Escape trajectories of the brown shrimp Crangon crangon, and a theoreticalconsideration of initial escape angles from predators

Tail-flip escape trajectories of the brown shrimp Crangon crangon have been investigated in response to a natural predator, the cod Gadus morhua, and an artificial stimulus. Shrimps escaped by rolling to their left or right d uring the initial tail-flip of a response, and thereafter swam on their sid e. As a result of the laterally directed first tail-flip, initial escape an gles always lay between 75 degrees and 156 degrees with respect to the (pre -escape) longitudinal axis (anterior=0 degrees) of the shrimp. Symmetrical attacks from either head-on or tail-on produced escapes to the shrimp's lef t or right in equal proportions, although a contralateral bias did occur if the shrimp experienced a looming object from one side before a symmetrical attack was applied. Lateral attacks produced a significantly greater propo rtion of contralateral responses than ipsilateral ones. Empirical and theor etical analyses indicate that the initial escape direction is influenced by an interaction between the range of first tail-flip escape angles that the shrimp is capable of performing and the risk of being intercepted by a pre dator during the initial stage of an escape. Thus, the unpredictability ('p rotean behaviour') of the response may be affected by the conditions of the interaction. Subsequent tail-flips of an escape usually directed the response away from the stimulus, but sometimes escapes were instead steered to the side of the stimulus and then behind it. The probability of each type of escape occurr ing changed with attack direction. The elements of protean behaviour that have been identified in both the ini tial and subsequent stages of the escape may prevent predators from learnin g a fixed pattern of response, but a trade-off occurs when escape trajector ies infringe upon zones of high capture risk.