Effects of predator‐induced spines on individual fitness in Daphnia pulex

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Abstract
We studied both the cues for spine induction and costs of spine production in Daphnia pulex (Cladocera). A significant number of D. pulex cultured individually in water treated with Chaoborus crystallinus larvae that were fed carried spines to the end of the fourth instar. Water treated with Chaoborus that were starved induced only minor morphological changes in D. pulex. Treatments with crushed D. pulex did not affect neck spine induction.Spine production carries a cost evident in the timing of first reproduction of D. pulex. Over half (58%) of D. pulex grown in culture conditions with a high density of Chaoborus postponed first reproduction from the fifth to the sixth instar. A decrease in growth of the spined morphs was attributable to an increase in duration of instars. Fitness effects persisted through the third reproductive instar. The intrinsic rate of increase was lowest in D. pulex populations cultured in Chaoborus‐treated water. Treatments with crushed D. pulex did not affect growth but did significantly increase clutch size of D. pulex.