A FRESH-WATER PROSOBRANCH, MELANOIDES-TUBERCULATA, IN A HYDROGEN-SULFIDE STREAM

Hydrogen sulphide is poisonous to aerobic organisms and consequently m olluscs are rarely found in streams with high concentrations of H2S. I n the Arava Desert (Israel) we found a freshwater snail, Melanoides tu berculata, in a stream with a sulphide gradient. The H2S gradient rang ed from 7.3 to 0.1 mg/litre, and adult (but not juvenile) M. tubercula ta were found to survive in oxygen-depleted water containing hydrogen sulphide concentrations of 3.4 mg/l. In these micro-habitats M. tuberc ulata may perhaps breath aerobic oxygen. Juveniles were found further down the gradient and their frequency in the population gradually incr eased, from 0% at 3.4 mg/l to 10% at 1.8 mg/l, and to 56-61% at <0.1 m g/l. These data suggest either that snails invading the high-sulphide micro-sites do not reproduce, or that they reproduce but the juveniles are unable to survive in the sulphide environment.