A neurophysiological analyses of aggressive behaviour in Carcinus maenas

Agonistic activities, that is behaviours associated with aggression and defence were studied in Carcinus maenas. Two particular agonistic interactions which employ the chelipeds were chosen for detailed analysis; the fast strike and threat display behaviours. The fast strike involves rapid flexion of the chelipeds and is completed within 30 to 60 ms. The limb is propelled forwards and downwards from the first two cheliped joints. Calculation of the energy required for a strike revealed that it is necessary for the muscles of the coxa--basi-ischium complex to develop energy before the strike is performed. It is found that this energy is produced by antagonistic muscles contracting together before the strike, allowing isometric tension development. The moments of the coxa promotor and remotor muscles about the thorax-coxa joint give rise to a bistable articulation in which the coxa may be rapidly remoted or promoted in a "flip-flop"? situation. Three of the basi-ischium muscles, the Anterior levator, Anterior and Posterior depressors, have an unusual geometry which gives them more than one function. The contributions of the coxal and basi-ischium muscles during the strike is discussed and a method by which the strike is performed, is suggested. A strike only occurs from a threat display position in which the chelipeds are extended and levated. The threat behaviour was examined to determine the muscle activity which precedes the strike. Characteristic patterns of motor activity correspond to different postures of threat display. Manipulating various stimulus parameters presented to the crab showed that threat displays are most readily released by the rapid approach of large objects. Neural activity recorded from the circumoesophageal connective nerves revealed that the presentation of stimuli representing rapidly approaching objects accompanies high frequency bursts of large spikes. This activity also corresponded to high frequency motor activity in the cheliped muscles which accompanies the adoption of extreme threat display positions.