Development of spontaneous and evoked behaviors in the medicinal leech.

ABSTRACT The ontogeny of behavior in an organism must reflect developmental events in the nervous system, and it thus provides a noninvasive measure of neuronal development. This approach may be particularly fruitful in the medicinal leech because the neuronal basis of several behaviors has been characterized in adult leeches, providing a rich background against which behavioral development can be interpreted. We have investigated the order in which behaviors arise during the period of embryonic development and have determined the time at which each behavior is first expressed. Some behaviors, such as lateral ridge formation, germinal plate bending, spiral twisting, and sidewinding, were produced spontaneously by embryos. Others, such as shortening, circumferential indentation, local bending, and elongation, occurred only when they were elicited by weak mechanical stimulation. Such stimulation rarely evoked a behavioral response in young embryos (at 45% of the time required for complete embryonic development, 45% ED), but by 80% ED embryos responded to nearly 100% of the stimuli presented. In embryos older than 50% ED, the behavior most frequently evoked by stimulation of the anterior end, the posterior end, or the rear sucker was shortening. Stimulation of the midbody usually evoked behavior other than shortening, illustrating that the body was behaviorally compartmentalized, at least in part. Some behaviors observed during embryogenesis are never seen in adult leeches. For example, in response to stimulation of the midbody, young embryos produced a behavior that we have called "circumferential indentation," whereas older embryos produced local bending, a response previously described for adults. The switch from circumferential indentation to local bending may signal the formation of new synaptic connections.