ELLIOTT, Glen RD; Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada: The contractile apparatus of a juvenile freshwater sponge, Ephydatia muelleri

Within the metazoa the Porifera is the only phylum that lacks a nervous and muscular system. Nevertheless, freshwater demosponges have a coordinated behaviour that is very like a slow sneeze: in response to specific stimuli they slowly expand and then rapidly contract the aquiferous canal system. Although the cells that line the pinacoderm and aquiferous canals are thought to be contractile, there has been no direct evidence of their role in contractions until now. Using fluorescent phalloidin to label filamentous actin in juvenile sponges, I found remarkable tracts of condensed actin filaments that span the length of elongate cells. The actin filaments form dense plaques at the junction with adjacent cells, and are aligned along what appear to be tension lines towards the tips of spicules that project from the sponge’s surface. Scanning electron microscopy of relaxed and contracted sponges shows fibroblast-like cells that line the aquiferous canal system and interlace with choanocyte chambers. Cells with such condensed actin fibres have not been shown before in sponges. The location of these cells and their immense tracts of contractile filaments suggests they are responsible for the ‘sneezing’ behaviour. These results support century-old hypotheses that sponges possess a contractile and coordination system that predates the kind of muscle and nerves observed other basal metazoans.