Fig. 1. The sponge Tethya wilhelma (A) on natural substrate, recorded from above, (B) a glass plate, recorded from below, with indicated directional change, and (C) on a plastic substrate, recorded from the side. Black dots mark the sponge centre at t=0 min, white dots and corresponding outlines mark the location of the sponge body after the indicated times (min). Bars=5 mm; indicated axes correspond to Fig. 2. The complete time-lapse series are displayed in supplementary material Movies S1-S3.