Correlative field studies have shown that the food quality of natural seston for Daphnia is highly correlated with the content of eicosapentaenoic acid (EPA), indicating that the growth of Daphnia in nature is at least seasonally limited by a low availability of this polyunsaturated fatty acid. Here we test the hypothesis that a putative limitation due to limited availability of EPA in natural seston should lead to genetic adaptation in physiological traits reflected in genotype-dependent differences in the ability of sympatric clones of Daphnia to cope with these shortages. We further test if this adaptation to the low availability of EPA provides another explanation for the coexistence of hybrids with their parental taxa. Standardized growth experiments were performed with and without dietary EPA using clones of three taxa from the Daphnia longispina complex: Daphnia galeata, Daphnia hyalina, and their interspecific hybrids. Fitness was estimated from juvenile somatic growth rates, g, and from intrinsic population growth rates, r. There was marked interclonal variability in the susceptibilities of g and r to the absence of EPA from the food, with considerable intraspecific variability. At the taxon level, we noted differences in susceptibility to EPA limitation with regard to the intrinsic rate of population increase r, which demonstrates that the availability of EPA affects the relative fitness of hybrids, but not of the parental species. On average, hybrids seemed less susceptible to the absence of EPA than do parental species. Environmental heterogeneity driven by EPA-mediated changes in the quality of food may in consequence contribute to the maintenance of genetic diversity in Daphnia and may be a reason for the temporal dominance of hybrids.
Herbivorous zooplankton avoid size-selective predation by vertical migration to a deep, cold water refuge. Adaptation to low temperatures in planktonic poikilotherms depends on essential dietary lipids; the availability of these lipids often limits growth and reproduction of zooplankton. We hypothesized that limitation by essential lipids may affect habitat preferences and predator avoidance behavior in planktonic poikilotherms. We used a liposome supplementation technique to enrich the green alga Scenedesmus obliquus and the cyanobacterium Synecchococcus elongatus with the essential lipids, cholesterol and eicosapentaenoic acid (EPA), and an indoor system with a stratified water-column (plankton organ) to test whether the absence of these selected dietary lipids constrains predator avoidance (habitat preferences) in four species of the key-stone pelagic freshwater grazer Daphnia. We found that the capability of avoiding fish predation through habitat shift to the deeper and colder environment was suppressed in Daphnia unless the diet was supplemented with EPA; however, the availability of cholesterol did not affect habitat preferences of the tested taxa. Thus, their ability to access a predator-free refuge and the outcome of predator-prey interactions depends upon food quality (i.e. the availability of an essential fatty acid). Our results suggest that biochemical food quality limitation, a bottom-up factor, may affect the top-down control of herbivorous zooplankton.
Crustaceans present a remarkable variety of forms that differ greatly in body size and growth strategies (determinate or indeterminate). This diversity reflects the long evolutionary history of this group and the variety of environments a crustacean may inhabit. It is rooted in a wide array of internal (physiological, structural) growth constraints and different extrinsic ecological factors determining the extent to which the body size of an individual crustacean attains its upper limit. We briefly review the combined effects of these factors with a focus on the effects of food quality and quantity, predation, and temperature on life histories in the context of an individual, as well as at the population and community levels. We discuss the discrepancy between the possible and the attained body size in an attempt to resolve the extent to which the observed pattern (1) is genetically based, (2) reflects the adaptive plasticity of the phenotype, and (3) is driven by global environmental changes.
A high genetic variation and recurrent changes in the genetic structure have been found in many pelagic populations. However, evidence that directly links these changes to differences in the ecological performance of particular genotypes is scarce. We hypothesized that within Daphnia, the specialization of clones occurring in a particular season to the food quality specific for that time of the year is responsible for the observed changes in the genetic structure of a population. This hypothesis was tested by comparing the fitness of spring and summer clones of the Daphnia longispina group, given food of biochemical quality relevant to these seasons. We identified significant intraspecific differences between clones of Daphnia that are specific for particular seasons, but there was no evidence that clones are adapted to the food quality available at the respective times of year. Summer clones reproduce at smaller size, and have a lower juvenile specific growth rate as compared to spring clones, irrespective of food quality. Spring clones invest more energy in somatic growth at the cost of reproduction, whereas summer clones invest more energy in reproduction at the cost of somatic growth. On the basis of the observed differences between spring and summer clones in their patterns of energy allocation, we suggest that other factors, most likely predation, are the major forces driving phenotypic and genetic diversity in the investigated Daphnia population of a large lake.
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