Multiple conducting systems and the control of behaviour in the brain coral Meandrina meandrites (L.)

Abstract

Extracellular suction electrode recordings from tentacles of Meandrina provide evidence for three conducting systems. One system may be the colonial nerve net. It is through-conducting and occasionally gives multiple responses to mechanical and electrical stimulation. The other two systems are also normally through-conducting but they conduct very slowly and are termed slow systems. One slow system (SSo) is in the oral disk and the other (SSc) is in the tissue covering the thecal ridges (coenosarc). SSo pulses travel throughout the interconnected oral disk regions of the colony but also enter the SSc. SSc pulses travel over the entire coenosarc but cannot enter the SSo. Both slow systems are present in the tentacles. Repetitive stimulation of the SSo evokes oral disk expansion and tentacle extension, seemingly identical to the expansion that normally occurs at night when the colony shows tentacular feeding. Repetitive stimulation of the SSc also evokes slight tentacle extension but more noticeably causes the coenosarc to become turgid. Many colonies show coenosarc expansion during the day and this may alter the exposure of symbiotic zooxanthellae to sunlight. The SSo and SSc are spontaneously active and both show a marked increase in pulse frequency when exposed to dissolved food substances. The level of activity in each system may also be modified by changes in light intensity and the SSc and SSo may thus control respectively diurnal and nocturnal expansion of different parts of the colony. The colonial nerve net appears to coordinate fast and slow contractions. Intense light can increase the level of spon­taneous activity in the nerve net.