Directions for teachers: After students have had a chance to review the article “Hermit crabs are drawn to the dead,” lead a classroom discussion based on the questions that follow. The questions are labeled according to concepts covered.
1. Energy and competition
What factors might determine whether a land-dwelling crab remodels its shell or takes over another shell? How might energy expenditure play a role in that decision? According to the article, suitable shells are a limited resource for land-dwelling hermit crabs. How might a sudden influx of shells affect a hermit crab population? What about a sudden decline in the availability of shells? What would graphs of crab population over time for these two scenarios look like?
2. Adaptation and speciation
The article says that there are some 850 known hermit crab species, most of which live in the sea. Why do you think there are so many hermit crab species? Why do you think most live in the sea? What variations in traits might you expect among these species? How could you use species information to diagram the evolution of hermit crabs over time? What environmental changes might lead to new species, or species extinction, over time? In what ways might humans influence speciation?
3. Advantageous traits and natural selection
How might the ability to smell their own dead benefit land-dwelling hermit crabs? Do you think this ability is a heritable trait? Describe how the crabs might have evolved this ability, and why the trait isn’t seen in marine-dwelling hermit crabs. How might you diagram how the number of organisms with an advantageous trait changes over time? Do you think it is more likely for crabs to evolve the ability to sense their own dead or for crabs to evolve the ability to grow their own shells? Why?
4. Chemical cues and structure
The scent given off by dead crabs is a chemical cue for land-dwelling hermit crabs. Explain how chemicals are used to communicate among organisms. What other organisms rely on chemical cues? (Be sure to consider a range of possible organisms, not just animals.) Look up the molecular formula of one chemical cue and draw its chemical structure. What behavior or process does the chemical cue initiate when an organism senses it? Now draw a diagram that shows how the chemical lets organisms communicate. Show where the chemical comes from, how it is transmitted to another organism and how it is received. Be sure to note which processes are chemical and which are physical.
5. Force diagrams as models
Draw a simple force diagram that demonstrates a shell’s buoyancy in water. Now draw a simple force diagram of a shell on land. How would hermit crabs living in the shells affect the force diagrams? What environmental factors might upset the balance of these simple systems? How would these factors affect your simple force diagrams? What assumptions did you make in your simple force diagrams? How might these assumptions break down in the real world?
6. Chemical and physical changes
What does it mean for hermit crabs to “remodel” their shells? How is the shell changing and how might the change benefit the crab? Is it a chemical change or a physical change? Assuming the shell is made mostly of calcium carbonate, can you predict more specifically how the change could occur by writing out a reaction explaining the process? How might environmental factors such as temperature or humidity affect the rate of the reaction?