INFLUENCE OF FOOD DENSITY ON THE EFFECTS OF A CHAOBORUS-RELEASED CHEMICAL ON DAPHNIA-AMBIGUA

1. Daphnia ambigua was reared individually at four different food levels in water conditioned by a predator, Chaoborus flavicans. Daphnia ambigua developed spike-like helmets during the first four instars in each treatment with the Chaoborus-conditioned water, but only during the first instar in untreated water. The helmet development at instars 2-4 was probably induced by a chemical released from the Chaoborus. 2. The helmets decreased in size with decreasing food levels, but never ceased to form, even at a very low food concentration. 3. Daphnids cultured in the Chaoborus-conditioned water exhibited reduced growth, reproduction and survival rates at low food levels. These results might be induced by the energy loss associated with producing the helmets. 4. It is suggested that the predator releases a chemical, which reduces the tolerance of the cyclomorphic Daphnia to food deficiency, thus accentuating the summer decline in the Daphnia population caused by food shortage.