SYNERGISTIC, EFFECTS OF LOW-OXYGEN CONCENTRATION, PREDATOR KAIROMONE, AND A PESTICIDE ON THE CLADOCERAN DAPHNIA-PULEX

The cladoceran Daphnia pulex was exposed to low oxygen concentration, a kairomone from the predator Chaoborus, and the pesticide carbaryl; the combined effects of these factors on Daphnia life-history characteristics were analyzed. Low oxygen reduced juvenile growth rate, mature size, clutch size, neonate body size, and neonatal neckteeth development; the effects depended on size at birth (neonate size), with a stronger influence on smaller neonates. The kairomone and carbaryl had similar effects, apart from the effect on neonatal neckteeth development, which was enhanced. In all treatments, positive relationships were found between individual body size and clutch size, mother's body size and neonate size, and neonate size and neonatal neckteeth development. The most important effect of the environmental factors is probably reduction in juvenile growth rate. Reduced growth rate induced small size at maturation and thus smaller adult sizes, leading to smaller clutch sizes and producing smaller neonates, which had poorer neckteeth. Synergism was detected statistically for the effects of any pair of the three factors and of all together, indicating that one factor reduces tolerance to the other factors.