Clonal previously reared on either (normal and apochlorotic zoochlorellae-containing stocks), , or were presented with either 15 cells each of normal or apochlorotic or 30 cells of either stock in 1 ml depression slides. Prey-choice, search period and handling (=ingestion) times were recorded. Didinia chose apochlorotic prey twice as often as normal paramecia. They located and attacked apochlorotic cells more rapidly than normal prey, and they ingested ‘bleached’ paramecia more quickly than normal cells. There was a consistent pattern of relatively increased search and handling times for primary control didinia reared on paramecia other than , compared with the experimental predators. The frequency distributions of both search and handling times were normal. Predatory efficiency of didinia attacking apochlorotic paramecia was significantly greater (> 10 times) than that exhibited for normal cells as measured by the escape frequency of prey. Similarly, the greatest loss of zoochlorellae by regurgitation or premature defecation occurred after normal prey were ingested. These results support the hypothesis that the mutualistic zoochlorellae within tend to discourage predation by by releasing distasteful metabolites that repel them. The reciprocal nature of this ciliatezoochlorella symbiosis includes a protective function for the latter partner.


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