A series of laboratory tests on the snapping shrimp,Alpheus heterochaelis(Say), revealed that after removal of the large chela or immobilization of its dactylus, the shrimp could not compete successfully with normal shrimp in acquiring or holding a shelter. Evidently, the action components of the dactylus play a primary role in the successful defense of a shelter. Unlike normal shrimp or shrimp with an immobilized dactylus, shrimp lacking the large chela often were allowed to encroach beneath the shelter by their like‐sexed competitors, suggesting that the structure of the chela itself and not its movements elicits aggressive behaviour by adversaries. The absence or immobilization of the chela did not seem to affect heterosexual pairings.