Daphnids

Physical description: Ceriodaphnia dubia are very small, never larger than one millimeter in length.  When in good health, they appear to be a brownish-orange color.  There is a large, dark spot on the anterior end of every Ceriodaphnia, this is the compound eye.  Males and females can be distinguished from each other by their shape and size.  Males are smaller and less rotund than are the females; the males are almost triangular in shape.  Movement is achieved by a powerful set of second antennae.  Because of this, movement is generally vertical and jerky.

Ecology: The water flea, Ceriodaphnia dubia, occurs in littoral areas lakes, ponds, and marshes throughout most of the world. 

Uses in Aquatic Toxicology: Ceriodaphnia dubia are an EPA recommended freshwater invertebrate used in both acute and chronic toxicity testing.  In acute toxicity testing, Ceriodaphnia are used at <24 hours old and survival rates are recorded.  In chronic toxicity testing, Ceriodaphnia are used at <24 hours old and all neonates must have been released within 8 hours of each other.  In chronic tests, survival and reproduction are recorded.

Other Uses: Live food source in freshwater larviculture and in the ornamental fish industry.

Photo: (2005) Are We Underestimating Species Extinction Risk? PLoS Biol 3(7): e253

Physical description: Daphnia magna are similar to Ceriodaphnia dubia but larger, usually five to six millimeters in length. 

Ecology: Daphnia magna is principally a lake dweller and is restricted to waters in northern and western North America.  Daphnia magna reproduce only by cyclic parthenogenesis in which the males contribute to the genetic makeup of the young during the sexual stage of reproduction. 

Uses in Aquatic Toxicology: Daphnia magna are used in freshwater acute toxicity testing at <24 hours old and survival rates are recorded.

Other Uses: Live food source in freshwater larviculture and in the ornamental fish industry.