Predator-mediated life history shifts in Daphnia:: enrichment and preliminary chemical characterisation of a kairomone exuded by fish

Daphnia responds to chemical cues released by fish with adaptive changes in life history traits, e. g. a reduction in size at first reproduction (SFR). We used effects on SFR of a clone of Daphnia magna to characterise the chemical nature of the kairomone released by Leuciscus idus. The kairomone was of low lipophilicity and was enriched from incubation water by C-18-solid phase extraction. The chemical cue was an anion at neutral pH and was reversibly inactivated by acetylation. Ester bonds, olefinic bonds or glucuronic acid can be excluded as essential for the biological activity. The effects on SFR increased with increasing concentrations of enriched kairomone. HPLC yielded only two adjacent active fractions that differed in retention times from those obtained for the diel vertical migration inducing kairomones released by other cyprinid fish species.