Evidence For One-Way Valves in the Water-Flow System of Sponges

Abstract

A water current over a marine sponge can increase flow through the sponge ; even a current below the active pumping rate is effective. To investigate the structural correlates of such flow induction, two series of models were tested in a flow tank. These models were hollow cylinders with either wall perforations serving as ostia or an apical orifice as osculum. They were connected to an external reservoir which acted as a water source with the osculum exposed to flow, or as a sink where ostia were exposed. The reservoir could be raised or lowered to simulate active pumping. The models differed from intact sponges in showing much lower flow induction in general and no induction with ambient currents much below the active pumping rate.The magnitude of flow induction could be raised to near normal by the addition of one-way valves in the model ostia. These valves prevented backflow out of the ostia lateral and downstream to the current while permitting the positive pressure on the upstream ostia to augment flow. To be effective at low ambient currents, the valves had to be isolated from the active pump ; if the valves were located in the ‘dermis’, the ‘subdermal space’ provided the requisite isolation.Direct evidence for functional valves was obtained by cannulating freshly collected Haliclona viridis and observing that water could be more easily drawn out of an osculum than forced into it.