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Branchiopoda, from Ancient Greek βράγχια ( bránkhia), meaning "gill", and πούς ( poús), meaning "foot", is a class of . It comprises Anostraca, 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.
The current upper-level classification of Branchiopoda, according to the World Register of Marine Species (2021), is as follows:
Class Branchiopoda Latreille, 1817
The following cladogram is based off a 2023 phylogenetic analysis using bayesian inference and maximum likelihood:
In addition, the extinct genus Lepidocaris is generally placed in Branchiopoda.
Anostraca
The fairy shrimp of the order Anostraca are usually long (exceptionally up to ). Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace. They live in and across the world, including pools in , in ice-covered mountain lakes and in Antarctica. They swim "upside-down" and filter feeder organic particles from the water or by scraping algae from surfaces. They are an important food for many birds and fish, and are cultured and harvested for use as fish food. There are 300 species spread across 8 families.
Lipostraca
Lipostraca contains a single extinct Early Devonian species, Lepidocaris, (2025). 9781840760415, Manson Publishing. ISBN 9781840760415 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 , but no carapace. It occurred chiefly among , probably in alkaline vernal pool.
Notostraca
The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, Triops and Lepidurus, are considered , having not changed significantly in outward form since the Triassic. They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. The telson is flanked by a pair of long, thin caudal rami. Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. The evidence of phenotypic plasticity of Arctic tadpole shrimp ( Lepidurus arcticus, Notostraca) has been observed in Svalbard. Notostracans are the largest branchiopodans and are living on the bottom of , ponds and shallow lakes.
Laevicaudata, Spinicaudata and Cyclestherida (once Conchostraca)
Clam shrimp are bivalved animals which have lived since at least the Devonian. The three groups are not believed to form a clade. They have 10–32 trunk segments, decreasing in size from front to back, and each bears a pair of arthropod leg which also carry . A strong muscle can close the two halves of the shell together.
Anomopoda, Ctenopoda, Onychopoda, and Haplopoda (once Cladocera)
These four orders make up a group of small commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. 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. (2025). 9780471358374, John Wiley and Sons. ISBN 9780471358374 There is a single median compound eye. Most species show cyclical parthenogenesis, where asexual reproduction is occasionally supplemented by sexual reproduction, which produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats. 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.
Evolution
The fossil record of branchiopods extends back at least into the Upper Cambrian and possibly further. The group is thought to be monophyly, with the Anostraca having been the first group to cladogenesis. It is thought that the group evolved in the seas, but was forced into vernal pool and by the evolution of Osteichthyes. Although they were previously considered the sister group to the remaining crustaceans, it is now widely accepted that crustaceans form a paraphyletic group, and Branchiopoda are thought to be sister to a clade comprising Xenocarida (Remipedia and Cephalocarida) and Hexapoda ( and their relatives).
See also
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