Branchiopoda is a group of primitive and primarily fresh water crustaceans, mostly resembling shrimp. There are over 900 known species worldwide. A few are well-known, including Artemia (brine shrimp, called Sea-Monkeys when marketed as novelties), and Daphnia, both of which are raised as aquarium food or as interesting pets in and of themselves. Branchiopods should not be confused with Brachiopods.
In older classifications, the superorder Diplostraca was made up of the two orders Conchostraca and Cladocera. According to newer data, however, these orders may be artificial (at least in their present state) as some of their members seem to have a paraphyletic origin within the Diplostraca. For this reason, the order Conchostraca is no longer used.
Although a few species of water flea have adapted to a life in the sea, these crustaceans belong primarily in fresh water. The oldest known species is a 500 million year old fossil fairy shrimp called Rehbachiella kinnekullensis. Fairy shrimps are also seen as the most original and primitive members of the class. Branchiopods in general are equipped with a ventral food groove, useful for suspension and filter feeding. The water current in the ventral food groove, used for breathing and feeding in most species, is produced by a battery of unspecialised legs. This is thought to resemble a very original way of living among the crustaceans.
They probably originated in marine environments, but only those who migrated to fresh water survived. The fact that they are especially adapted to temporary pools and waters that are too extreme for other animals to live in (like salt lakes) indicates that they could have sought refuge in these places because they were unable to compete with or evade the more advanced groups of animals evolving in their original habitats. This resulted in a short generation cycle and small body size. The large branchiopods, Anostraca, Notostraca and Conchostraca (even if it the last one seems to be a partially artificial order) are considered to be the most primitive, and most of them are still unable to live in waters where there are fish and other advanced predators, since they are too slow and vulnerable to survive them. A few of them, however, have evolved to cope with this problem well enough to survive, even with predators around them. The small branchiopods, mostly represented by water fleas, have succeeded in becoming zooplankton in such a degree that waters filled with fish and other threats are no longer a problem. Their main defence is their high number thanks to their small size, ability to produce many offspring and short life cycle.
Clam shrimp live up to their name as they are often seen burrowed in the mud like mussels at the bottom of temporary ponds. They are so well adapted to this extreme way of life that they are able to reach adulthood within a few days after inundation under optimal conditions. They can reproduce sexually, hermaphroditically or parthenogenetically.