Festering food: chytridiomycete pathogen reduces quality of Daphnia host as a food resource.

When parasitic infections are severe or highly prevalent among prey, a significant component of the predator's diet may consist of parasitized hosts. However, despite the ubiquity of parasites in most food webs, comparisons of the nutritional quality of prey as a function of infection status are largely absent. We measured the nutritional consequences of chytridiomycete infections in Daphnia, which achieve high prevalence in lake ecosystems (>80%), and tested the hypothesis that Daphnia pulicaria infected with Polycaryum laeve are diminished in food quality relative to uninfected hosts. Compared with uninfected adults, infected individuals were smaller, contained less nitrogen and phosphorus, and were lower in several important fatty acids. Infected zooplankton had significantly shorter carapace lengths (8%) and lower mass (8-20%) than uninfected individuals. Parasitized animals contained significantly less phosphorus (16-18% less by dry mass) and nitrogen (4-6% less) than did healthy individuals. Infected individuals also contained 26-34% less saturated fatty acid and 31-42% less docosahexaenoic acid, an essential fatty acid that is typically low in cladocera, but critical to fish growth. Our results suggest that naturally occurring levels of chytrid infections in D. pulicaria populations reduce the quality of food available to secondary consumers, including planktivorous fishes, with potentially important effects for lake food webs.

Intraspecific variation in a predator affects community structure and cascading trophic interactions.

Intraspecific phenotypic variation in ecologically important traits is widespread and important for evolutionary processes, but its effects on community and ecosystem processes are poorly understood. We use life history differences among populations of alewives, Alosa pseudoharengus, to test the effects of intraspecific phenotypic variation in a predator on pelagic zooplankton community structure and the strength of cascading trophic interactions. We focus on the effects of differences in (1) the duration of residence in fresh water (either seasonal or year-round) and (2) differences in foraging morphology, both of which may strongly influence interactions between alewives and their prey. We measured zooplankton community structure, algal biomass, and spring total phosphorus in lakes that contained landlocked, anadromous, or no alewives. Both the duration of residence and the intraspecific variation in foraging morphology strongly influenced zooplankton community structure. Lakes with landlocked alewives had small-bodied zooplankton year-round, and lakes with no alewives had large-bodied zooplankton year-round. In contrast, zooplankton communities in lakes with anadromous alewives cycled between large-bodied zooplankton in the winter and spring and small-bodied zooplankton in the summer. In summer, differences in feeding morphology of alewives caused zooplankton biomass to be lower and body size to be smaller in lakes with anadromous alewives than in lakes with landlocked alewives. Furthermore, intraspecific variation altered the strength of the trophic cascade caused by alewives. Our results demonstrate that intraspecific phenotypic variation of predators can regulate community structure and ecosystem processes by modifying the form and strength of complex trophic interactions.

Genetic architecture of the cryptic species complex of Acanthocyclops vernalis (Crustacea: Copepoda). II. Crossbreeding experiments, cytogenetics, and a model of chromosomal evolution.

Collectively, populations of Acanthocyclops vernalis, a species complex of freshwater copepods, are remarkably similar as to morphology and DNA content, despite variability in chromosome number. Reproductive isolation had been reported among some populations, but with each new investigation the species boundaries and factors that may influence them appeared less clear. To clarify the pattern of biological species within this group of populations, we adopted a comprehensive approach and examined patterns of reproductive isolation in populations for which morphology, chromosome number, DNA content, and 18S rDNA sequences are known. In this study we established nine isofemale lines from four sites in Wisconsin and performed 266 crosses. Crosses within and among these lines were used to relate the degree of reproductive isolation to chromosome differences and to construct a model to explain the origin and maintenance of chromosome number variability. Different gametic and somatic chromosome numbers were observed among specimens within some isofemale lines. In a few cases, gametes with different haploid numbers were produced by a single female. Matings within isofemale lines always produced at least some reproductively successful replicate crosses (produced viable, fertile offspring). Crosses between lines from the same site showed reduced success relative to within-line crosses. Crosses between populations from distant sites showed limited genetic compatibility, producing viable, fertile F1 offspring but infertile F2 adults. One cross between lines with different chromosome numbers (one with 2n = 8 and one with 2n = 10) produced fertile viable offspring, which reproduced for at least 60 generations. These hybrids had either eight or nine chromosomes in the third generation of inbreeding, and eight chromosomes after 20 generations. These hybrids also had reduced nuclear DNA contents at the third generation, a level that persisted through the 20th generation. Successful backcrosses between some hybrids and their parental lines further demonstrated the potential for genetic compatibility among forms with different chromosome numbers. We propose a model in which alterations due to Robertsonian fusions, translocations, and/or loss of chromosomal fragments generate heritable variation, only some of which leads to reproductive isolation. Hence, some of the criteria traditionally used to recognize species boundaries in animals (morphology, DNA content, chromosome number) may not apply to this species complex.

Effects of pharmaceuticals on Daphnia survival, growth, and reproduction.

Pharmaceuticals have been globally detected in surface waters, and the ecological impacts of these biologically-active, ubiquitous chemicals are largely unknown. To evaluate the aquatic toxicity of individual pharmaceuticals and mixtures, we performed single species laboratory toxicity tests with Daphnia magna, a common freshwater zooplankton. We conducted acute (6-day) and chronic (30-day) exposure pharmaceutical bioassays and evaluated survivorship and morphology of adults and neonates, adult length, resting egg production, brood size (fecundity), and the proportion of male broods produced (sex ratio). In general, exposure to a single pharmaceutical in the 1-100 microg/l range yielded no apparent effects on the normal life processes of Daphnia. However, chronic fluoxetine exposure (36 microg/l) significantly increased Daphnia fecundity, and acute clofibric acid exposure (10 microg/l) significantly increased sex ratio. A mixture of fluoxetine (36 microg/l) and clofibric acid (100 microg/l) caused significant mortality; the same fluoxetine concentration mixed with 10 microg/l clofibric acid resulted in significant deformities, including malformed carapaces and swimming setae. Mixtures of three to five antibiotics (total antibiotic concentration 30-500 microg/l) elicited changes in Daphnia sex ratio. We conclude: (1) individual and mixtures of pharmaceuticals affect normal development and reproduction of Daphnia magna, (2) aquatic toxicity of pharmaceutical mixtures can be unpredictable and complex compared to individual pharmaceutical effects, and (3) timing and duration of pharmaceutical exposure influence aquatic toxicity.

Effects of vertebrate hormones on development and sex determination in Daphnia magna.

Daphnia (Crustacea) are extensively used as model organisms in ecotoxicology; however, little is known regarding their endocrine system. This study examines Daphnia vulnerability to vertebrate hormones. Twelve natural or synthetic vertebrate hormones were screened for activity on developmental and reproductive processes in Daphnia magna. Natural hormones tested included: beta-estradiol, gonadotropin, hydrocortisone, insulin, melatonin, progesterone, somatostatin, testosterone, and thyroxine at concentrations ranging from 1 to 100 microg/L. Synthetic hormones tested included diethylstilbestrol (estrogenic), R-1881 (androgen), and ICI-182,780 (antiestrogen); all hormones were screened with a 6-d assay. Additionally, progesterone, insulin, testosterone, and thyroxine were screened for 25 d. Diethylstilbestrol decreased D. magna growth rate while thyroxine increased it. Short-term testosterone exposure reduced D. magna fecundity; however, long-term exposure did not, potentially indicating testosterone hydroxylation with long-term exposure. Hormones commonly considered sex-hormones (estrogens and androgens) in vertebrates do not appear to control sexual differentiation in D. magna; however, several vertebrate hormones do affect reproduction and development in D. magna making D. magna a potentially useful tool in monitoring for the presence of these hormones or compounds that mimic them.

Medium-term and long-term priorities in ecological studies.

Research priorities in ecology and environmental sciences for the future are formulated. The priorities for both fundamental and applied ecology are proposed. The list of priorities includes 50 items. The priorities are relevant to terrestrial, aquatic, and general ecology. The list of priorities is helpful when grant proposals are being prepared, evaluated, and selected for funding.

Effects of common-use pesticides on developmental and reproductive processes in Daphnia.

Daphnia magna were evaluated for use as a screen for pesticides that have been demonstrated to have estrogenic (o'p'-DDT, di-n-butyl phthalate, toxaphene), anti-androgenic (p'p-DDE, linuron), thyroid (acetochlor, alachlor, metribuzin), insulin (amitraz) or lutenizing hormone (2,4-D) activity in vertebrates, and to establish daphnid sensitivity to these compounds. Pesticides with unknown effects on vertebrate endocrine systems (chlorosulfuran, cyanazine, diflubenzuron, metolachlor, and diquat) were also evaluated. Compounds were assayed for six days at environmentally relevant concentrations ranging from 0.001 to 100 mirog/L, using female Daphnia and their offspring. Sublethal endpoints included offspring sex (sex determination), clutch size (fecundity), and adult size (growth rate). Toxaphene was the only compound that affected sexual differentiation, increasing male production. Daphnia fecundity declined with exposure to toxaphene, and daphnid growth rates were reduced by acetochlor exposure. Diflubenzuron, o'p'-DDT, and p'p-DDE significantly reduced Daphnia survival. No correlation existed between affected reproductive or developmental processes and specific endocrine systems or subsystems. Results from this study indicate that Daphnia make a good screen for assessing potential environmental impacts but are not a useful indicator of pesticide hormonal activity in vertebrates. This assay consistently detected sublethal but ecologically relevant effects of these pesticides on Daphnia at environmentally relevant concentrations typically below their listed EC50 value.

THE EVOLUTIONARY ECOLOGY OF AN ANTIPREDATOR REACTION NORM: DAPHNIA PULEX AND CHAOBORUS AMERICANUS.

Ponds containing the parthenogenetic zooplankter Daphnia pulex with and without chaoborid predators were sampled over the course of a season. A significant (P < 0.05) Spearman rank correlation was found between predator density and the expression of an antipredator defense (neckteeth) by the Daphnia. The reaction norms (percent induction of a single genotype versus predator density) of clones isolated from predator-free and predator-rich habitats were determined in a laboratory setting. There was a statistically significant different response among the six clones tested (P < 0.05). Clones isolated from chaoborid ponds showed significantly greater sensitivity to the presence of predator than clones from predator-free ponds (P < 0.05). In the laboratory, food levels under which prey were cultured affected induction of the antipredator response. Highest induction was found at the lowest food level used.

Demographic costs of Chaoborus-induced phenotypic plasticity in Daphnia pulex.

It has been proposed that morphological defenses against predation have demographic costs. We measured the cost of a predator-induced morphological defense, using predaceous phantom midge larvae Chaoborus americanus (Insecta, Diptera) and the prey species Daphnia pulex (Crustacea, Cladocera). The induced defense is a neck tooth (and other pleiotropic structures) developed in juvenile D. pulex in the presence of C. americanus. Laboratory life table experiments, in the absence of predation, indicated the population growth rate of typical D. pulex was 11% to 39% greater than that of D. pulex exposed to C. americanus extract, or C. americanus-conditioned water. The reduction in population growth rate was most frequently associated with an increase in the time between birth and first reproduction. Induced individuals required twenty more hours at 23°C, and twenty five more hours at 20°C, to develop to the age of first reproduction. Under limiting food conditions age-specific survivorship and the number of offspring produced per female by the induced form were reduced relative to the typical form. As a result, the difference in population growth rates among forms was greater at the low food level as indicated by a highly significant food by form interaction effect. In addition to neck teeth and lowered reproductive rates, the offspring of induced form individuals had significantly longer tail-spines (7.2-7.5%), and primiparous adults from the induction treatment were significantly shorter than controls (3-8%).

The ecological role of chemical stimuli for the zooplankton: predator-induced morphology in Daphnia.

Numerous adaptive predator-induced responses occurred when eight clones representing seven Daphnia (Crustacea: Cladocera) species were tested against three common predators: fourth instar larval phantom midge Chaoborus americanus, adult backswimmer Notonecta undulata, and small sunfish Lepomis macrochirus. The predators were confined within small mesh bags, suggesting that the signal for induction is chemical. The induced responses included longer tail spines, longer heads, smaller bodies, increased egg clutches, and decreased lipid reserves. Each Daphnia species responded to each of the three predators in a unique manner. Induced responses in the above characters showed no significant association. The induced morphological changes are generally consistent with current theories of what is an adaptive response for the various sizes of Daphnia exposed to tactile and visual predators. The abundance of induced responses in these experiments suggests that predator-induced responses are a widespread and ecologically important phenomenon of the freshwater zooplankton.