Key to Australian Freshwater and Terrestrial Invertebrates
Phylum Mollusca
Class Gastropoda
Informal group Pulmonata
Informal group Hygrophila
Common names: freshwater snails, pond snails, freshwater limpets
Overview
Hygrophila contains four families of freshwater snails and limpets. They have a diversity of shell forms and do not have an operculum. All species have an enclosed, air-breathing lung, although several species have also evolved external respiratory gills. The eyes are located at the base of a single pair of non-retractable sensory tentacles attached to the head.
Distribution and diversity
Hygrophila contains four families: Lymnaeidae, Planorbidae, Ancylidae and one introduced cosmopolitan family, Physidae, represented by the northern European Physa acuta, which is found in temperate and sub-tropical south-eastern Australia, New Zealand, and other pacific islands. Lymnaeidae is an almost cosmopolitan family of snails and is widespread throughout Australia, including Tasmania with around ten species including at least three introduced species. The Planorbidae are the largest and the most diverse family of freshwater pulminate snails and has a worldwide distribution. In Australia it is represented by a diverse, widely distributed fauna with over 40 described species. The freshwater limpet family Ancylidae contains three species in two genera occurring in Australia, and is now considered part of the Planoribidae.
Life cycle
Hygrophila are hermaphroditic with many species requiring fertilisation from another individual, while others can self fertilise. Some species can reproduce parthenogenetically, laying unfertilised eggs that develop into adults. Eggs are typically laid in a gelatinous capsule that is attached to plants or other appropriate surfaces. Development is advanced, with no free-swimming larval stage and juveniles emerge from the capsule looking like miniature adults.
Feeding
Hygrophila are herbivorous or omnivorous, mostly grazing on living and dead plant matter, and algae, with some species also scavenging on the carcases of dead animals.
Ecology
Most species of Lymnaeidae inhabit slow or non-flowing freshwater, even stagnant eutrophic conditions, with some found in mildly saline waters. Lymnaeids are potentially of major economic importance as some species are the main intermediate hosts for sheep liver fluke (Fascicle hepatica). In Australia, Planorbidae are typically the dominant molluscs at a site, often to the extent they are the only group of freshwater molluscs found in many areas. They inhabit mildly saline stagnant pools to fast flowing creeks and rivers. Some species occur among algal growth or vegetation, dead leaves or other debris and some among or on the underside of 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. The northern European Phyla acute (Physidae) occurs in slow to non-flowing water. Many species of Hygrophila are subjects of conservation concern due to habitat destruction as a result of water management and pollution. Several species are assumed extinct in Australia due to draining of wetlands and changes to water flow.
