Used as alternative to Gammarus in: Environments, Populations and Ecosystems
Daphnia – Daphnia magna
Background. Daphnia are tiny crustaceans, never growing larger than a couple of millimeters no matter how well they eat. They are sometimes called water fleas because they are about that size and superficially look like fleas. But unlike fleas they are nearly transparent, live in water, have a shell, and are graced with an interesting array of modified appendages for sensing, feeding, and swimming. Daphnia are free swimmers, propelling themselves with surprising speed, considering they use a pair of modified antennae to swim. As they travel they filter even tinier organisms from the water. They feed on single-celled algae, yeast, and bacteria. Daphnia in turn are eaten by fish and aquatic insects. It seems that their role in life is to provide a snack for a larger organism.
Reproduction. Reproduction is an interesting business with Daphnia. Under optimum conditions of ample food and moderate temperatures, most of the Daphnia in a colony are female. They produce eggs without mating by a process called parthenogenesis. The 50 or so eggs are held in the brood chamber for a few days until hatching, and then out comes the new generation of Daphnia. This will continue indefinitely as long as conditions are favorable. When Daphnia are stressed, however, they produce about equal numbers of males and females, and the offspring mate. After mating, the female Daphnia produce hard-shelled eggs (cysts) that can withstand harsh conditions for a long time, even complete drying out. When conditions are once again favorable, the cysts hatch and the process repeats.
Habitat. An aquarium full of green water spells good conditions for Daphnia. In the absence of predators, a modest number of Daphnia can explode into a colony of thousands in a few weeks. When this happens, it is possible to use them for food-chain investigations. A hand lens will clearly reveal the green color (algae) in the gut of the transparent Daphnia, and fish will demonstrate the next part of the food chain in which Daphnia play a role.
Feeding. Daphnia can be kept in green water (water with a high concentration of single-celled algae) at room temperature or a little cooler. Don’t place them in direct sunlight. If no green water is available, they can be fed bacteria or yeast. To prepare bacteria, mash half of a hard-boiled egg yolk in 1 liter of dechlorinated or spring water. Let it sit for a couple of days. The cloudy liquid will be full of bacteria. A yeast suspension can be prepared by stirring half a package of dry baker’s yeast in a liter of warm water. To feed the Daphnia, simply remove 1/2 liter of water from the Daphnia culture (pour it through a net to save the Daphnia), and put 1/2 liter of the food suspension into the Daphnia culture. When the water clears, feed them again.
What to do when they arrive. Upon arrival, transfer into a larger container of dechlorinated or spring water, using a large baster or by pouring contents directly from the shipping jar. Keep container at room temperature out of direct sunlight. Daphnia are scavengers and feed on microscopic algae and protozoans normally found in pond water. If keeping for longer periods of time, introduce aquatic plants into the aquarium which, as they break down, will provide food.
What to do with them when the investigation is completed. Daphnia can be kept in an aquarium of pond water with plants, or maintained as described above. Offer the daphnia to another classroom for further investigations. Aquarium fish will also appreciate the daphnia as food. Do not release into the wild unless they were collected from a local pond.