Tonic immobility in the blue crab (Callinectes sapidus, Rathbun) its relation to threat of predation.

The tonic immobility. response in the blue crab (Callinectes sapidus, Rathbun) was investigated in a series of seven experiments. Although reported to be a powerful variable in other species, preinduction electric shock produced inconsistent increases in the duration of tonic immobility with the blue crab; Manipulations that were more directly relevant to the fear of predation had considerably greater effects than shock. Physical damage to the chelipeds, mirror image stimulation, and immobilization beneath artificial glass eyes all produced significant prolongation of the immobility episode; Crabs immobilized on a bed of sand rather than on a hard surface showed shorter immobility durations, suggesting that opportunity for escape is an important variable affecting the immobility reactionmthe present results support the contention that threat of predation is the organizing principle behind tonic immobility