Epixenosomes, peculiar Epibionts of the hypotrich Ciliate Euplotidium itoi, defend their host against predators

Euplotidium itoi harbors on its dorsal surface peculiar episymbionts (referred 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 Lironotus lamella when preying upon Euplotes crassus, E. itoi without epixenosomes, and E. itoi with epixenosomes. Litonotus discharges its toxicysts upon direct-cell-to cell contact, and paralyzes the three types of prey with the same efficiency. Nevertheless, Litonotus can ingest Euplotes, Euplotidium without epixenosomes, and to a certain extent, Euplotidium with epixenosomes whose ejecting capability has been inhibited, while it never eats Euplotidium with unaltered epixenosomes. In each prey-type, about 60% of the individuals attacked by Litonotus toxicyst discharge are able to recover their normal behavior once transferred into pure sea water. This percentage for E. itoi with epixenosomes that are never eaten by the predator corresponds to the probability 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 epixenosome’s ejecting apparatus in a defensive function.