NAKAYA, F.; SAITO, Y.; MOTOKAWA, T.; Tokyo Institute of Technology; Univ. of Tsukuba; Tokyo Institute of Technology: Scaling analysis of physiological time of the colonial ascidian Botrylloides violaceus

The physiological time, such as the duration of cardiac cycle and respiratory cycle, lengthens in proportion to the 0.25 power of the body weight in homoeothermic animals. Little is known about the physiological time of poikilothermic animals and we know nothing about the physiological time of modular organisms. We investigated the scaling of time-related parameters of the heartbeat in the colonial ascidian Botrylloides violaceus. The colony consists of zooids that are identical in age, size, and shape regardless of colony size. The body of the zooid and tunic is transparent enough for the observation of heartbeats. Zooids in a colony are connected each other by a vascular system sharing the blood flow between them. The heart of ascidians periodically reverses the direction of blood flow: it alternates between advisceral direction and the abvisceral direction. We measured the duration of a single heartbeat (Tb) of both directions and the duration of the blood-flow reversal (Tr) of both directions in the various-sized colonies (zooid number ranging 1 – 700) and in tadpole larvae. The hearts of zooids in a colony behaved identically: Tb and Tr did not differ irrespective of the location of the zooids in a colony. Tr toward the abvisceral direction was longer than that toward the advisceral direction. The colony size affected Tb and Tr differently. Tb was constant irrespective of colonial size, whereas Tr shortened in proportion to the 0.15 power of colony size. Tb of larva was the same as that of adults; Tr of larva was shorter than that expected from the same-sized adult colony. The present results showed that the heartbeat frequency was conserved throughout the development and colonial growth, whereas the time parameters of heart reversal changed. The factors that may affect the reversal will be discussed.