Predator‐induced morphological changes in Euplotes (Ciliata): Isolation of the inducing substance released from Stenostomum sphagnetorum (turbellaria)

The turbellarian Stenostomum sphagnetorum releases a morphogenetically active substance (S‐factor) into the surrounding medium. Ciliates of the genus Euplotes, which are potential prey of Stenostomum, are able to recognize the S‐factor. In response to this factor, each Euplotes cell becomes transformed into an enlarged circular morph with extended lateral wings that make its engulfment by the predator more difficult. The S‐factor was purified to chromatographic and electrophoretic homogeneity. From the observation that its biological activity can be eliminated by treatments with proteases and from the absorption spectrum of the purified S‐factor, it appears that the S‐factor is a protein. Upon heating at 95°C for 10 min, the S‐factor lost 50% of its biological activity, and after 30 min at 95°C the activity had disappeared completely. Electrophoretic analysis revealed that the S‐factor has a molecular mass of 17.5 kDa. It is active at concentrations as low as 10–11 M. The factor from Stenostomum is not identical to the Lembadion factor, which is released from the predatory ciliate Lembadion bullinum and has a morphogenetic effect on Euplotes.