Limnol. Oceanogr., 44(5), 1999, 1248–1258

We studied life-history characteristics of 22 genetically distinct Daphnia hyalina 3 galeata clonal lineages isolated from samples taken at different depths in the Schöhsee 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 a 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 lifehistory 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. Habitat selection has been identified as a potentially important force in maintaining genetic variation in natural populations (Jaenike and Grimaldi 1983; Rausher 1984; Hedrick 1986; Jaenike and Holt 1991). Zooplankton, particularly the cladoceran microcrustacean, Daphnia, have proven to be very useful model organisms in the study of the relationship between habitat selection and ecologically important traits. A number of studies have reported interor intrapopulational genetic variation in a multitude of ecologically relevant traits in Daphnia (reviewed by Carvalho 1994; De Meester 1996b). Several recent studies have provided important contributions to our knowledge of genetic variation in life-history traits in zooplankton (e.g., Ebert et al. 1993a,b; Spaak and Hoekstra 1995). Furthermore, habitat selection in zooplankton can be realized along temporal (King 1972, 1980) 1 Present address: The University of Oklahoma Biological Station, HC 71, Box 205, Kingston, Oklahoma 73439.