Microbial Symbionts for Defense and Competition among Ciliate Hosts
Like multicellular organisms, ciliates are attacked by predators and also affected by harmful microorganisms. Ciliates have only limited means to escape predators; some are able to enhance their swimming speed when getting into contact with a predator or turn backward rapidly. Some ciliates are able to release nasty cell organelles called extrusomes, attacking predators with volleys of tiny arrows. The extrusomes best studied are the trichocysts of Paramecium. Trichocysts are effective in defending Paramecium against predators such as the ciliate Didinium (Harumoto and Miyake, 1990). Extrusomes of other ciliates may contain microbicidal substances (Masayo et al., 1999; Miyake et al., 2001); predators tend to avoid these ciliates. A further possible method of defense is the morphological deformation of ciliates, e.g., Euplotes may alter its shape in response to the presence of kairomones released by certain predators of this protozoon (Kusch, 1993). Most ciliates, however, lack means of defense against predators. In the ciliate Euplotidium, this de ciency is overcome by its epixenosomes, epibiotic bacteria that play a signi cant role in defending its host against predators.