Water flea

Water fleas are an order of crustaceans who are so named because they tend to swim by making little jumps. Water fleas are small, between 0.2 and 3 mm in length, and found in most freshwater habitats. Some are kept as live food for aquarium fishes, or as interesting creatures to study because their bodies are almost transparent.

The division of the body into segments is nearly invisible. The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species the rest of the body is covered by a carapace, with a ventral gap in which lie the five or six pairs of legs. The most prominent features are the compound eyes, the second antennae, and a pair of abdominal setae.

A few water fleas are predatory on tiny crustaceans and rotifers, but most are filter feeders, ingesting various sorts of organic detritus including protists and bacteria. Daphnia can be kept easily on a diet of yeast. Beating of the legs produces a constant current through the carapace which brings such material in. Swimming, on the other hand, is powered mainly by the second antennae, whose strokes are responsible for the jumps.

Males are only found during spring and autumn, and even then may make up considerably less than half the population, in some species being unknown entirely. The rest of the time females reproduce exclusively by parthenogenesis. The young that hatch from the eggs are similar to the adult. Once sexual maturity is reached eggs are produced between each subsequent molt.