Daphnia, commonly called water fleas, are a freshwater zooplankton found in ponds and lakes all over the world. Daphnia received the name water flea due to their jerky swimming motions. Some types of Daphnia can be seen with the human eye, while others must be identified with a microscope. Depending on the species they can range in size from 0.5mm to 1cm. Their outer covering, or carapace, is transparent, so many internal organs can be seen, especially the beating heart. On the head there is a compound eye and a pair of antennae, which are used for swimming. Females are usually larger than males and have a brood chamber under their outer carapace where eggs are carried.
Daphnia are an extremely important part of aquatic food chains. They eat primary producers such as algae, yeast, and bacteria. Daphnia are the prey of tadpoles, salamanders, newts, aquatic insects, and many types of small fish. Fluctuations in Daphnia populations can cause algae overgrowth or even a drastic drop in fish populations. Through the food chain, larger animals caught or eaten by humans can be affected by large changes in Daphnia populations
Source: Anne Deken, Missouri