Tropical and sub-tropical freshwater snails of the family Ampullariidae (AKA Pilidae). They have round shells (hence the name apple), and commonly grow to over an inch (2.5 CM) in diameter; the species Pomacea maculata can get as big as 6 inches (15 cm). Apple snails are the largest of the fresh water snails. The primary distinguishing feature of apple snails is that, while they are largely aquatic snails, they have both gills and lungs. To supplement this, they have a long breathing tube, called a siphon, that they can use to breath air while staying submerged. They also have long labial tentacles around their mouths.
Apple snails come in a wide range of colours and sizes; they are often sold as mystery snails in aquarium supply shops. Because of their bright colours and large size, apple snails are one of the more common aquarium snails, but many species will eat aquarium plants.
Apple snails can be eaten by humans, and have been introduced to many areas (SE Asia and Oceania in particular), as a potential food source*; unfortunately, they are also a pest which will invade rice and taro fields, eating the seedling plants. Both the introduction of snails and the pesticides used to control them (which also eliminate other, indigenous snails) will no doubt have some negative environmental impact.
Apple snails are specifically adapted to tropical and sub-tropical areas that undergo times of comparative drought. The retention of lungs aid in their survival during the dry times, but more importantly, they can aestivate for a period of months (the exact amount of time varies with the species).
Apple snails are unusual in that they have separate sexes; most other types of snails are hermaphroditic. (Apple snails may be protandric, although there is little evidence for this). Another comparative oddity is that many species of apple snails will lay their eggs out of the water, which is rare in aquatic snails. These oddities seem to work well for the snail; they can spread with disturbing speed.
Phylum Mollusca, class Gastropoda, subclass Prosobranchia, order Caenogastropoda, superfamily Ampullarioidae, family Ampullariidae.
Asolene (South America)
Marisa (SA, has been introduced to Central and North America)
Pomacea (South and Central America, introduced to S. Asia)
Afropomus (West Africa)
Saulea (West Africa)
Pila (Africa, also native to Asia)
Pila (South Asia, also native to Africa)
* These are not the type of snail traditionally used for escargot; those are land snails of the family Helix.
http://www.applesnail.net/ is the best over-all site I've found.
http://www.applesnail.net/content/species/genera_maps.htm shows the ranges of the genera.
http://www.fcsc.usgs.gov/sofla/Apple_Snail/apple_snail.html were also used as references.