Swimming in Aplysia brasiliana: analysis of behavior and neuronal pathways.

Swimming of Aplysia brasiliana was analyzed using time-lapse video and computer graphics techniques to quantify the cyclical movements of different regions of the parapodia. Both the speed of swimming and the period of oscillation are temperature dependent, whereas the metachronal offset is not. The parapodial arterial supply is described; ligation of parapodial arteries does not affect the parapodial motions. Peripheral and central lesions indicated that (1) the anterior parapodial nerve plays the major role in parapodial flapping, (2) a separate neuronal oscillator resides in each pedal ganglion, (3) bilateral coordination is mediated via the pedal commissure, and (4) the swimming "command" pathway is the cerebro-pedal connective. During regular parapodial flapping the speed of level midwater swimming is constant throughout the cycle, suggesting that swimming is not produced by jet propulsion. An alternative propulsion model is advanced.