Cladocera or cladocerans are small crustaceans commonly called water fleas, part of the Class Branchiopoda. They form a monophyletic group, which is currently divided into 4 suborders, 11 families, 80 genera, and about 400 species. The most commonly known genus is Daphnia (freshwater water fleas), which is the most researched in this group; Daphnia is commonly used to test the toxicity of chemicals in solution or for water pollution.
Cladocera are free-swimming organisms, and most orientate themselves with dorsal side up. They have a two-valved carapace covering most of the body except the appendages. In some families, the carapace only covers the brood pouch area. The head is typically separated from the body by a deep indentation, but also may not be separated. It projects forward as a beak or rostrum. On the forehead is an unpaired compound eye, a result of two fused eyes, and, in most species, a simple naupliar eye.
The number of ommatidia that make up the compound eye varies from species to species according to their food preferences. The plankton-filtering Daphnia has an eye of only 22 ommatidia, while the carnivorous Evadne and Leptodora have 80 and 300, respectively. The eyesight of carnivorous water fleas plays an important role in catching food. The first pair of antennae that contains sensory organs is quite small and is attached to the rostrum. The second pair, however, is disproportionably large compared to the body. They have two branches, and both branches bear long, feathery bristles. The second pair of antennae is the primary organ of locomotion and catching food in water fleas. In many species the two pairs of antennae are attached to each other. Sweeping both antennae downward simultaneously, the crustaceans swim by making short jumps. On the lower side of the head is the mouth, surrounded by the upper lip in front, the mandibles at the sides, and the maxillae (jaws) at the back.
The thorax of water fleas is very short and consists of 4 to 6 segments. In females, there is a large space between the back and the dorsal side of the carapace that functions as a brood pouch. The eggs are laid into the pouch and develop there. In carnivorous water fleas the shell is reduced in size, leaving most of the body exposed, and is attached to the dorsal side ; in these species it is only used as a brood pouch.
The structure of the thoracic legs is varied according to food preferences. These legs are generally similar to those of fairy, tadpole and clam shrimps. In most cladoceran crustaceans they are used to filter small particles of food suspended in water, which they eat. Accordingly the lobes of the legs are equipped with numerous feathery bristles, which together make up the filtering apparatus. The legs sweep 300-500 times a minute.
Most species also have a tail, or spiny projection of the carapace, while some have hairs projecting. Only a few groups have "true tails" that are segmented and are a projection of the body instead of the carapace.
Order Cladocera Latreille, 1829