Before the trip, spend some class time talking about ecology and other concepts, such as carrying capacity and competition. The distribution of the shells graph should be discovered by the students themselves and a discussion of the results can follow. Each student carefully observes tidepools at low tide and collects 100 snail shells for examination in the field. Students measure the shells and record their lengths. Students record if there is a hermit crab inside or not, being careful to take good care of the animals.
Students create a data table and plot the data on the graph.
Discuss the results. When hermit crabs are placed in cool fresh water, they will exit their shells. They will reenter their shells when placed back into salty ocean water. Ask the students to observe shell preference. The class may collect new kinds of shells from a non-hermit crab habitat and present them as choices as well.