Freshwater Molluscan Shells / Pulmonata / Basommatophora
Freshwater Pulmonate Gastropods
Freshwater pulmonate snails are characteristically thin and light shelled. Many can regulate their buoyancy to neutral and even positive by taking air into a "lung," which allows them to glide upside down on the air/water surface. Their life cycle is rapid. In nature, one or two generations per year may be produced, with complete replacement of individuals (Brown, 1991). All are hermaphroditic; that is, each individual has the capacity to function as both male and female. Unlike the prosobranchs, with many endemic species and varieties, the pulmonate species are fewer but much more widespread. Freshwater pulmonates reach their greatest size and diversity in the northern latitudes, and many species are commonly found in shifting and ephemeral habitats.
Although all are in the subclass Pulmonata, the freshwater pulmonate snails are not closely related to the land snails. Terrestrial pulmonates ("true" land snails) are in the Order Stylommatophora, while the freshwater pulmonates, considered as the single superfamily Lymnaeacea by some authors, are placed in the Order Basommatophora, along with the marine pulmonate "false limpets," superfamily Siphonariaceae. Freshwater limpets are also Lymnaeacean, but their limpet-like form has evolved several times within this group, from different coiled ancestors. The marine superfamily Melampoidea (Ellobioidea) are included as a third grouping within Basommatophora by some authors, but are given a separate order, Archaeopulmonata, in more recent sources.
Abbott (1989) uses:
Order Archeopulmonata, with a single superfamily,
Ellobioidea (littoral marine "melampus" shells)
Order Basommatophora, with seven superfamilies:
Siphonarioidea (marine "false limpets")
Acroloxoidea (freshwater limpets [in part])
Lymnoidea (dextral* pond snails, limpets)
Physoidea (sinistral* pond snails)
Planorboidea (ramshorn pond snails, limpets).
Banarescu (1990) for freshwater taxa only, uses:
Order Basommatophora with a single freshwater superfamily;
Lymnaeacea, containing six families in two sublineages:
* Sinistral and dextral refer to the mapping of rotation direction to a straight line vector. Most snail shells, as well as common screws, etc., are dextral, or right-handed. Take your right hand. Point the thumb in the general direction the screw moves as it rotates or the shell grows as it spirals downwards (down), and your curled fingers will define the direction of rotation for right-handed shells, screws, etc. Likewise for sinistral or left-handed snail shells, but in reverse.