VERTICAL-DISTRIBUTION OF YOUNG POND SNAILS (BASOMMATOPHORA, PULMONATA) - IMPLICATIONS FOR SURVIVAL

The vertical distribution of juvenile pond snails (Lymnaea stagnalis and Stagnicola elodes), hatched from egg masses containing known numbers of viable embryos, was recorded daily in artificial containers under outdoor conditions for up to 25 days after hatching. Both species showed an initial vertical movement to the surface of the water with a subsequent return to the bottom sediments after 10-15 days. This pattern was consistent despite differences in the depth of the water column (13 vs. 18.5 cm). At 25 days after hatching, > 85% of the population was in the lower half of the container at any one time, almost all being associated with the bottom sediments. The presence of an adult snail of either species in the same container significantly reduced the survival of young snails (from between 85-94% to between 1-18%). Consumption of the young snails during their 1st 10 days out of the egg mass, when they were moving to the surface, accounted for the observed differences in survival. Young snails appeared to be ingested incidentally by the foraging adults, most often when the former were concentrated at the air-water-glass interface.