Low-density anti-predation refuge in Daphnia and Chaoborus?

We used a flow-through experimental system ("plankton organ") to test if Daphnia hyalina x galeata (Dhg), D. cucullata (Dc) and Chaoborus flavicans (Ch) take higher risks of predation at low population density levels than at high ones, i. e. to test whether or not they select shallower depths with higher temperatures and higher food levels when they do not receive chemical information on crowding (added to the medium with fish kairomones). Neonates of Daphnia and Chaoborus 4 th instar larvae were first grown for 55 h in batch cultures: Daphnia were reared at high food levels (Scenedesmus obliquus) and at two different population densities of 1 and 100 ind. L -1 (Dhg and Dc); Chaoborus were kept at densities of 1 and 10 ind. L -1 without food. After 55 h, individual Daphnia 2 nd instar juveniles were transferred into the narrow vertical tubes of a "plankton organ" with constant flow of fresh medium. The medium contained chemical information on either high or low population density; all media had the same food and fish kairomone levels. The same setup was used for individual Chaoborus 4 th instar larvae, but using media without food. Whereas no difference was found in Dc, different depth selection behavior was apparent in Dhg. The daytime depth selected by the animals was greater in the presence of chemical information on high population density, and this behavior became more pronounced with increasing duration of the experiment. The difference was highly significant for the entire dataset. The calculated duration time of egg development was up to 17% longer and the calculated birth rate 12% reduced for Daphnia receiving high-density information, as compared to control animals enjoying higher temperatures of the shallower and warmer strata. Only minor and inconsistent difference was found for Chaoborus.