The effect of predator and prey density on the induced defence of a ciliate

1. The level of antipredator defence should be proportional to the actual attack probability to minimize the cost of defence and maximize the net benefit. 2. The hypothesis that the induced antipredator morphology of Euplotes octocarinatus is a graded response to the actual risk of predation by Stylonychia mytilus was tested by manipulating the density of both prey and predator populations. 3. The magnitude of the response was graded according to both predator and prey density. A dense prey population may be protective since a prey is more exposed to a predator's attack as a solitary individual. 4. The results suggest that Euplotes is able to 'estimate' the real risk of predation and respond appropriately, without mobilizing more resources than needed. 5. Separation of the prey and predator with a nylon net revealed that the response was not induced by a water-transmitted factor but that direct cell-to-cell contacts were important. This finding departs from those of other studies.