were brought to the laboratory. They were reared in three 60 L aquaria (60 cm 25 cm 40 cm) and fed with lettuce leaves, cabbage and duckweed in room temperature and natural photoperiod).
Tap water was replaced every other day. Female and male snails were randomly selected to 30 pairs in each aquarium. When the snails began to lay the eggs, the initial time was recorded (usually in the night). After they finished, the eggs were removed from the wall of aquaria with a razor blade, and put into three petri dishes (diameter 7.5 cm) which were lined with filter papers.