EFFECTS OF A CHAOBORUS-RELEASED CHEMICAL ON DAPHNIA-AMBIGUA - REDUCTION IN THE TOLERANCE OF THE DAPHNIA TO SUMMER WATER TEMPERATURE

Daphnia ambigua was reared at eight different temperatures (5-degrees, 10-degrees, 15-degrees, 20-degrees, 23-degrees, 25-degrees, 28-degrees, and 30-degrees-C) in water conditioned by a predator, Chaoborus flavicans. D. ambigua developed spikelike helmets in juvenile stages probably in response to a chemical released from the Chaoborus larvae. Helmet size was affected by water temperature (max size at 23-degrees-C) and decreased with decreasing temperature from 23-degrees to 10-degrees-C. At high temperatures (28-degrees and 30-degrees-C), survival rate of juveniles exposed to the Chaoborus-released chemical decreased greatly. Molting to the high-helmeted morph might increase the risk of mortality at those temperatures. Results suggest that the predator releases a chemical that reduces the tolerance of cyclomorphic Daphnia to high temperature, thus inducing the summer decline in the Daphnia population.