Feeding Behavior in the Ciliate Pseudomicrothorax dubius is a Series of Morphologically and Physiologically Distinct Events 1

Based upon light and electron microscopical observations, the feeding behavior of the ciliate Pseudomicrothorax dubius , when fed the cyanobacterium Oscilatoria formosa , is resolved into two principal phases, contact swimming and phagocytosis, the latter being separable into two steps, attachment aad ingestion. Following collision with an O. formosa filament. cells swim along the filament with their ventral cilia in contact with it during the contact swimming phase. Phagocytosis commences with the attachment of the cytostome to the filament, which initiates lysosomal streaming in the cytostomal‐cytopharyngeal region. The filament then enters the cytopharynx concomitant with food vacuole formation during the ingestion step. Treatment of cells with trypsin or modification of the extracellular ionic medium inhibits the attachment step of phagocytosis but does not affect contact swimming. Behavior of cells when fed different cyanobacterial species as well as artificial food substrates is also examined. Contact swimming is a form of contact guidance since the shape of the food substrate determines the direction of cell movement. Additionally, a chemical factor may be present in or on the cyanobacteria and play a role in contact swimming. Evidence is presented that suggests that during the attachment step, two phenomena are involved: direct adhesion between cell surfaces and adhesion due to material liberated by exocytosis.