Electrophysiology of two parallel conducting systems in the colonial Hexacorallia

Extracellular polythene suction electrodes have been used to record electrical activity in four species of Madreporaria - Dendrogyra cylindrus, Meandrina meandrites, Mussa angulosa and Eusmilia fastigiata. A colonial conduction system, believed to be the nerve net, was found in all species. It conducted without decrement between all polyps. A second colonial system was found in Meandrina, Mussa and Eusmilia. Pulses could be recorded only from tentacles or oral disks though the system could be excited by electrical or mechanical stimuli to any part of the colony. In the tentacles and oral disk, this conduction system had a refractory period of about 60 ms while in the column or interpolyp regions the refractory period was much longer - up to several seconds. The effect of these differences was to limit the frequency of conduction of pulses in this system between polyps. The second system is compared to the s. s. 1 (ectodermal slow conduction system) of the sea anemone Calliactis parasitica. It is the first demonstrated example of a colonial slow conduction system in the Hexacorallia and is similar in properties to a colonial slow conduction system previously described for Pennatula phosphorea (Octocorallia). The slow conduction system may have a rôle during feeding behaviour by promoting expansion of tentacles and the production of mucus.