chemical factors. Very few species tolerate salinity. Indeed salt runoff may make ditches near roads uninhabitable. Neutral water (pH 7) suits all our snails, but most thrive better in alkaline water, as in chalk streams and limestone areas. Others tolerate mildly acidic waters (pH 5-7), but peat bogs are too acid for most. Water hardness (dissolved mineral content) is closely related to pH. Several species are associated with soft and hard water.

Nutrient pollution (phosphates and nitrates) from domestic effluents, faming and industry has become very widespread. Different species are more or less tolerant, and thus can be used as bioindicators.

The new book was produced by Amgueddfa Cymru – National Museum Wales, the Conchological Society of Great Britain and Ireland, and others, with the support of the National Lottery Heritage Fund.

The Freshwater Snails guide is available to order from the FSC website: https://www.field-studies-council.org/shop/publications/freshwater-snails-aidgap