The contraction-expansion behaviour in the demosponge Tethya wilhelma is diurnal and light-controlled

Sponges (phylum Porifera) are metazoans without muscles and nervous system. Still, they perform coordinated behaviours, such as whole body contrations. Previous studies have indicated diurnal variability in number of contractions, and in expression of circadian clock genes. Here we show that diurnal patterns are present in the contraction-expansion behaviour of the demosponge Tethya wilhelma using infrared videography and a simulated night/day-cycle including sunset and sunrise mimic. In addition, we show that this behaviour is at least strongly influenced by the ambient light intensity and therefore implicates light-sensing capabilities in this sponge species. This is backed by our finding that T. wilhelma consistently contracts at sunrise, and that this pattern disappears both when the sponge is kept in constant darkness and when in constant light.