Neural redundancy and regularity of swimming beats in scyphozoan medusae.

The swimming beat of scyphozoan medusae can be generated by any one of the marginal ganglia present in the intact animal (Bullock & Horridge, 1965, summarize the data used in this Introduction). This finding of multiple neural units which all perform similar or perhaps even identical functions is relatively common. It raises the question, 'What function is served by the apparent neural redundancy?' Several possible functions can be suggested for the multiplicity of pacemakers for the jellyfish swimming rhythm. First, the multiplicity of ganglia which can trigger a beat may facilitate responses of beat frequency to local sensory stimuli. This speculation is suggested by the following two observations: (1) each ganglion serves as an integration centre at which the swimming beat frequency can be influenced by excitation induced in the diffuse nerve net by local stimulation; (2) a ganglion is found near each major sensory area (rhopalium) with the number of ganglia varying in different species according to the number of rhopalia. Indeed, the marginal ganglia represent considerable centralization of neural tissue and of function as compared to the kind of neural organization believed to be phylogenetically oldest, namely, local neural co-ordination of local responses to local stimuli. We have made no attempt to test this possible function of redundancy, but have concentrated instead on ways in which the redundancy might contribute to the regularity of jellyfish beating. Several jellyfish have arrived in our laboratory with more or less normal swimming beats but with one or more marginal bodies that appeared damaged. Activity of an individual ganglion was tested by cutting out a piece of jellyfish that contained no other ganglia. In such tests the apparently damaged ganglia often proved incapable of initiating swimming beats. In these cases the redundancy of pacemakers permitted swimming to continue despite serious damage to part of the nervous system. Horridge (1959) has suggested that even in intact animals the multiplicity of ganglia may play an important role in increasing the frequency and regularity of the swimming beat. (Note that this is an adaptive function of the redundancy only if fast and regular swimming beats are more useful to the animal than slow and irregular swimming beats. Ecological data to test this assumption are not available to us.) Horridge'8 suggestion that redundancy of pacemakers contributes to regularity of beating was based initially on the observation that pieces of jellyfish containing only