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Abstract

We studied life-history characteristics of 22 genetically distinct Daphnia hyalina X galeata clonal lineages isolated from samples taken at different depths in the Scohsee Lake (Germany) in the presence and absence of fish kairomones. substantial genetic variability was observed for all traits investigated. The presence of fish kairomones had a significant effect on all traits except relative length of the tail spine and age at maturity. All genotypes (clones) had. significantly smaller size at maturity and produced smaller offspring in the presence of fish kairomones, an observation that is consistent with the hypothesis of an adaptive response. Overall, reproductive output was less for fish-induced animals, but the effect was relatively small for the size of the first clutch and for the intrinsic rate of increase compared to the differences observed for size of the second and subsequent clutches. Adult body size was related to habitat selection behavior. Clones established from animals that were caught in the epilimnion during the day and early in the evening (termed "risk-tolerant" [RT] depth selection behavior) were found to be smaller, both in the absence and presence of fish kairomones, than clones isolated from the hypolimnion (termed "risk-averse" [RA] depth selection behavior). This relationship suggests a coadaptation between size-related life-history traits and habitat selection (depth selection) in zooplankton. There were no significant differences in the shifts in life-history characteristics between clones exhibiting RT and RA depth selection behavior in response to the presence of fish kairomones.