Branchiopoda

Branchiopods Fossil range: 500–0 Ma PreЄ Є O S D C P T J K Pg N
Triops, a notostracan
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Branchiopoda Latreille, 1817
Orders
Subclass Sarsostraca Anostraca Subclass Phyllopoda Lipostraca† Notostraca Infraclass Diplostraca Laevicaudata Spinicaudata Cyclestherida Cladocera

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.

Evolution

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Adult fairy shrimp, Branchinecta packardi, an anostracan

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.

Characteristics

The defining feature of the branchiopods is the presence of flattened leaf-like abdominal appendages, to which the gills are attached. These appendages also have hairs for filtering food from the water, and in some groups are used to aid swimming. In many branchiopods, however, the main source of propulsion however, comes from a long second pair of antennae.^[1]

The number of thoracic segments varies from species to species.

The structure of the reproductive, nervous and circulatory systems is primitive compared to other crustaceans. Some branchipods do possess a small amount of haemoglobin in their blood, but this may depend on the amount of oxygen in the water, with the pigment disappearing when the animal is placed in well-oxygenated water, and returning in stagnant water. The heart is tubular or sac-like, and the aorta is the only blood vessel, with blood otherwise circulating freely through the body. Nitrogenous waste is excreted through maxillary glands.^[1]

Almost all branchiopods are free-living, although there are a small number of ectoparasitic species.

The female broods the eggs for a time after fertilisation, usually either beneath the carapace or in a sac derived from the uterus. The eggs hatch into miniature versions of the adult, with no free-living larval stage. Many species are partially or wholly parthenogenetic.^[1]

Clam shrimp

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.^[2]

References