A Hermit Crab's Next Move

A Card Near a Crab

A fourth-grade class visits a beach field lab beside rocky tide pools. An educator cares for several marine hermit crabs in seawater trays with sand and shelters. The class investigates whether a crab's view of a moving object affects its actions. Students watch without touching the animals or their water.

Animals receive information through sense organs. Eyes detect light, including information about shapes and movement. Structures on a hermit crab's feelers can detect touch. Sense organs send information to the brain. The brain processes that information and can use information from past experiences. It then sends signals that guide a response, such as moving or becoming still. A model can represent these steps even when observers cannot see them happening inside an animal.

First, the educator chooses six active crabs and places a screen outside each tray. Next, each crab experiences three conditions. For Still card, a dark card stays still behind a clear screen. For Moving card, the educator slowly moves the card toward the tray behind the clear screen, stopping well outside the tray. For Hidden card, the educator makes the same movement behind an opaque screen. The opaque screen blocks the crab's view of the card.

The card never touches a tray or casts a shadow on it. Water conditions, lighting, screen position, and card size stay the same. The educator changes the order of the conditions among crabs and waits until each crab is active again before another trial. Each crab has one trial in each condition. Trials are brief, and the educator stops if an animal needs a break.

Finally, students record whether each crab pulls into its shell within five seconds after a trial begins. The graph shows their counts. Students can observe the card, the screen, and each crab's actions. They cannot directly observe the information traveling inside a crab or its brain processing that information.