Daphnia are members of a genus of tiny crustaceans that belong to the order Cladocera. Commonly called water fleas, Daphnia are very prolific and are extremely important as a food source for juvenile fish and other aquatic animals.

Populations of Daphnia usually consist of parthenogenetic females that produce only other females. However, during periods of stress, such as cold weather, drought, or overpopulation, males are produced along with haploid eggs. After sexual fertilization, the eggs remain in the brood chamber of female Daphnia until the next molting period. Encased in modified and thickened cases known as ephippia, the eggs can survive many years in severe conditions after they are released. Resistant to freezing, drying, and digestion, the eggs do not hatch until favorable circumstances develop.

Recently, studies of Daphnia 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.