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

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Abstract
Extracellular suction electrode recordings from tentacles of Meandrina provided evidence for 3 conducting systems. One system may be the colonial nerve net; it was through-conducting and occasionally gave multiple responses to mechanical and electrical stimulation. The other 2 systems were normally through-conducting but conducted very slowly and were termed slow systems. One slow system (SSo) was in the oral disk and the other (SSc) was in the tissue covering the thecal ridges (coenosarc). SSo pulses travel throughout the interconnected oral disk regions of the colony but entered the SSc. SSc pulses travelled over the entire coenosarc but could not enter the SSo. Both slow systems were present in the tentacles. Repetitive stimulation of the SSo evoked oral disk expansion and tentacle extension, seemingly identical to the expansion that normally occurred at night when the colony showed tentacular feeding. Repetitive stimulation of the SSc evoked slight tentacle extension but more noticeably caused the coenosarc to become turgid. Many colonies showed coenosarc expansion during the day and this could alter the exposure of symbiotic zooxanthellae to sunlight. The SSo and SSc were spontaneously active and both showed a marked increase in pulse frequency when exposed to dissolved food substances. The level of activity in each system could be modified by changes in light intensity and the SSc and SSo could control, respectively, diurnal and nocturnal expansion of different parts of the colony. The colonial nerve net appeared to coordinate fast and slow contractions. Intense light increased the level of spontaneous activity in the nerve net.