Cladocerans are also known as water fleas. They are crustaceans in the Class Branchiopoda and Order Cladocera. All but a few of the 400 species live in freshwater.
Cladocerans are distinctive looking with a large unpaired eye in a foreward facing head on an upright, kidney-shaped body. There is a protruding rostrum with the mouth at the front. They have antennae with long feathery bristles on their heads and a large protruding abdomen. They use their eyes and antennae for locating food and avoiding predators. Their bodies are transparent so one can see their heart beating, what they have eaten and if they are carrying any eggs. They have five or six pairs of legs which they use to move around in the plankton in freshwater ponds and lakes. They can swim by thrusting downward with their second pair of antennae. Most have a tail or spiny projection at the end of the abdomen. Cladocerans are protected by a carapace which, as in other crustaceans, must be shed in order for growth to occur.
Some cladocerans are herbivores, grazing on phytoplankton,or filtering them out of the water, while others are carnivorous, eating other small animals. They are important in fresh water food chains with many small fish and other predators feeding on them.
The most commonly recognised water flea is Daphnia. Several species of Daphnia are grown as fish food and also used to test the pollution levels in water. Daphnia that live in low oxygen water have red, haemoglobin-rich blood. Those that exist in oxygen-rich water have clear blood.
Cladocerans have a brood pouch in which their fertilised eggs are protected. This is a space in the dorsal part of the carapace known as an ephippium and is unique to this group of crustaceans. The brood pouch usually holds five to a dozen eggs. These hatch into juvenile forms. These grow and moult into adolescents and then into mature adults. The life span is variable depending on temperature with the higher the temperature, the shorter the life. On average, the animals live four or five weeks.
They aren't very big or noticeable but if you spend time looking at pond water with a microscope, you will see them.
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