The Role of Trichocyst Discharge and Backward Swimming in Escaping Behavior of Paramecium from Dileptus margaritifer1

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Abstract
Trichocyst discharge is an effective defense of Paramecium against Dileptus margaritifer. The possible defensive function of backward swimming, which often follows trichocyst discharge upon Paramecium‐Dileptus encounters was studied. Mutants incapable of backward swimming (pawnA in P. tetraurelia, cnrA in P. caudatum) escaped from dilepti nearly as frequently as wild‐type cells. Double mutants (pawnA‐nd7, cnrA‐tnd2) were eaten nearly as frequently as mutants incapable of trichocyst discharge. Thus, in the defense of Paramecium against D. margaritifer, the role of backward swimming is minor, if any, compared to trichocyst discharge. Among escaped cells, about a half of wild‐type and essentially none of pawnA (cnrA) cells showed backward swimming. Paramecium behavior during the encounter can be mimicked by the local, not global, application of lysozyme which is a strong secretagogue of trichocyst.