Defensive function of pigment granules in Blepharisma japonicum

The defensive function of pigment granules in the red heterotrichous ciliate Blepharisma japonicum was studied by comparing normally-pigmented red cells, albino mutant cells and light-bleached cells (a phenocopy of the albino mutant) as prey for a carnivorous ciliate Dileptus margaritifer. Albino and bleached cells were eaten more readily than red cells by dilepti. Under certain conditions, dilepti were killed by red cells, but not by albino and light-bleached cells. The cell-free fluid of red cells, but not of albino and bleached cells, was toxic to dilepti. We conclude that one of the functions of pigment granules in B. japonicum is defense against predators. The offense-defense interaction between D. margaritifer and B. japonicum is, therefore, mediated by their extrusomes, toxicysts in the former and pigment granules in the latter. Other extrusomes of unknown function might also participate in offense-defense cell-cell interaction in protists.