Short Communication Interneurones Mediating the Escape Reaction of the Marine Mollusc Clione Limacina

Neural mechanisms underlying the escape reaction have been extensively studied for over 20 years in many species of animals belonging to several different phyla (for a review, see Eaton, 1984). Among molluscs, the escape reaction has been most thoroughly studied in Tritonia. In this animal a strong mechanical or chemical stimulus applied to the posterior part of the body evokes swimming (for a review, see Getting, 1983; Getting and Dekin, 1985). Electrophysiological experiments on the isolated nervous system revealed that neurones constituting the central pattern generator (CPG) for swimming were strongly depolarized in response to sensory stimulation and that this depolarization caused rhythmic activity in the CPG. However, attempts to identify command neurones mediating the escape reaction in Tritonia have failed (Getting, 1983). This paper deals with the escape reaction in another gastropod mollusc, Clione limacina (subclass Opistobranchia, order Pterapoda). We have been able to identify some of the interneurones involved in this animal's escape reaction. Clione swims using rhythmic movements of two wings, which are controlled by the locomotor CPG located in the pedal ganglia. The CPG can generate the locomotor rhythm (fictive swimming) in vitro, i.e. after isolation of the pedal ganglia from the rest of the nervous system, and the neuronal organization of the CPG has been analyzed (Arshavsky etal. 1985a-c; Satterlie, 1985, 1989). The CPG consists of two groups of interneurones (7 and 8) controlling various groups of wing motoneurones, including two large cells, 1A and 2A, which innervate the dorsal and ventral wing muscles, respectively (Arshavsky etal. 1985a,b). Locomotor activity is not continuous in freely swimming Clione: periods of locomotor activity (lasting 1-3 min) usually alternate with periods of inactivity during which the animal sinks passively; this results in 'vertical migrations' (Litvinova and Orlovsky, 1985). The effects of tactile stimulation depend on the phase of vertical migration in which the stimulus is applied. In a passive animal,