The terrestrial snail Helix lucorum crawls using waves of muscular contraction (pedal waves) that spread along the sole of its foot. Crawling speed depends on wave generation frequency (step frequency) and the distance the snail moves forwards during each wave (step length). In a previous study, video recordings of a crawling snail showed that its sole length varied over a wide range and was directly correlated with mollusc speed. Speed depended on step length, which was directly related to sole length, rather than on step frequency, which remained rather constant. In the present study, the effects of dopamine, ergometrine (a blocker of dopamine receptors in molluscs) and serotonin injection on the linear relationship between sole length and locomotor speed in Helix lucorum were studied. In crawling snails, dopamine caused sole contraction, and locomotion slowed down or ceased. Ergometrine stimulated locomotion, which resembled rapid crawling with an extended sole, as observed under normal conditions. Serotonin stimulated locomotion and accelerated crawling significantly without causing changes in sole length. The acceleration of locomotion induced by serotonin injection was due to pedal wave (step) elongation. It is proposed that, during each locomotor episode, dopamine controls snail speed by regulating sole length, which determines the amplitude of contraction of the muscle cells involved in pedal waves and, as a result, step length; serotonin determines the basic step length and shifts the linear relationship between sole length and mollusc speed upwards along the axis of mollusc speed. The efficiency of the serotonergic system depends on the physiological state of the mollusc (e.g. that characteristic of summer or winter).

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