Limpet Shell Exercise

Author: Crissy Huffard, adapted from Matt Wedel’s bone exercise

Concepts covered:
In this lesson we review the terms and concepts: evolution, selection, biotic, and abiotic factors.
We come up with hypotheses about limpet shell types based on what we discuss about intertidal environments. We use ratios to compare shell types, and (at some point) test the hypotheses.

Grade span: 6-8

Teacher background:

In California, and most paces on earth, the water along the coasts rises and falls twice a day. This happens because of the pull of the moon (and a little bit the sun) on the oceans, and the movement/ sloshing of a bulge of water as the earth rotates.

For a good explanation of tides visit this online tutorial:

The Intertidal zone includes the shoreline that spans from what gets wet at high tide (short periods, twice/24hr here) to what dries out during low tide (also short periods twice/24 hr here). The sub-tidal zone is always underwater, with fish swimming around, some surge, maybe waves during a storm. When the tide comes up, the fish come in to feed in areas that were just recently high and dry. When the tide recedes, the most (but not all) fish swim back to the sub-tidal zone and intertidal animals hang out, either in tide pools (pools left in/between rocks), of they hang out on the rocks. Some animals (like anemones, mussels, barnacles) can’t move and must deal where they are. Others, like limpets and snails can move, but tend not to.

Common intertidal organisms: sea stars, mussels, barnacles, crabs, fish (even eels!-they hang out in the pools), chitons, snails, limpets, sea urchins, sea slugs, clams.

Limpets are gastropod mollusks (so are snails and slugs) that have a single, uncoiled, cap-like shell. They are important grazers of algae along rocky intertidal shorelines (such as along the California coast). They graze along the rocks when the water comes up, and go back to a “home spot” when the tide goes out (water falls). This home spot on the rock fits their shell well, so when they clamp down on a rock it makes a good fit, and a predator or wave can’t get under the lip to flip it over or peel it off. Limpets don’t move very far from their home spot, so they tend to stay in one zone of the intertidal. They offer an interesting system for investigating vertical zonation of body types in the intertidal. We’re going to look at the shell types.


  1. Download a PDF version of the handout. Adjust the handout to meet your class’s needs.

  2. Make copies of the handout. I copied a small ruler at the bottom of the last page so that everyone had a ruler and they didn’t have to pass them around. They’ll need calculators, but tend to have their own.

  3. Using a mechanical pencil (so you don’t write on the shell itself), trace the outline of each limpet on a couple of big note cards, giving each limpet a number. Keep the same numbering system for each bag. Put the cards in the bags.

In retrospect:
It sparked their curiosity well enough, but this is a lot to fit into one class period! Take two class periods or cut some of it out. The writing took especially long, and most needed to take it home to work on it. Measuring height was a challenge. Show them how to put the shell on its side, on the ruler, and look at it from above.

Also- Dave was on his way out the door when I got these limpets/shells and I don’t know which zone they’re actually from, (or even if some technically aren’t limpets- our little secret) so we couldn’t test our hypotheses. There was no time for it anyway.