Litter shredding in a desert oasis by the snail Melanopsis praemorsa

The freshwater snail Melanopsis praemorsa is veryabundant inoases of the Judean Desert. To determine its role as littershredder, experiments were carried out in Nahal Arugot. Boxeswereplaced in the stream (29 °C), each containing 1 g ofdrywillow leaves together with one snail; or six snails.With one snail per box, dry leaf weight after two daysdeclined to72% of the initial dry weight, after twelve days to 58% andaftereighteen days to only 40%. During this period thesnails in the experiment increased by 12% in shell-height, by20%in snail weight (including the shell), by 33% in live freshbiomass. Each snail consumed about 10 mg litter per day.With six snails per box, dry leaf weight after two daysdeclined to62% of the initial dry weight and after twenty days to only15%.In natural populations where density was 44 snails per quadrat(of25×25 cm), 1 g of dry leaves was reduced to 25%theinitial weight within three days. Overall plant cover along a600×10 m stretch of the stream was found to be45%, ofwhich 65% was willow. During one year about 1 kg litter wasshedper m² willow; by extrapolation, this would be about1800 kglitter throughout the stretch. Much of this vast amount oflitteris consumed by the snails, who thereby reduce summer dammingof theoasis. Our study suggests that M. praemorsa, a generalistfeeder, may be a major litter shredder in certain desert oasisecosystems.