The New Zealand mud snail (Potamopyrgus antipodarum) is a species of very small freshwater snail with a gill and an operculum. This aquatic gastropod mollusk is in the family Tateidae. It is native to New Zealand, where it is found throughout the country, but it has been introduced to many other countries, where it is often considered an invasive species because populations of the snail can reach very high densities. Shell description The shell of Potamopyrgus antipodarum is elongated and has dextral coiling, with 7 to 8 whorls. Between whorls are deep grooves. Shell colors vary from gray and dark brown to light brown. The average height of the shell is approximately 5 mm ( in); maximum size is approximately 12 mm ( in). The snail is usually 4–6 mm in length in the Great Lakes, but grows to 12 mm in its native range. It is an operculate snail, with a 'lid' that can seal the opening of its shell. The operculum is thin and corneus with an off-centre nucleus from which paucispiral markings (with few coils) radiate. The aperture is oval and its height is less than the height of the spire. Some morphs, including many from the Great Lakes, exhibit a keel in the middle of each whorl; others, excluding those from the Great Lakes, exhibit periostracal ornamentation such as spines for anti–predator defense. Taxonomy This species was originally described as Amnicola antipodarum in 1843 by John Edward Gray: Inhabits New Zealand, in fresh water. Shell ovate, acute, subperforated (generally covered with a black earthy coat); whorls rather rounded, mouth ovate, axis 3 lines; operculum horny and subspiral: variety, spire rather longer, whorls more rounded. This species is like Paludina nigra of Quoy and Gaimard, but the operculum is more spiral. Quoy described the operculum as concentric, but figured it subspiral. Paludina ventricosa of Quoy is evidently a Nematura. Forms Potamopyrgus antipodarum f. carinata (J. T. Marshall, 1889) Distribution This species was originally endemic to New Zealand where it lives in freshwater streams and lakes in New Zealand and adjacent small islands. It has now spread widely and has become naturalised, and an invasive species in many areas including: Europe (since 1859 in England), Australia, Tasmania, Asia (Japan, in Garmat Ali River in Iraq since 2008), and North America (USA and Canada: Thunder Bay in Ontario since 2001, Washington State since 2002, British Columbia since July 2007),