Body size and neck spine development in Daphnia greatly influence this animal's vulnerability to predation by the size-selective invertebrate planktivore Chaoborus. We develop a stage-classified matrix population model for Daphnia that investigates the interaction and evolution of these two traits in situations (fishless lakes and ponds) where Chaoborus predation constitutes the major source of mortality. This model produces fitness landscapes for these traits in ten distinct Daphnia

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... s that are characterized by Chaoborus size (medium-sized Chaoborus americanus or large Chaoborus trivittatus), Chaoborus density (0-1.0 L -1 ) and food level (high or low). Larger Daphnia phenotypes are favored in both high and low food environments that contain C. americanus, and also in a high food situation with C. trivittatus. The environments with C. trivittatus and low food availability, however, select for very small, as well as very large, Daphnia phenotypes (small phenotypes are favored more at high Chaoborus densities), but not those that are intermediate in size. The development of neck spines is advantageous in all situations with Chaoborus, but high food environments that contain C. americanus favor their elimination following juvenile development, while the other model environments favor their retention (to various degrees) after maturity. These model predictions describe alternative antipredator strategies, two of which correspond closely with phenotypic patterns exhibited by two species of Daphnia (Daphnia pulex and Daphnia minnehaha) that commonly coexist with Chaoborus in fishless lakes and ponds.