Excitability and electrical signalling in hexactinellid sponges.
Sally P. Leys and George O. Mackie
Biology Department, University of Victoria, Victoria, B.C. V8W 3N5, Canada.
Sponges arose very early in metazoan evolution and lack a nervous system but some hexactinellids respond to touch by turning off their feeding currents. This response spreads through the body of the sponge at about 0.2 cm. per second on an all-or-none basis, much like the neuronally-mediated ciliary arrests seen in the branchial sac of ascidians and in other filter-feeders.
Earlier attempts to record propagated electrical events from sponges have been unsuccessful largely because of technical problems associated with attachment or insertion of electrodes in the delicate, porous, filamentous tissues of these animals.
We have adopted a new approach. This involves dissociating sponge tissue, letting it reaggregate to form a compact mass, and grafting this back on to the original sponge. The graft fuses with the sponge, and serves as a "handle" for attachment of recording electrodes. Recordings from such preparations made from Rhabdocalyptus dawsoni show action potentials (the first to be recorded from any member of the Porifera) spreading into the graft following stimulation at remote points (S.P.Leys & G.O.Mackie, 1997, Nature 387, 29-30). The pathway for impulse spread is probably the trabecular reticulum, a syncytial tissue permeating all parts of the sponge including the flagellated chambers, where the feeding currents are generated.
Arrests of the feeding current are seen following any mechanical disturbance of the sponge, and also when excessive particulate matter is present in the ambient water. Cessation of pumping presumably helps to prevent clogging of the internal water passages.
These results show that this group of sponges is capable of responding protectively to potentially harmful environmental conditions, and that they make use of propagated electrical impulses like higher animals, despite their lack of a nervous system. It appears likely that contact-excitability and electrical impulse conduction arose in the Metazoa prior to the evolution of nerves.