Predatory Copepods and Bosmina: Replacement Cycles and Further Influences of Predation Upon Prey Reproduction

Abstract

In the days of large lakes, populations of cladocerans and rotifers often appear to undergo seasonal morphological changes. In the past, these transformations were thought to reflect phenotypic plasticity within lineages (cyclomorphosis). However, in Union Bay populations of Bosmina, the transformations clearly involve regular seasonal replacements (termed "taxocene cycles") by genetically different lineages. In Union Bay, Washington, USA, selective predation by both visually foraging and blindly grasping predatrory copepods, combined with reproductive and competitive costs associated with prey defense, created a spatial and temporal environment where the seasonally fluctuating selective regimes exceed the ability of individual lineages to adapt phenotypically to changes. Genetically different clones, some derived from biologically isolated populations, rapidly replace one another in time and space. While most clones remain extant throughout the year, even times of relatively small density differences are punctuated by rapid clonal replacement. Detailed study of the clonal replacements disclosed that: (1) phenotypes generally fall into three modal classes (short—, intermediate—, and long—featured forms); (2) the short and longs are separate species; and (3) intermediates are not the results of crosses between extremes, but are heterogeneous mixtures of convergent lines derived from the other two species. While there is limited sexual activity, and certainly opportunity for mating between clones, there is no electrophoretic evidence for crossing. The distinctive nature of certain secondary sexual features seems associated with mate choice and thus aids the isolation of various lineages. In general, the frequency of sexual activity, the evolution of the brood chamber, and the transparent ephippium all appear intimately associated with predation and the selective advantage of rapid reproductive responses.