Diet-dependent induction of life-history changes in Daphnia: 5α-cyprinol sulfate is not the kairomone

Abstract Daphnia responds to chemical cues released by fish with adaptive changes in behavior (DVM), morphology and life-history traits, e.g., a reduction in size at first reproduction (SFR). For cyprinid fish it has been revealed that DVM and changes in morphology of Daphnia are induced by the bile salt 5α-cyprinolsulfate (CPS), while the substance that induces life-history changes is unknown. Here we show that the unknown kairomone from fish incubation water of roach ( Rutilus rutilus ) that have been fed Daphnia (RfD) can be entirely extracted using lipophilic C 18 -solid phases. Such extracts induce a reduction of SFR in D. magna while similar extracts with chironomid larvae as fish prey do not. Subsequent fractionation of RfD by HPLC revealed that the major bile salts of cyprinid fish, CPS and taurochenodeoxycholic acid, eluted outside the active fractions, while taurocholic acid (TCA) had its peak within the active fractions. A dose–response experiment revealed that up to 2 µM TCA did not reduce SFR, which excludes TCA as potential kairomone. In conclusion, none of the major cyprinid bile salts in fish incubation water accounts for the observed reduction in SFR, implicating an as yet unidentified, negatively charged kairomone.