Epixenosomes: Peculiar epibionts of the hypotrich ciliate Euplotidium itoidefend their host against predators

Euplotidium itoi harbors on its dorsal surface peculiar episymbionts (refer red to as epixenosomes) equipped with a complex extrusive apparatus. In the laboratory, E. itoi stocks without epixenosomes behave and reproduce like symbiotized stocks. The hypothesis that epixenosomes play a defensive role against predators was tested by comparing the behavior of Litonotus lamella when preying upon Euplotes crassus, E. itoi without epixenosomes, and E. i toi with epixenosomes. Litonotus discharges its toxicysts upon direct-cell- to cell contact, and paralyzes the three types of prey with the same effici ency. Nevertheless, Litonotus can ingest Euplotes Euplotidium without epixe nosomes, and to a certain extent, Euplotidium with epixenosomes whose eject ing capability has been inhibited, while it never eats Euplotidium with una ltered epixenosomes. In each prey-type, about 60% of the individuals attack ed by Litonotus toxicyst discharge are able to recover their normal behavio r once transferred into pure sea water. This percentage for E. itoi with ep ixenosomes that are never eaten by the predator corresponds to the probabil ity of survival. This probability is lower for the other two prey-types in which the prey engulfed by the predator do not have the chance to recover. These data support the hypothesis and suggest the involvement of the epixen osome's ejecting apparatus in a defensive function.