Daphnia Genotypes Research Articles

Predation Risk, Prey Behavior, and Feeding Rate in Daphnia pulex

Exposure to chemical exudates from the planktonic invertebrate predator Chaohorus americanus causes the prey Daphnia pulex to migrate upwards in the water column of laboratory experimental chambers. This behavior is an antipredator defense that results in spatial separation of Daphnia from Chaoborus, since this predator tends to remain near the bottom of the chambers. We test the effects of prey vulnerability, genotype, prior exposure to predators, and predator density on antipredator behavior of D. pulex. To study differences in vulnerability, we compared two genotypes of Daphnia that differed in their ability to produce a morphological defense (growth of neck spines) when exposed to Chaohorus chemical. Overall, behavioral responses to Chaoborus were stronger at higher predator densities. Juvenile Daphnia that are most vulnerable to Chaoborus predation had stronger responses than the less vulnerable adults. Neither genetically determined ability to produce neck spines nor phenotypic expression of neck spines affected strength of antipredator behavior. Vertical migration to avoid Chaoborus may have an ecological cost. Presence of Chaoborus reduced ingestion rates of juvenile Daphnia at low food levels (5 × 103 algal cells∙mL−1). Feeding experiments also showed differences in feeding characteristics of two different clones of D. pulex.

Measuring molecular variation in zooplankton populations: DNA extraction from smallDaphnia species

We present methods developed for the small-sizedDaphnia species,D. galeata, D. hyalina, D. cucullata, and their hybrids, which facilitate the use of DNA variation in population studies. Described are large-scale production of material from single genotypes ofDaphnia and subsequent extraction of total DNA. The average DNA yields ranged between 6 µg per gram wet weight (D. cucullata) and 12 µg per gram wet weight (D. galeata). For comparison, the large-sized speciesD. pulex was tested and yielded an average of 28 µg DNA per gram wet weight. The DNA isolated in this manner lends itself well to molecular genetic techniques suited for population studies.

Phototaxis in Daphnia magna: the influence of temperature and acidity on the phototactic behaviour of Daphnia genotypes

Phototaxis in <i>Daphnia magna</i>: the influence of temperature and acidity on the phototactic behaviour of <i>Daphnia</i> genotypes

Differential response of Daphnia genotypes to oxygen stress: respiration rates, hemoglobin content and low-oxygen tolerance.

Laboratory respiration rate experiments using three electrophoretically identified clones of the fresh water, planktonic cladoceran, Daphnia pulex, from an eutrophic farm pond, indicated that clones acclimated to both low and high oxygen levels, regulated oxygen consumption across a wide range of oxygen concentrations (1.0-9.0 mg· liter-1). A "threshold" oxygen level of 0.5-1.0 mg·liter-1 was reached, where animals succumbed to oxygen stress, regardless of hemoglobin content. No significant clonal differences in respiration rates were found. These data suggest that members of this Daphnia population are able to regulate oxygen metabolism across a wide range of ambient oxygen concentrations, and indicate a well-adapted respiratory system.Low-oxygen tolerance experiments and hemoglobin measurements indicated further that physiological differences indeed exist between clones; one clone produced the lowest amount of hemoglobin and was least tolerant of low oxygen levels. These data imply that spatial and temporal changes in dissolved oxygen concentration may be an important selective force influencing the clonal (genotypic) composition of natural cladoceran populations.