Hypnotised Shark

Tonic immobility has also been used to describe the paralysis which often immobilizes animals, such as rodents or birds, when they feel threatened by a predator. Tonic immobility plays a role in survival if it helps a hunted animal to blend in with its surroundings. This tonic state is common with invertebrates as well.
Inducing tonic immobility in some animals requires extreme treatment such as electric shock, along with other elements which produce stress, while in others the state can be induced without exposure to apparently significant stress (stroking a particular area of a lobster's shell, focusing a hen's attention on a line drawn in the dirt, etc.).
Tonic immobility has also been hypothesized to occur in humans that are subject to intense trama such as rape or sexual assault.