It is well known that the cladoceran Daphnia changes its morphology and life history parameters in response to infochemicals released from predators to maintain the population in the presence of predators. This daphnid response is called ‘inducible defense’. Lots of studies have been conducted about the relationship between Daphnia and planktivorous fish via infochemicals. However, they are based on individual level experiments but not on population level ones. To understand actual dynamics of Daphnia population in the field with abundant fish, it is necessary to make clear how the Daphnia population respond to the fish infochemical. We conducted laboratory experiments to reveal the response and mechanism of Daphnia population against the fish infochemical.

When exposed to the fish infochemical, Daphnia population did not increase after the first peak density. Moreover, the mortality of Daphnia tended to increase when the animals were exposed to both the fish infochemical and low food level. The development of filter screen area, which is an important feeding apparatus, was interfered by the fish infochemical.

Our results show that Daphnia population negatively affected by the fish infochemical. These results suggest that the fish infochemical decrease the tolerance of starvation of Daphnia.