Assignment Mollusks: What determine a snail´s size before reproduction?

 Goal: Design an experiment that will provide information about how smails detect the presence of crayfish in their water.

Biologists studying snails found that the size of a particular species of snail reaches before beginning reproduction depends on the environment.  If the water in which the snail lives comes from an area in which snails were prayed on by crayfish, the snail grows to be about 10 mm in length before reproducing.  If the water comes from an area containing only snails, only crayfish, or only crushed snails, the snail grows to a length of only about 4mm before reproduction.

Analysis: 

    Biologists know that larger snails are not eaten by crayfish.  Growing large quickly before reproducing has survival value for the snail because it then will not be eaten by the crayfish.  How might the young snail detect the presence of crayfish in the water? 

Thinking Critically: Design an experiment that will provide information about how smails detect the presence of crayfish in their water.