Vesiculation of food vacuoles and segregation of ingesta: Peculiarities of the digestive cycle in the raptorial ciliate Homalozoon vermiculare

摘要

The carnivorous gymnostome ciliate Homalozoon vermiculare feeds on other ciliates, e.g, Paramecium and Colpidium. The gulper ingests the complete prey organism within one minute and encloses the food in a single, relatively large food vacuole. During the next 20 min, this big food vacuole becomes vesiculated into numerous small vacuoles of 15-20 mu m in diameter, after which a condensation of their contents takes place. The different food components are separated more or less simultaneously. Finally, about one hour after phagocytosis, the different food vacuoles contain almost exclusively, e.g., mitochondria or extrusomes (from predator-discharged toxicysts and/or from prey discharged trichocysts) or mainly food vacuoles of the prey filled with bacteria. At this time the contents are greatly condensed and degradation of the food starts with the appearance of dictyosomes. After 4-24 h the digestion is completed and numerous vacuoles with indigestible prey remnants are found in the caudal region of the predator, where they are defecated. This type of digestive cycle - vesiculation of the food vacuole, segregation of their contents, differential degradation of the separated ingesta - is in contrast to the digestive cycle known for filter feeders like Paramecium, in which the different food particles are not segregated and not differentially digested. Food uptake and subsequent food vacuole vesiculation is inhibited by cytochalasin B but not by colchicine, indicating that microfilaments such as actin are involved in phagocytosis and digestion. Polymerization of microtubules does not play a crucial role in food uptake and digestion.