Daphnia are small in size, but large in purpose. These microscopic crustaceans, belonging to the order Cladocera, are more populous than ants, found worldwide, and essential for the fishing industry. They are nicknamed water fleas due to their characteristic hopping motion and resemblance to the common flea.
Like other Cladocerans, Daphnia are an important link in the food chain. Most species ofDaphnia are found in freshwater habitats, but a few occur in marine environments. The microscopic organisms feed upon the microscopic particles and phytoplankton they encounter in their aquatic surroundings. In turn, Daphnia act as food for juvenile fish. These tiny crustaceans are one of the most popular live food for aquarium fishes, but are also important on a much grander scale. In the Great Lakes of North America, water fleas are the basic food of nearly all commercial fish species.
The reproduction and survival of the species, therefore, is of great interest. Daphnia is parthenogenetic most of the year, producing unfertilized eggs that develop into females. Towards the end of seasonal population peaks, in a mechanism not well understood, parthenogenetic females produce parthenogenetic males and sexual females. The sexual females and males copulate and produce a small number of resilient eggs. These eggs can overwinter, withstand drying and freezing, etc., and will hatch when conditions permit. Consequently, the eggs aid the organism in dispersal between bodies of water and allow the species to endure in ephemeral habitats.