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Class of crustaceans

Branchiopoda is a class of crustaceans. It comprises fairy shrimp, clam shrimp, Diplostraca (or Cladocera), Notostraca, the Devonian Lepidocaris and possibly the Cambrian Rehbachiella. They are mostly small, freshwater animals that feed on plankton and detritus.

Members of the Branchiopoda are unified by the presence of gills on many of the animals' appendages, including some of the mouthparts. This is also responsible for the name of the group (from the βράγχια, gills, akin to βρόγχος, windpipe; πούς, foot). They generally possess compound eyes and a carapace, which may be a shell of two valves enclosing the trunk (as in most Cladocera), broad and shallow (as in the Notostraca), or entirely absent (as in the Anostraca). In the groups where the carapace prevents the use of the trunk limbs for swimming (Cladocera and clam shrimp), the antennae are used for locomotion, as they are in the nauplius. Most branchiopodans eat floating detritus or plankton, which they take using the setae on their appendages.

In early taxonomic treatments, the current members of the Branchiopoda were all placed in a single genus, Monoculus. They live in vernal pools and hypersaline lakes across the world, including pools in deserts, in ice covered mountain lakes and in Antarctica. They swim "upside down" and feed by filtering organic particles from the water or by scraping algae from surfaces.

Lipostraca contains a single extinct Early Devonian species, Lepidocaris rhyniensis, which is the most abundant animal in the Rhynie chert deposits. It resembles modern Anostraca, to which it is probably closely related, although its relationships to other orders remain unclear. The body is long, with 23 body segments and 19 pairs of appendages, but no carapace. It occurred chiefly among charophytes, probably in alkaline temporary pools.

The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, Triops and Lepidurus, are considered living fossils, having not changed significantly in outward form since the Triassic. The evidence of phenotypic plasticity of Arctic tadpole shrimp (Lepidurus arcticus, Notostraca) has been observed in Svalbard. Notostracans are the largest branchiopodans and are omnivores living on the bottom of temporary pools, ponds and shallow lakes. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down turned head, and a carapace covering the apparently unsegmented thorax and abdomen. There is a single median compound eye. In the water bodies of the world, a lot of Cladocera are non native species, many of which pose a great threat to aquatic ecosystems.

The fossil record of branchiopods extends back at least into the Upper Cambrian and possibly further. The group is thought to be monophyletic, with the Anostraca having been the first group to branch off. It is thought that the group evolved in the seas, but was forced into temporary pools and hypersaline lakes by the evolution of bony fishes.