Swim pacemakers in cubomedusae

provision for integrative sensing of environmental information so that appropriate reactions can follow. In most metazoans, specialized sensory systems make specific connections within integrative centers which, in turn, pass on sorted information to appropriate effectors. However, in the generalized cnidarian, integrative centers are considered to be poorly developed (in a comparative sense). In scyphozoan and cubozoan jellyfish, ganglion-like neuron accumulations are found in the marginal rhopalia; however, in most of these jellyfish, the conduction of information between rhopalia is by diffuse, non-polarized nerve nets (Pantin and Vianna Dias, 1952; Horridge, 1954; Horridge, 1956a; Horridge, 1956b; Passano, 1965; Passano, 1973; Satterlie, 1979; Anderson and Schwab, 1981; Anderson and Schwab, 1982; Anderson, 1985). This raises interesting questions concerning radial coordination of motor output in situations in which asymmetric sensory input produces directed locomotory movements. In particular, the lack of cephalization raises the question of how a series of such pacemakers interact to produce asymmetric locomotory responses to move the animal towards or away from environmental stimuli. Swim pacemakers in cubomedusae and scyphomedusae are restricted to a finite number of distinct marginal neural structures (rhopalia). Removal of all the rhopalia leaves the medusa unable to produce spontaneous contractions of the swimming musculature (Romanes, 1876; Romanes, 1877; Pantin and Vianna Dias, 1952; Passano, 1965; Passano, 1973; Passano, 1982; Satterlie, 1979). Transmission of electrical information throughout the muscle sheet is via subumbrellar nerve nets, which use symmetrical or reciprocal chemical synapses (Anderson, 1985; Anderson and Schwab, 1981; Satterlie, 1979; Satterlie and Spencer, 1987; Westfall, 1987). Coordination of rhopalial pacemakers is presumed to be due to a simple dominance hierarchy in which a discharge in one pacemaker not only triggers a through-conducted contraction of the swim musculature but also resets all the other pacemakers (Horridge, 1959; Passano, 1965; Passano, 1973; Passano, 1982). Cubomedusae and scyphomedusae are under a developmental constraint of tetramerous symmetry, so the rhopalia occur in multiples of four (see Hyman, 1940). Cubomedusae have four rhopalia, while scyphomedusae have at least eight, with some species having up to 64 rhopalia. The resultant pacemaker redundancy was examined in three modeling studies of scyphomedusan jellyfish in which contractile activity in a piece of jellyfish containing a single rhopalium was used to construct models of multiplepacemaker networks (Horridge, 1959; Lerner et al., 1971; Murray, 1977). In all three models, pacemakers were 1413 The Journal of Experimental Biology 204, 1413–1419 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3205