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Ecology

Life History Notes

Daphnia are small crustaceans of the suborder Cladocera. They inhabit freshwater, and are generally found in open water among the weeds of lakes and ponds, although some species can be found in rivers and streams. Daphnia are small (general 0.2-3mm) and laterally flattened. Their body is covered by a carapace, but a head and antennae are usually apparent. The body ends in a spine. A second pair of antennae is used primarily for propulsion of the animal through the water. Movement is generally vertical, with the head directed upwards. Most species are known for their vertical migration, generally consisting of upward movement in the dark and downward migration during daylight hours. This is a possible response to avoid predation by fish.

Daphnia are commonly maintained in laboratories for testing toxic substances in water. They have the advantage of being small, with a maximum size of 5mm, and a high reproduction rate. At 20 C, they have a relatively short life span (about 8 weeks) and mature within 6 to 8 days after leaving the brood pouch. Under normal breeding conditions, adult females produce 3 to 9 young per brood.

Daphnia have a unique system of reproduction. They and other Cladocerans are capable of producing three types of eggs during their life cycle. During most of the year, when environmental conditions are favorable, females produce eggs that do not require fertilization by a male, a strategy called parthenogenesis. These eggs develop into parthenogenic females which are genetic copies of their mothers. The eggs hatch within a few days inside a brood pouch formed dorsally by the two carapace halves of the mother. The young are in a form much like that of the adult, only smaller. As environmental conditions become more unfavorable (food supplies fail or ponds begin to dry up), some parthenogenetically created male eggs are also formed along with the female eggs. The brood chamber surrounding these eggs changes to form a thick, protective shell, the ephippium, which is sealed and shed when the mother molts. Protected in this way, the eggs can dry and freeze without losing viability. The eggs remain in this state until favorable conditions for development reoccur. Eggs of these first Daphnia hatched in the newly favorable conditions do require fertilization. Male Daphnia are smaller than females and usually of similar form. They are distinguished by larger antennules and their first foot is frequently armed with a stout hook which serves to clasp females.

Daphnia feed on algae, bacterial flora and smaller zooplankton (mainly protozoan and rotifer species). Daphnia themselves are an important part of the diets of small fish and predaceous insects and their larvae.

Raising Daphnia in the Classroom

Suitable containers for raising Daphnia range from glass jars to aquariums. The instructions given can be applied to almost any receptacle, but an aquarium will be used in this example.

Equipment needed:

  • aquarium
  • aquarium air pump
  • standard airline tubing
  • air or bubble stone
  • dechlorinated water
  • Daphnia spp.

Feeding:

Almost any organic matter in suspension will be acceptable. Some examples include:

  1. Sprinkle dry yeast on the water surface.
  2. Mix 1 part fish food or other animal chow such as rabbit or dry dog food with 2 parts water to create a Daphnia "milkshake" (keep refrigerated). Pour some of this mixture into the water (about 3 Tbls. Per feeding for a 10 gallon aquarium).
  3. When allowing the water to stand, add a head of lettuce. At the end of the week the water should be a nice green color and contain organic particles for the Daphnia to eat.

Daphnia may possibly be acquired from Battelle staff. The other (expensive) alternative is to order them from a science catalog such as the Carolina Biological Supply Catalog.

have the aquarium set up before the Daphnia arrive. It must be filled with dechlorinated water. The best method is to let tap water stand for several days before adding the animals. It is also perfectly acceptable to use pond water, but it may be more difficult to see the animals. You must also set up the aerator system. The air stone may simply lie on the bottom of the tank.

When the Daphnia arrive, it is a good idea to acclimate them to the water temperature in the tank (Ambient air temperatures of 18 - 22 C are perfectly acceptable). Simply put the Daphnia, still in the container they arrive in, into your aquarium. After about 15 minutes the Daphnia may be allowed to swim free in the aquarium. You should then feed them. Thereafter, allow the water to clear before feeding the Daphnia again. Also, at least once a month, remove half the water in the tank and replace it with fresh (dechlorinated!) water. The Daphnia multiply rapidly enough that it will not hurt to remove half the Daphnia along with the water. In fact, it may prove beneficial to the population.

Individual animals may be selected for observation with an eyedropper. Your Daphnia should grow and multiply. Enjoy!

*It may be a good idea to keep one or more backup cultures in case something happens to the original. To do this, simply place several Daphnia in a quart jar with the culture water from the main aquarium and watch the population explode!

Ecology

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