Why do cubomedusae have only four swim pacemakers?

The classic view of swimming control in scyphozoan and cubozoan jellyfish i nvolves a diffuse motor nerve net activated by multiple pacemaker sites tha t interact in a simple resetting hierarchy. Earlier modeling studies of jel lyfish swimming, utilizing resetting linkages of multiple pacemakers, indic ated that increases in pacemaker number were correlated with increases in t he rate and regularity of network activity. We conducted a similar study us ing the cubozoan jellyfish Carybdea marsupialis, concentrating not only on the adaptive features of multiple pacemaker networks but also on the mechan ism of pacemaker interaction, The best fit for our experimental data is a m odel in which pacemakers express a degree of independence. Thus, our result s challenge the idea that pacemaker interactions in scyphozoan and cubozoan medusae are based on a strict resetting hierarchy. Furthermore, our data s uggest that the combination of semi-independent linkage of pacemakers with the small pacemaker number characteristic of cubomedusae is important in (i ) maintaining a biphasic modulatory capability in the swimming system, and (ii) allowing behaviorally appropriate directional responses to asymmetrica l sensory inputs in the radially arranged jellyfish nervous system.