Colonial behaviour and electrical activity in the Hexacorallia

The behaviour of the polyps of eight coral species in the subclass Hexacorallia is described with reference to electrical activity recorded with extracellular suction electrodes. Following repetitive mechanical or electrical stimulation, waves of polyp retraction spread over the colony from the point of stimulation. In response to a single electrical or mechanical stimulus, a single electrical potential was evoked. It is suggested that this pulse represented activity in the colonial nerve net. The pulse was conducted without decrement over large areas of the colony. The conduction velocity was 15-25 cm s-1 at 25°C. There was no evidence for multiple firing following single threshold stimuli. Polyp retraction only occurred after two or more ‘nerve net' pulses had impinged upon the neuromuscular junctions of the retractor muscles within a short time interval. Colonial polyp retraction responses occurred, therefore, when a number of nerve net pulses were conducted across the colonial nerve net. A burst of pulses showed a reduction in frequency as it was conducted from the point of stimulation. A new explanation of colonial polyp retraction patterns in response to electrical stimulation is proposed. The different patterns may be explained in terms of the way in which the frequency (but not the number) of the pulses in a burst of nerve net activity changes with the distance conducted. Changes in nerve net pulse frequency affect the degree of facilitation of the neuromuscular junctions which, in turn, affects the size of the muscle contraction evoked. The possibility is considered that ‘slow conduction systems’ similar to those found in sea anemones may also be present in the colonial Hexacorallia.