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.

The history of the genus Eubosmina in Lake Mondsee (Upper Austria)

“…mixta group, one can recognize morphological successions from long-featured populations to short-featured popu- lations or vice versa, which seem to be linked to environmental changes of the lakes . Our results support that changes are accomplished by 'clonal succession' (Black, 1980 ;Brock, 1980) or 'fluctuating regimes' (Kerfoot & Peterson, 1980) . Among the factors that may or may not favour one or the other Bosmina species, food supply can be expected to be a prominent one .…”

“…Food supply may control species composition in the first hand, but predation pressure is more likely to influence the shape of individuals of a population . Since Dodson (1974) and others postulated size selection by predators as an important steering factor of population morphology, it has been shown by numerous authors that the shape of bosminids is squeezed between fish predation, which points at large specimens more than at small ones (Zaret & Kerfoot, 1975 ;Stenson, 10 1 1976 ;Johnson & Raddum, 1987 ;Sandlund et al ., 1987), and invertebrate predation, which strikes harder on small specimens than on big ones (Kerfoot, 1977 ;O'Brien, 1979 ;Kerfoot & Peterson, 1980 ;Wong, 1981 ;Johnson & Raddum, 1987), but that the latter also provokes defense mechanisms such as formation of long spines, etc . (Sprules et al ., 1984 ;Kerfoot, 1987) .…”

Predation of freshwater jellyfish on Bosmina: the consequences for population dynamics, body size, and morphology

“…In contrast, large copepods, like Epischura sp. and Cyclops sp., are already known to cause life-history and morphological changes in small cladoceran species (Kerfoot, 1980(Kerfoot, , 1987. Life-history plasticity of prey organisms can strongly influence the outcome of food web interactions (Chase, 1999).…”

Response of zooplankton communities to liquid creosote in freshwater microcosms

“…This trend is somewhat surprising, because many adult copepods, including those collected in the current study, are predatory and may have been expected to benefit from the increased food supply. Indeed, Bosmina has been shown to be a primary food source for copepods, including members of the genus Cyclops [53]. Environ.…”