Hermit Crab Housing

The Search for Housing is a lesson plan created by 2000 HOST Teacher Michael Collins.  


Students will collect 100 snail shells, measure and record the length of the shell, and determine whether or not it is occupied by a hermit crab.  The data will then be compiled and graphed to see if there is a difference in the sizes of the shells between the occupied and the unoccupied snail shells.

Grade level:  7 to 10


The hairy hermit crab (Pagurus hirsutiusculus) is extremely common at the Coal Oil Point Reserve and other locations along the Southern California coast.  It ranges in length from 1 to 4 inches; those found in tidepools rarely exceed 1 inch, and the larger ones are found in deeper water.  In this region, hairy hermit crabs are most commonly found occupying the shells of the black turban snail (after the snail has died).  However, as the crabs grow, they need to “molt” several times each year by finding new shell homes and moving into them.  After they back themselves into a new shell, their soft bodies become protected.  At the entrance of the shell, the crab’s armored wrists and claws help provide a defensive shield.  Their antennae are brown or gray with white bands.  Their walking legs have no bands.

Most hairy hermit crabs seem to prefer living in snail shells.  In fact, their abdomen has a natural twist which corresponds to the spiral of most snail shells.  However, almost any sort of hollow object can make a home for a hermit crab;   some of them will move into straight worm shells.  They have even been seen in plastic pill bottles.

When selecting a snail shell, most hermit crabs seem to prefer a particular size and shape (which depends on the snail species).  Shells that are too big or too small can make the hermit crab easy prey.  Sometimes a hermit crab may find a shell that is the optimal size and shape, but it is already occupied by another hermit crab.  When this happens, the hermit crab will knock its shell against the other crab’s shell.  The dominant crab almost always gets the shell it wants, and the subordinate crab needs to go house hunting again.

The availability of empty snail shells is a limiting factor to hermit crab distribution.  It can be difficult to find an empty snail shell in the tidepool habitat, in part, because snail shells only last about 6 months after the death of the snail.  After the snail dies and its shell has been in circulation for a while, polychaete worms foul along the opening (or “aperture”) of the shell.  This fouling looks like little white specks.  So if there are many specks, then the snail shell has been persisting for a long time in the environment and may not be available for very much longer.  These specks do not appear when the snail is still alive, because the snail will remove them.

The carrying capacity of the tidepool habitat for hermit crabs is therefore based on the availability of the correct size and species of snail shells.   This experiment will test this hypothesis by graphing the data to compare the size of shells that are empty to those that are presently being used by the crabs.

Location and Time

Coal Oil Point Reserve Coal Oil Point Reserve

TIME REQUIRED: The Hermit Crab collection could be completed successfully in as little as 45-60 minutes. It could also be extended to allow time to identify other species in the area. Allow another class period for discussing results and possible adaptations.

Teacher Preparation

Before the trip, spend some class time talking about ecology and other concepts, such as carrying capacity and competition.  The expected results are that there is a difference between the sizes of the shells on the graph as the bell curve of the occupied shells is shifted to the right (as values get larger).  The distribution of the shells graph should be discovered by the students themselves and a discussion of the results can follow.


  1. Each student carefully observes tidepools at low tide and collects 100 snail shells for examination in the field.  IT IS IMPORTANT THAT THE STUDENTS PAY CLOSE ATTENTION TO WHERE THE SHELLS ARE COLLECTED SO THAT THEY MAY BE RETURNED WHERE FOUND.   DO NOT REMOVE THE SHELLS FROM THE RESERVE.
  2. Students measure the shells and record their lengths.
  3. Students record if there is a hermit crab inside or not, being careful to take good care of the animals.  DO NOT HANDLE SHELLS WITH CRABS FOR TOO LONG.
  4. Students create a data table and plot the data on the graph.  The heading will be “The Distribution of Snail Shells Available for Use by Hermit Crabs.”   Students plot data of occupied and unoccupied shells:  the y-axis is the number count, and the x-axis is the shell size in millimeters.
  5. Discuss the results.

Possible Extensions:  The class can examine what kinds of shells hermit crabs prefer.  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.  Here are the steps:   (1) place a small number of hermit crabs in fresh water noting the sizes and kinds of shells they already occupy, (2) wait for the crabs to exit their shells, (3) offer different kinds of shells of different shapes and sizes to the crabs, and (4) place the crabs back into salty ocean water and observe the crabs as they pick a shell, and (5) make observations of which types of shells the crabs selected, both the size and shape.   Ask the students to describe the kinds of shells the crabs appear to be using:   their shape, their size, their age, and other attributes.  The class may collect new kinds of shells from a non-hermit crab habitat and present them as choices as well.

These are two genera of snails whose shells are most commonly used by hermit crabs at Coal Oil Point:  Black turban snail (Tegula spp.) and Olive snail (Olivella spp.)


  1. Students will present their plotted results.
  2. Ask the students why the distribution of shells changes with the shell size.
  3. Ask the students how competition affects the distribution of snail shells available for use by hermit crabs.
  4. Ask students to explain why snails are important to the tidepool ecology.

Teacher Materials

  • paper
  • pencils
  • metric rulers
  • containers that hold water
  • graph paper