Kuhlmann, H-W, Bräucker R, Schepers, AG (1997) Phototaxis in Porpostoma notatum, a marine scutociliate with a composed crystalline organelle 

The histophagous scuticociliate Porpostoma notatum passes through five developmental stages during its life cycle: theronts, trophonts, protomonts, tomonts and tomites. The stages are characterized by both, a different morphology and behaviour. When cells are exposed to radiant white light of 10 klx, negative phototaxis occurs in trophonts, which represent the feeding stage of the reproduction cycle, as well as in protomonts and young tomonts, the two successive stages after food uptake. Tomites, resulting from one or two divisions of a tomont, show no phototactic orientation until they have differentiated into theronts. Theronts may show either a positive or negative phototactic response. Individuals of all life cycle stages, except for young tomites, commonly bear a well-developed cup-like organelle, which faces the oral cavity with its concave side. This organelle is composed of alternating layers of cytoplasm and crystals. By this architecture, the organelle resembles a multilayer interference reflector, a structure which has been described in multicellular organisms several times. A possible function of the crystalline organelle in photo-orientation of P. notatum is discussed. Key words: Porpostoma notatum; Life cycle; Orientation; Phototaxis; Crystalline organelle.