PIPELINES AS A REFUGE FOR FRESH-WATER SNAILS

In the lower River Murray, Australia, populations of gastropods have d eclined markedly over the past 40 years. Of the native fauna of about 18 species, only the ancylid Ferrissia petterdi remains common (but in patches), and another five species feature in sporadic local resurgen ces. Until recently the viviparid Notopala was thought to be extinct. The introduced physid Physa acuta is widespread, but in low numbers. T he decline probably is associated with flow regulation, high turbidity and the introduction of common carp. Snail populations have persisted , however, in irrigation pipelines fed from the river, where they peri odically cause blockages. This paper reports the discovery of the only known population of Notopala hanleyi in a pipeline where the inner wa ll accumulates microbial and other organic matter, providing high qual ity food for snails. In the Murray before regulation most of the perip hytic biomass was probably microbial, as fluctuating water levels woul d have maintained littoral communities in a highly productive, early s tate of succession. It is postulated that, by stabilizing seasonal wat er levels, regulation has promoted the production of filamentous green algae, an inferior food source, and so displaced snail populations fr om the open aquatic environment.