Introduction

Anatomy of Daphnia

Daphnia are small crustaceans that inhabit lakes, ponds and streams. They eat algae, and in turn, serve as an energy source for larger predators such as fish or other zooplankton (i.e., small animals and protists of aquatic environments, like Hydra!). They capture algae (e.g., Volvox and Chlamydomonas) with their legs, which also serve as gills, helping them breathe oxygen from the water. Daphnia swim in a jerky fashion by thrusting downward with of a pair of powerful, double branched antennae, which is the reason for their common name “water flea” (Buchsbaum et al., 1987).

Daphnia have a single compound eye and an ocellus (a simple eye). They have two pairs of antennae (4 total), one pair for locomotion and one for sensing their environment. Their thoracic appendages produce a current of water to carry food particles to the mouth and oxygen to the gills. Because Daphnia exoskeletons are transparent, internal organs such as the beating heart are visible without dissection. Also, note that if one has eaten green algae, its gut may look green. (See Micscape Magazine photos.) A post-abdominal pair of claws is used to remove unwanted material that may have accumulated in the carapace. Males are smaller than females, but have hooks on their first pair of thoracic appendages to grasp females. The males have larger first antennae than females. (Clare, J. and Micscape Magazine)

Examine Daphnia in water. Place a single Daphnia in a drop of water on a depression slide and observe at low magnification. Sketch what you see and label the structures highlighted in boldface, above.

Experiment

Ecology: Daphnia are sensitive to poor water conditions and therefore serve as bio-indicators (i.e., organisms which when present or absent reflect on water quality; See Clare, J.) By studying the effect of common pollutants such as alcohol, caffeine, and nicotine on Daphnia's heart rate, we can learn more about how chemicals affect aquatic organisms, and thereby the ecosystems of which they are a critical part. We can also compare the effect of chemicals on Daphnia to their effects on human physiology.

We will observe the effects of certain chemical compounds (see TABLE 1, below) on the heart rate of Daphnia.

1. Before beginning the experiment, predict what you think the effect of each chemical will be on the heart rate of Daphnia. Each group should fill in Table I with their own group data. We will then put class data on the board.
2. Count and record the number of heartbeats per/15 seconds. Do this twice and then calculate the average number of beats per minute. This is the baseline heart rate. If Daphnia moves too much, remove it from the well, put a tiny dab of Vaseline at the bottom of the depression well, and then add the Daphnia.
3. Use a tiny piece of Kimwipe to remove most of the water drop and then add single drop of the chemical compounds listed on the chart below.
4. Wait 30 seconds after adding the compound, and then record the number hearbeats/15 seconds (make sure to conduct two counting trials per chemical).
5. Between chemicals, use a tiny piece of Kimwipe to remove the drop of one chemical solution, then add a new drop of water to rinse the slide and Daphnia, then remove the rinse water and add the next chemical solution.

Questions

1. Do your observations match your predictions?
2. Are the number of beats in Trial 1 and Trial 2 the same? What might account for differences? Is the average of two trials meaningful?
3. What would happen if you did the same experiment at a different temperature? If ice is available, try the experiment again at a lower temperature.
4. What would you add to or change in this draft of the experiment to make it easy for students in future semester to repeat and obtain the same results?

Additional information

Daphnia reproduce asexually and sexually. When food is abundant, Daphnia reproduces parthenogenetically. Parthenogenesis is a form of asexual reproduction where unfertilized eggs develop into zygotes. Under ideal environmental conditions, eggs produced this way all develop into females. If conditions are less favorable, some unfertilized eggs develop into males, and the subsequent generation will reproduce sexually. Fertilized eggs produced by sexual reproduction develop within the carapace. Like other crustaceans, a carapace covers the body, but in the female Daphnia, it also serves as a brood chamber for developing embryos. The egg-housing part of the carapace is called the ephippium because it is shaped like a saddle. When sexually-produced eggs leave the female, they enter a suspended state where they are able to resist drying and freezing, and they can survive the winter and hatch in the spring. (See Clare, J.)

References

• Daphnia: An Aquarist's Guide, Version 3.2, (written & compiled by John Clare.) www.caudata.org/daphnia/
• Micscape Magazine: www.microscopy-uk.org.uk/mag/indexmag.html. Click on “Pond Life ID Kit” at the left side of the page, then “Crustacea,” “Water Fleas.”
• Why study Daphnia? ei.cornell.edu/toxicology/bioassays/daphnia/
• Buchsbaum, R., Buchsbaum, M., Pearse, J., and Pearse, V. Animals Without Backbones (3rd ed.) 1987. University of Chicago Press. p. 346.
• Ebert D, 2005. Ecology, Epidemiology, and Evolution of Parasitism in Daphnia [Internet]. Bethesda (MD): National Library of Medicine (US), National Center for Biotechnology Information. www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=daph

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