The predatory cladocerans, Leptodora kindtii (Focke, 1844) and Bythotrephes longimanus (Leydig, 1860), express markedly different life-history traits. Leptodara 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 large-bodied 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.
Bibliographical noteFunding Information:
I thank M. Brown and two anonymous reviewers for their constructive comments on the manuscript. Ideas advanced here benefited from discussions with J. T. Lehman and students in my Plankton Biology classes at the University of Minnesota Duluth. M. Brown assisted in the collection and maintenance of Bythotrephes. This work is the result of research sponsored by the Minnesota Sea Grant College Program supported by the NOAA Office of Sea Grant, United States Department of Commerce, under grant No. NOAA-NA16-RG1046. The US Government is authorized to reproduce and distribute reprints for government purposes, not withstanding any copyright notation that may appear hereon. This article is journal reprint No. JR 510 of the Minnesota Sea Grant College Program. This research was also financially supported by the University of Minnesota Duluth and through a Grant-in-Aid award from the University of Minnesota to D. K. Branstrator.