Freshwater Biology (1997) 38, 353–364 Phenotypic plasticity of Daphnia life history traits: the roles of predation, food level and toxic cyanobacteria M. WALLS,† C. LAURE ´ N-MA ¨ A ¨ TTA ¨ ,*‡ M. KETOLA,* P. OHRA-AHO,*§ M. REINIKAINEN¶ANDS. REPKA*# *Laboratory of Ecology and Animal Systematics, Department of Biology, University of Turku, FIN-20014 Turku, Finland †Department of Biology, University of Turku, Turku, FIN-20014, Finland ‡Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berks SL5 7PY, U.K. §Plant Production Inspection Centre, Pesticide Division, PO Box 42, FIN-00501 Helsinki, Finland ¶Department of Biology, Åbo Akademi University, BioCity, FIN-20520 Turku, Finland #Netherlands Institute of Ecology, Centre for Limnology, Rijksstraatweg 6, 3631 AC Nieuwersluis, The Netherlands Address for correspondence: Dr Camilla Laure ´n-Ma ¨a ¨tta ¨, Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berks SL5 7PY, U.K. SUMMARY 1. We studied the life history responses of Daphnia pulex under different biotic conditions. In a factorially designed experiment, we tested the impacts of water conditioned by the invertebrate predator Chaoborus, low and high food level (10 000 and 100 000 Scenedesmus cells ml –1 ), and exposure to toxic Microcystis (5000 cells ml –1 ) on twelve D. pulex clones originating from different habitats. Our aim was to compare the phenotypic plasticity of different clones, and to study the interactions among biotic factors. 2. Individuals cultured in Chaoborus-conditioned water started to reproduce at a larger size than individuals cultured in water not conditioned by the predators. We found interactions between food level and Chaoborus-conditioned water on age at first reproduction and total offspring number. In addition, the impact of the Chaoborus treatment on the size at first reproduction was reduced by Microcystis exposure. 3. Clonal differences were found in the degree of phenotypic plasticity of different life history traits. However, there was no obvious connection between the original habitat of the clone and the degree of plasticity. Introduction Organisms must tolerate a range of environmental conditions in order to persist in a specific habitat. Several factors (predation pressure, food quality and quantity, competition, temperature, pH and nutrient level) may change during the growing season and genotypes differ in how well they are adapted to fluctuations in environmental conditions (Vanni, 1987; Pijanowska, Weider & Lampert, 1993). Phenotypic plasticity is a major mechanism contributing to popula- tion persistence in temporally fluctuating environ- ments (Bradshaw, 1965; Caswell, 1983; Stearns, 1989; © 1997 Blackwell Science Ltd 353 Lampert, 1993; Yampolsky & Scheiner, 1994). Selection on different mean trait values in different environ- ments seems to be a sufficient prerequisite to create phenotypic plasticity (Via & Lande, 1985; Via, 1993). The evolution of phenotypic plasticity requires condi- tions favouring a generalist strategy (Thompson, 1991), such as may be useful in both temporary and perman- ent Daphnia ponds, where abiotic and biotic conditions often fluctuate. Clonally reproducing organisms, such as Daphnia pulex (de Geer) are especially suited for studying phenotypic plasticity. By using clonal organ-