By Judy Follo and Daphne G. Fautin
Approximately 800 species of branchiopods are found worldwide in freshwater ponds, lakes, and inland saline waters such as the Great Salt Lake in Utah. Their fossil record includes the extinct order Lipostraca and dates back to the Devonian period (approximately 400 - 360 million years ago). Some references recognize four extant orders: Anostraca (fairy shrimps), Notostraca (tadpole shrimps), Cladocera (water fleas), and Conchostraca (clam shrimps). Members of these orders are commonly used for aquarium fish food, scientific research, and once were marketed as pets called sea monkeys. These small crustaceans are a very important source of food for fish and birds in nature.
Distinguishing features of a branchiopod include a small body (0.25 mm -
10 cm long), paired compound eyes, single simple eye, simple mouth parts,
leaflike or phyllopodous appendages, and minimal body tagmosis. The
nervous system and sensory system are simple, although some species
vibrate their compound eyes to gather more visual information.
Branchiopods use their leaf-like appendages for feeding, locomotion, and
respiration. Gathered food particles are pushed into a 
ventral food
groove that leads to the mandibles , then the mouth and, in turn, a
complete gut . Undigested particles exit through the anus and nitrogenous
wastes are eliminated through maxillary glands. These glands, located
near the maxillae , also function in osmoregulation. Locomotion is
generally achieved by metachronal beating of the appendages. The leaflike
portions of the appendages have a large surface area that functions in gas
exchange. Since most branchiopods are small with a thin cuticle, gas
exchange can occur across the body wall as well. The circulatory system
includes a heart that pumps blood into an open body cavity or hemocoel.
The pigment in the blood of some species is hemoglobin.
Brine shrimp, of the order Anostraca, are most noted for their desiccation-resistant eggs that will hatch in salt water. Most species of branchiopods are gonochoric and some are parthenogenetic. Those species that have indirect development produce nauplius larvae.
References:
Brusca, R.C. and G.J. Brusca. 1990. Chapter 18: Phylum Arthropoda: The Crustaceans. Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusetts.
Kozloff, E.N. 1990. Chapter 17: Subphylum Crustacea. Invertebrates. Saunders College Publishing. Philadelphia and other cities.
Encyclopedia Britannica, 2003. Branchiopod. Encyclopedia Britannica Online 
http://search.eb.com/eb/article?eu=118970
Contributors
Judy Follo (author). Daphne G. Fautin (author).
