PHOTOTAXIS IN PORPOSTOMA-NOTATUM, A MARINE SCUTICOCILIATE WITH A COMPOSED CRYSTALLINE ORGANELLE

The histophagous scuticociliate Porpostoma notatum passes through five developmental stages during its life cycle: theronts, trophonts, prot omonts, tomonts and tomites. The stages are characterized by both, a d ifferent morphology and behaviour. When cells are exposed to radiant w hite light of 10 klx, negative phototaxis occurs in trophonts, which r epresent the feeding stage of the reproduction cycle, as well as in pr otomonts and young tomonts, the two successive stages after food uptak e. Tomites, resulting from one or two divisions of a tomont, show no p hototactic orientation until they have differentiated into theronts. T heronts may show either a positive or negative phototactic response. I ndividuals of all life cycle stages, except for young tomites, commonl y bear a well-developed cup-like organelle, which faces the oral cavit y with its concave side. This organelle is composed of alternating lay ers of cytoplasm and crystals. By this architecture, the organelle res embles a multilayer interference reflector, a structure which has been described in multicellular organisms several times. A possible functi on of the crystalline organelle in photo-orientation of P. notatum is discussed.