There are approximately 72 British species of fresh- and brackish-water molluscs. It is difficult to give an exact figure for several reasons. Firstly, where is brackish water considered to give way to fully marine conditions? A number of brackish species such as the Laver Spire Snail, Hydrobia ulvae, that appear on non-marine lists (such as the Conchological Society's non-marine Mollusca recording card) do survive for periods in fully salt water in estuaries and tidal creeks.