Swimming behaviour in crabs is electromyographically described in relation to the involvement of the discharge of the equilibrium interneurones. In intact crabs or crabs with only the fifth legs remaining, swimming consists of cyclic out-of-phase sculling movements of the last pair of pereïopods (P5). In these legs, all muscles are involved within a single swimming cycle; antagonistic muscles burst alternately, as do bilateral pairs of muscles. Bursting in the four proximal muscles ensures the appendage rotation while distal muscles set the scull in the best propulsive position.

Swimming evoked by tilt in the sagittal plane starts with symmetrical remotor activity before alternate bursting begins. Tilt in the plane of a statocyst vertical canal leads to asymmetrical onset of remotor bursting, starting with the muscle contralateral to the stimulated statocyst.

Tilt in defined vertical planes elicits the discharge of identified equilibrium interneurones. Of these, interneurones C and D are active before and during swimming. Sensory inputs from the statocysts and/or the leg proprioceptors to these interneurones are both adequate to drive swimming. Moreover, our experiments suggest that cell C activity is strongly involved in the onset and the maintenance of swimming behaviour.

Swimming can be altered by autotomy of legs on one side performed a few days in advance, and leads to the same turning tendency as does contralateral cutting of a connective. This may be due to modification in the strength of the remaining central connections to compensate for those lost.

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