They are interesting animals for the microscopist. Because you can see through them, you can study for instance the beating of the heart and follow the course of the blood cells round part of the body. Their sizes differ from several hundred microns to more than five millimetre for the larger species. They reproduce in summer mostly parthenogenetic, that is, the eggs develop without undergoing fertilization. These eggs can also be found in populations under stress, such as during the drying up of a pond.
Daphnia feeds on bacteria, fine detritus and very small algae.
But not all water fleas are peaceful herbivores. Some species are voracious predators. Go to the next page where you can see Polyphemus pediculus giving birth. Only some species inhabit marine waters. They differ considerably in appearance from their freshwater relatives.