Life history and multiple antipredator defenses of an invertebrate pelagicpredator, Bythotrephes longimanus

Multiple antipredator defense strategies and their interactions were examin ed in a field study of the predatory planktonic waterflea Bythotrephes long imanus in Lake Constance, at the northern fringe of the European Alps. Beca use 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 t o fish predation. Bythotrephes population dynamics were characterized by pr onounced population growth in late spring, maximum abundances in June, and a steady decline toward the end of the season. In late spring, high populat ion growth rates were achieved by means of large clutches, low investment i n individual offspring, and small size at first reproduction. While the pop ulation was still increasing, a marked life history shift occurred. The rep roductive strategy of females switched toward high per-offspring allocation at the expense of clutch size. This change in reproductive behavior result ed in an increase in the size of neonates and was accompanied by an increas e 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 im portance 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 sea sonally increasing mouth gape size of juvenile fish. In contrast to the pre dictions of the predator-avoidance theory, the migration amplitude of large and conspicuous Bythotrephes was small as compared to other zooplankton sp ecies. We argue that theory and data can be reconciled ii: the migration be havior of Bythotrephes is considered as a result of an interaction of preda tor 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.