Active Use of Ambient Flow by a Deep-Sea Glass Sponge

Abstract

How flow moves through porous structures like sponges has long intrigued physical and biological scientists. Despite sponges having specialized cells that function as biological pumps, their porous bodies are proposed to passively take advantage of ambient currents via ‘inducedflow’. This hypothesis relies on the fact that flow external to perforated or tube-like structures drives flow through the structure, but much of the support for this comes from work on dead specimens. A modern understanding of sponge morphology and physiology however, shows that sponges possess a sophisticated sensory system, even in their canals. We used custom flow and oxygen sensors at a 175m deep sponge reef to test the hypothesis of current-induced passive flow through living glass sponges. Evidence to support a passive flow hypothesis was only found in one of six individuals that filtered more water during periods of higher ambient current. As expected, all individuals stopped pumping independently of ambient currents, illustrating their control over pumping using the well-described electrical conduction system. However, at higher ambient currents, sponges removed 30% less oxygen, suggesting a mechanism by which the sponge senses the ambient flow rates and reduces the metabolic expenditure of filtration. The underlying mechanism by which this happens remains unknown, but it may involve a feedback loop through the canals, potentially via primary cilia that have been shown to sense flow in other sponges. Our experiments reveal that while sponges can take advantage of ambient flow, water movement through these animals is controlled by their complex physiology.