Litter shredding in a desert oasis by the snail Melanopsis praemorsa

The freshwater snail Melanopsis praemorsa is very abundant in oases of the Judean Desert. To determine its role as litter shredder, experiments were carried out in Nahal Arugot. Boxes were placed in the stream (29°C), each containing 1 g of dry willow leaves together with one snail; or six snails. With one snail per box, dry leaf weight after two days declined to 72% of the initial dry weight, after twelve days to 58% and after eighteen days to only 40%. During this period the snails in the experiment increased by 12% in shell-height, by 20% in snail weight (including the shell), by 33% in live fresh biomass. Each snail consumed about 10 mg litter per day. With six snails per box, dry leaf weight after two days declined to 62% of the initial dry weight and after twenty days to only 15%. In natural populations where density was 44 snails per quadrat (of 25 x 25 cm), 1 g of dry leaves was reduced to 25% the initial weight within three days. Overall plant cover along a 600 x 10 m stretch of the stream was found to be 45%, of which 65% was willow. During one year about 1 kg litter was shed per m² willow; by extrapolation, this would be about 1800 kg litter throughout the stretch. Much of this vast amount of litter is consumed by the snails, who thereby reduce summer damming of the oasis. Our study suggests that M. praemorsa, a generalist feeder, may be a major litter shredder in certain desert oasis ecosystems.