Cladocerans

Cladocerans are small crustaceans belonging to the orders, Anomopoda, Ctenopoda, Onychopoda or Haplopoda. Commonly called water fleas, due to their small size and jerky swimming motion, cladocerans are extremely abundant in most freshwater habitats. Only in swiftly flowing or grossly polluted water are they absent. Although there are a few estuarine species this group has not been successful in the oceans. Some species are planktonic living in the open water of lakes, while others live on or near the bottom or on aquatic vegetation. Some are found primarily in small ponds or saline lakes which lack fish predators.

Ceriodaphinia acanthina with shell open showing legs used for filter-feeding

A few are predacious but most species are herbivores, feeding on phytoplankton, attached vegetation or decaying organic material. Most feed using complex movements of the legs producing a constant current of water between the valves. The legs have many hairs on them which filter food particles from the water as it passes between the valves and collect them in a groove at the base of the legs. This stream of food is fed to the mouthparts where the particles may be ground between the surfaces of the mandibles before being taken into the mouth. Although there is some evidence that certain types of food, such as particular types of algae, Protozoa, or bacteria may be selected by some species, it is generally believed that all organic particles of suitable size are ingested without any selective mechanism. When undesirable material or large tangled masses are introduced between the mandibles, they may be removed by spines on the first legs and then kicked out of the carapace by the postabdomen.

Most cladocerans can reproduce by parthenogenesis, females producing eggs that develop into young without fertilization by males. As a result, males are not present in the population for much of the year. As long as environmental conditions remain favourable females will continue to reproduce in this manner, producing only female offspring. If the environment starts to deteriorate due to overcrowding, lack of food or oxygen depletion eggs are produced that develop into males, and females capable of sexual reproduction. These females produce eggs that once fertilized by the males are capable of surviving extended periods of harsh conditions such as drying or freezing. In this manner cladoceran populations survive through winter and periods of drought. When environmental conditions are again favourable the eggs hatch and produce parthenogenic females.

An important link in food chains of virtually every inland body of water, cladocerans convert phytoplankton, and decaying organic matter into animal tissue that can be used by larger animals. In large lakes they are a major food source for many kinds of fish such as sticklebacks, minnows and young Sockeye salmon. Many aquatic insect larvae and other invertebrates also feed on cladocerans and species.

Cladocerans of the Thompson-Okanagan Region  
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