Escape behaviour in the stomatopod crustacean Squilla mantis, and the evolution of the caridoid escape reaction

The mantis shrimp Squilla mantis shows a graded series of avoidance/escape responses to visual and mechanical (vibration and touch) rostral stimuli. A low-threshold response is mediated by the simultaneous protraction of the thoracic walking legs and abdominal swimmerets and telson, producing a back wards 'lurch' or jump that can displace the animal by up to one-third of it s body length, but leaves it facing in the same direction. A stronger respo nse starts with similar limb protraction, but is followed by partial abdomi nal flexion, The maximal response also consists of limb protraction followe d by abdominal flexion, but in this case the abdominal flexion is sufficien tly vigorous to pull the animal into a tight vertical loop, which leaves it inverted and facing away from the stimulus. The animal then swims forward (away from the stimulus) and rights itself by executing a half-roll. A bilaterally paired, large-diameter, rapidly conducting axon in the dorsal region of the ventral nerve excites swimmeret protractor motoneurons in se veral ganglia and is likely to be the driver neuron for the limb-protractio n response. The same neuron also excites unidentified abdominal trunk moton eurons, but less reliably. The escape response is a key feature of the malacostracan caridoid facies, and we provide the first detailed description of this response in a group t hat diverged early in malacostracan evolution, We show that the components of the escape response contrast strongly with those of the full caridoid re action, and we provide physiological and behavioural evidence for the biolo gical plausibility of a limb-before-tail thesis for the evolution of the es cape response.