Multiple antipredator defense strategies and their interactions were examined in a field study of the predatory planktonic waterflea Bythotrephes longimanus in Lake Constance, at the northern fringe of the European Alps. Because of its large body size and conspicuousness, Bythotrephes is a preferred prey of freshwater fish. We observed seasonal changes in life history and morphology and diel vertical migration, all best understood as a response to fish predation. Bythotrephes population dynamics were characterized by pronounced population growth in late spring, maximum abundances in June, and a steady decline toward the end of the season. In late spring, high population growth rates were achieved by means of large clutches, low investment in individual offspring, and small size at first reproduction. While the population was still increasing, a marked life history shift occurred. The reproductive strategy of females switched toward high per‐offspring allocation at the expense of clutch size. This change in reproductive behavior resulted in an increase in the size of neonates and was accompanied by an increase in the size at first reproduction. Such a life history shift is typically observed in the presence of gape‐limited predators, which points to the importance of juvenile fish as the principal vertebrate planktivores in Lake Constance. The length of Bythotrephes defensive spina increased throughout the season, reflecting increasing predation pressure, probably owing to seasonally increasing mouth gape size of juvenile fish. In contrast to the predictions of the predator‐avoidance theory, the migration amplitude of large and conspicuous Bythotrephes was small as compared to other zooplankton species. We argue that theory and data can be reconciled if the migration behavior of Bythotrephes is considered as a result of an interaction of predator defenses in the presence of predominantly juvenile, gape‐limited fish. As Bythotrephes achieved protection because of its life history and spina, the costs of large diel migrations may outweigh the benefits.

Abstract. Multiple antipredator defense strategies and their interactions were examined in a field study of the predatory planktonic waterflea Bythotrephes longimanus in Lake Constance, at the northern fringe of the European Alps. Because of its large body size and conspicuousness, Bythotrephes is a preferred prey of freshwater fish. We observed seasonal changes in life history and morphology and diel vertical migration, all best understood as a response to fish predation. Bythotrephes population dynamics were characterized by pronounced population growth in late spring, maximum abundances in June, and a steady decline toward the end of the season. In late spring, high population growth rates were achieved by means of large clutches, low investment in individual offspring, and small size at first reproduction. While the population was still increasing, a marked life history shift occurred. The reproductive strategy of females switched toward high per-offspring allocation at the expense of clutch size. This change in reproductive behavior resulted in an increase in the size of neonates and was accompanied by an increase in the size at first reproduction. Such a life history shift is typically observed in the presence of gape-limited predators, which points to the importance of juvenile fish as the principal vertebrate planktivores in Lake Constance. The length of Bythotrephes defensive spina increased throughout the season, reflecting increasing predation pressure, probably owing to seasonally increasing mouth gape size of juvenile fish. In contrast to the predictions of the predator-avoidance theory, the migration amplitude of large and conspicuous Bythotrephes was small as compared to other zooplankton species. We argue that theory and data can be reconciled if the migration behavior of Bythotrephes is considered as a result of an interaction of predator defenses in the presence of predominantly juvenile, gape-limited fish. As Bythotrephes achieved protection because of its life history and spina, the costs of large diel migrations may outweigh the benefits.