Left-handed pond snails, flat-coils, ramshorns
Class Gastropoda: Subclass Pulmonata: Order Basommatophora.
A cosmopolitan family of mainly left-coiled freshwater snails, which also includes some flat-coiled, very few secondarily right-coiled and one or two non-coiled, limpet-like species. The Australian fauna comprises 42 named species in 10 genera and appears to consist of northern (south-east Asian), southern (Gondwanan, endemic) and introduced components. All but one Australian species are left-coiled (sinistral) or flat-coiled. The exception is a Tasmanian species with a limpet-like shape but a vestige of left-coiling at the shell apex.
Planorbidae are confined to waters of low salinity, usually with algal growth or water weed on which the animals feed. Some species occur among dead leaves or other debris and some among rocks. A few inhabit highly polluted, oxygen depleted or very deep water and utilise haemoglobin or carry other respiratory modifications for such conditions. Some exhibit considerable drought resistance. Many species are self-fertilised, and for this reason are readily dispersed. Planorbids often are the dominant molluscs at a site.
A major revision of this group is badly needed, as synonymy is high because of the variation in morphology which stems from environmental plasticity and isolation due to selfing. The Australian Planorbidae were previously classified as Physidae. These families are distinguished only by differences in the radulae, though as only one species of Physidae is known from Australia identification of specimens generally can proceed using other, more readily observed characters.
Smith, B.J. (1996) Identification keys to the Families and Genera of Bivalve and Gastropod Molluscs found in Australian Inland Waters . Cooperative Research Centre for Freshwater Ecology Identification Guide No. 6., Albury, NSW.
Walker, J.C. (1988) Classification of Australian buliniform planorbids (Mollusca, Planorbidae). Records of the Australian Museum 40 : 61-89.