Studies on hexactinellid sponges. II. Excitability, conduction and coordination of responses in rhabdocalyptus dawsoni (lambe, 1873)

Abstract

Rhabdocalyptus can arrest its feeding current. The response is initiated by mechanical or electrical stimulation, and is coordinated through the sponge by a conduction system, having a precise excitability threshold and conducting on an all-or-none basis. All parts are excitable and conduct. Individuals in colonial assemblages are coordinated. Spontaneous as well as evoked arrests are observed. There is evidence of scattered pacemaker sites. Conduction is diffuse and unpolarized, and occurs with a velocity of 0.26 ± 0.07 cm s -1 at 11 °C. The conduction system is probably the trabecular syncytium. Isolated dermal membrane (‘pure’ trabecular tissue, without flagella or contractile elements) conducts. Mechanical and chemical signalling mechanisms are discussed. It is concluded that they cannot account for the phenomena observed, but that conduction must involve electrical impulses. The effectors responsible for current arrests are almost certainly the flagella of the flagellated chambers. It is assumed that they stop beating on receiving an arrest signal through the conduction pathway. The waveforms of arrests recorded with a thermistor flowmeter are best interpreted in terms of sudden, all-or-none cessation of pumping, with slow, gradual recovery of full pumping power. The flagella probably beat feebly at first on becoming active again following an arrest. The effector response shows a refractory period of 30 s. Responses occur with short latency. Delays are attributable to conduction time. The system is fatigueable. Numerous parallels exist with the behaviour of the stigmatal cilia in the ascidian branchial sac, both in the characteristics of the effector response and in the mechanism of coordination.