Behavioral and neural responses of juvenile crayfish to moving shadows.

One of the most important decisions any animal has to make is how to respond to sensory cues that suggest an imminent attack by a predator. We measured behavioral and neural responses of juvenile crayfish to moving shadows of different velocities while the animals were searching for food. In all experiments, and independent of shadow velocity, each crayfish produced one of two discrete behavioral outputs: it either tail-flipped backwards by rapid flexion of its abdomen or it immediately stopped its forward locomotion. The probability of each behavioral response was dependent on the velocity of the shadows that were presented. While most animals responded with tail-flips to slow-moving shadows and stops were rarely observed, the number of tail-flips decreased as shadow velocity increased. Tail-flips were almost absent for very fast-moving shadows and stopping behavior became the dominating response. By using a non-invasive technique to record neural activity, we were able to identify the underlying neural circuit that controlled the observed tail-flips. All tail-flips were mediated by activation of the medial giant neurons, which are part of a hardwired neural circuit previously described to produce reflexive responses to tactile stimulation.