Swimming in Opisthobranch Mollusks: Contributions to Control of Motor Behavior: Introduction to the Symposium1

Abstract

Beginning with the pioneering work of Dennis Willows (Willows and Hoyle, 1969; Willows et al. , 1973), studies utilizing opisthobranch mollusks have been particularly useful in the description of rhythmic motor behaviors and their mechanisms of central pattern generation. General mechanisms that contribute to central pattern generator function in other animal groups, such as reciprocal inhibition, postinhibitory rebound, multi-component synaptic potentials, delayed excitation, and extrinsic modulation (to name a few), have been found to contribute to the generation of rhythmic motor behavior in opisthobranchs. In addition, some novel concepts that have since been substantiated in other animal groups were developed through work on opisthobranch locomotory systems: some CPG neurons are multifunctional and may participate in different motor behaviors (Getting, 1989); networks can be intrinsically modulated by components within the CPG networks (Katz et al. , 1994; Katz and Frost, 1995). The Opisthobranch Swimming symposium was organized with a primary goal of providing details of motor control from four specific Opisthobranch orders that have swimming members. This comparative approach is particularly interesting since behavioral observations of swimming activity in the four orders reveal three distinct types of swimming. In the Anaspids ( Aplysia ) and Gymnosomes ( Clione ), swimming is accomplished by flapping of parapodia. In the latter group, parapodia are highly modified into wings. In the Notaspids ( Pleurobranchaea ) and dorso-ventrally flattened Nudibranchs ( Tritonia ), swimming consists of alternate dorsal and ventral flexions of the body. Finally, in the laterally flattened Nudibranchs ( Melibe ), swimming occurs with alternating lateral flexions of the body. In all but one case ( Clione ), the central neural circuits and effectors for swimming also participate in other behaviors. For example, the parapodia of Aplysia undergo local protective contractions in response to directed stimulation and produce coordinated respiratory pumping contractions in addition to their role in swimming (Kandel, 1979). In …