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Guide to New Zealand's freshwater invertebrates
Potamopyrgus is a genus of native snails, the most common of which (P. antipodarum) is our most widespread freshwater snail. They are small, black, brown or pale snails with an obvious spire, an operculum (protective shield) covering the aperture (shell opening) when the animal withdraws, and sometimes with nodules around the shell. The aperture is on the right when facing toward you and when the spire points upwards (lower photo).
Potamopyrgus antipodarum is found in most New Zealand streams and rivers. P. estuarinus is restricted to sites with tidal influence.
Like most snails they are grazers, scraping biofilms from submerged surfaces.
High population densities are an indication of a long period since any significant flood event. They are found in pristine and polluted waters, and have been assigned tolerance values of 4 (hard bottom sites) and 2.1 (soft bottom sites).