Many demosponges can propagate contractions across the whole body in response to mechanical stimuli (perhaps sediment, or disturbance by waves or fishes). The stereotypical manner in which these contractions occur implies that cells must communicate both to trigger the start of the contraction and to propagate contractions. The image on the left shows a small (5mm diameter) freshwater sponge hatched from the round gemmule (overwintering cyst). This model is wonderful for studying the complex behaviours of coordination in sponges because of its small size and transparency. Glen Elliott (PhD 2009) cultured thousands of these and filmed their response to various stimuli. He found that a rapid shake, or inedible particles (ink) added to the water trigger a sequence of events that makes the sponge inflate its canals and then contract them so as to expell all the water vigorously through the chimney-like osculum.
To learn more about this see: Elliott & Leys J.Exp.Biol 2007

He has now found that a precise concentration of Glutamate can also trigger this behaviour, and that blockers of metabotropic glutamate receptors block the action of Glutamate. GABA and Nitric Oxide are also involved. Elliott and Leys in review

Glass sponges are so far the only sponges known to be able to conduct electrical signals, i.e by propagating an action potential. They can do this becasue their tissues are syncytial (there are no membrane boundaries to prevent this). In cellular sponges (Demosponges, Calcarea and Homoscleromorphs) we are not aware of any junctions (e.g. gap junctions) that would allow electrical current to pass, and there is so far no evidence of any behaviour rapid enough to suggest that electrical signalling occurs. That said... with more work, such a system may be found.

It is very difficult to record from the tissues because they're so thin (~1-2um), so we have been using a molecular approach. Gabrielle Tompkins-MacDonald was able to isolate potassium channels from a sponge, and express them in frog oocytes to record their ability to control current passage into and out of the cell. The first channels analyzed are inward rectifier K channels. Our analysis suggests sponge Kir channels pre-date the diversification of K channel families. Recordings show they are strong inward rectifiers which suggest a potential ability to recover membrane potential after depolarization. To read more see: Tompkins-MacDonald et al. JExpBiol 2009