Contrasting life histories of the predatory cladocerans Leptodora kindtii and Bythotrephes longimanus

The predatory cladocerans, Leptodora kindtii (Focke, 1844) and Bythotrephes longimanus (Leydig, 1860), express markedly different life-history traits. Leptodora produce small-bodied neonates that mature at small sizes but continue to grow throughout life. Bythotrephes produce larger neonates in both relative and absolute terms that grow rapidly to a large size at maturity whereupon they curtail somatic growth and divert resources mainly to reproduction. Despite their remarkable differences, the sets of life-history traits of both species appear to be solutions to the same basic selection pressures imposed by visually discriminating gape-limited fishes and foraging constraints imposed by prey size. Leptodora stresses pre-contact (transparency) while Bythotrephes stresses postcontact (caudal spine) modes of morphological defense against fishes. Mounting these disparate modes of defense has consequences for selection on timing and allocation to body growth that may underlie competitive imbalance between the species. Owing to the production of largebodied neonates that grow rapidly, Bythotrephes quickly attain body sizes that both admit them to a broader prey base in size and taxonomic variety, and allow shorter prey handling times, in comparison to Leptodora. This provides Bythotrephes with a wider and more exploitable prey base from an earlier age and may explain why Leptodora has declined in density following Bythotrephes invasion into some North American lakes. The divergent sets of life-history traits expressed by Leptodora and Bythotrephes parallel two dominant life-history strategies evolved by phytoplanktivorous species of the order Cladocera.