Water fleas are tiny crustaceans that belong to the order Cladocera. Approximately 150 species of water fleas have been identified in North America, almost all of which inhabit fresh water.

Some varieties of water fleas are planktonic, but others live on the bottoms of lakes or attached to aquatic plants. Extremely prolific, thousands of the small animals may inhabit a single square meter of water and are, therefore, an important part of the food chain. Most water fleas feed upon phytoplankton or decaying organic material, but some are predaceous and may feed on smaller members of their own order. In turn, water fleas are a fundamental diet component of a variety of fish, insect larvae, and other aquatic organisms.

Recently, studies of Daphnia, a common genus of water flea, have demonstrated that offspring may react to environmental factors endured by their parents. Predators of Daphnia release chemicals referred to as kairomones, which cause the water fleas to grow a protective structure around their heads, similar to a helmet. Interestingly, young Daphnia are more likely to have a larger defensive shield if their mothers have been exposed to kairomones, than those that have never been exposed to the chemical. Scientists have suggested that the phenomena may occur as a “switching on” of genes that are always present, though not active, in the organisms.