Complex effects of a kairomone of Chaoborus and an insecticide on Daphnia pulex

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Abstract
Daphnia pulex were reared in Chaoborus-conditioned water containing the insecticide carbaryl, and their life history parameters and morphologies were investigated. The insecticide inhibited the animals' growth and reproduction and delayed their maturation time more intensely in the chaoborus-conditioned water than in the control Chaoborus-free water, indicating that a kairomone of Chaoborus made the Daphnia more sensitive to the insecticide. The Chaoborus conditioned water induced neckteeth formation of D.pulex in instars 1–2 and elongated the intermoulting period of juveniles. The moulting to the spined morphs and elongation in duration of juvenile stages seemed to increase the risk of damage from the insecticide. The potential population growth rate of D.pulex in treatments was estimated as a possible fitness indicator of the animals. It was reduced synergistically by the kairomone of Chaoborus and the insecticide. Some individuals kept neckteeth until the third or fourth instar stage when they were exposed to sublethal concentrations of the insecticide in the Chaoborus-conditioned water. This was considered as a result of synergistic effects of both the kairomone and the insecticide. Insecticides may be a factor inducing further development of protuberant structures in cyclomorphic Daphnia in natural water bodies.

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