Sponges (phylum Porifera) are sessile filter feeders that lack both nervous and muscular systems. Despite the absence of conventional mechanisms of coordination, sponges carry out a range of behaviours that are both innate and in response to disturbances.

How sponges sense signals and co-ordinate contractions remains little understood and has bearing on the evolution of nervous systems. In glass sponges (Hexactinellida), syncytial tissues propagate electrical currents to arrest the feeding current. Glass sponges are a deep sea group, and their syncytial tissues and coordination system are a particular adaptation to the food poor habitat.

Electrical signaling is unknown in cellular sponges (Demospongiae, Calcarea, and Homoscleromorpha) and yet they are active animals that carry out range of whole-body contractions. Our previous work has shown that short cilia, strategically located in the excurrent canals, may sense changes in flow through the animal. The neuroactive chemical L-glutamate triggers stereotypical contractions, which via NO signaling activates a smooth muscle-like pathway.

We have been asking how signals propagate through the sponge and hypothesize that signaling occurs through a paracrine manner much as in glia. In other animals purinergic (>ATP) signaling is essential for the function of various tissues, and is the most prominent form of communication between glia and neurons. ATP often works together with glutamate in signaling. Using a pharmacological approach, we found that sponges respond to exogenous ATP in a concentration-dependent manner, and that ATP signaling occurs downstream of glutamate signaling.

Our results suggest that changes in flow through the sponge are sensed by hair cells, which trigger a cascade of downstream effects including, but not limited to, Ca²⁺ signaling, glutamate release and subsequent auto- and paracrine functions, and more ATP release. Our finding that glutamate and ATP signaling work in concert to coordinate contractions of the sponge indicate a role for ATP-mediated signaling that predates the evolution of the nervous system in animals.