RHYTHMICITY OF A SCYPHOZOAN JELLYFISH, CASSIOPEA XAMACHANA BIGELOW

Rhythmic contractions of the Scyphomedusan bell are initiated by pacemakers located around the margin. How these pacemakers interact to produce activity patterns of a specific frequency and regularity and what effect the environment has on such output are considered.;Whole animals exhibit a wide range of spontaneous rhythms which are not always significantly affected by light changes. The pacemakers within an individual are functionally heterogeneous in spontaneous output, response time, percent response and effect on remaining system when removed. These characteristics are correlated, resulting in some faster and more responsive pacemakers. Contrary to expectation, the degree of heterogeneity of the entire set of pacemakers is proportional to intact output regularity. Conversely, pacemaker number is proportional to contraction frequency and responsiveness to light. Heterogeneity has no effect on system response while pacemaker number has no effect on rhythm regularity.;The only prior quantitative model proposed to explain how these patterns are generated is the "redundancy hypothesis" developed by Horridge in 1959. This hypothesis is insufficient on the basis of the current results which are contrary to its assumptions and in part inconsistent with its predictions. The present work supports an alternate hypothesis, pacemaker heteropotency. Measures of pacemaker state are correlated in a way which enhances pacemaker heteropotency. The greater the tendency for functional heterogeneity, the more regular is the system output. This new hypothesis explains heretofore anomalous observations and individual behavioral variability.;Computer assisted analysis of Cassiopea xamachana inter-contraction intervals in field subjects, intact and sequentially segmented laboratory subjects under various stimulus conditions was used to assess spontaneous output. Pacemakers were individually subjected to physiological testing to determine comparative functional state. A method was developed for reversible addition or subtraction of pacemakers of known activity levels in the intact nervous system.