"The second major group of zooplanktors are the Cladocerans, commonly known as waterfleas. There are many different kinds of cladocerans, but one common and very important group is the daphnia. Daphnia are important in the food web of the slower reaches of rivers because they are very efficient filter feeders, grazing mainly on algae, and therefore can keep algal growth in check. The larger daphnia species are also an important food item for many species of fish.
"Daphnia are very prolific and can reproduce both sexually and asexually. During most of the year, they reproduce asexually, the females producing unfertilized eggs in a brood sac, which develop into female daphnia, kind of like clones. When conditions become less favorable (say decreasing temperature, oxygen, or light intensity) some females produce eggs that become males. Some of the females then produce fertilized resting eggs or what we call epphipia, which can survive freezing and/or drying conditions. Most cladocerans, including daphnia, overwinter as epphipial eggs. These epphipial eggs are very resistant and actually can be transported from water body to water body on duck feet.
"Other species of cladocerans that are common in slower reaches of the river include Bosmina (typically a small round cladoceran with an antenna that resembles an elephant's trunk), and Diaphanosoma, which is more elongate and has a large, thick antenna. Both of these species are filter feeders, and in turn are important in the diets of larval fish, and also many minnow species.
Leptodora is one of the largest cladocerans common in slow reaches of large rivers. While most cladocerans are microscopic in size (that is less than 2.0 mm in length) Leptodera can reach a length of 13 mm. This particular zooplanktor is predatory in nature and feeds on other cladocerans and copepods. The large size of the Leptodora makes it an ideal food for adult fish."