(ə′näs·trə·kə)

(invertebrate zoology) An order of shrimplike crustaceans generally referred to the subclass Branchiopoda.

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(Fairy shrimps)

Phylum: Arthropoda

Subphylum: Crustacea

Class: Branchiopoda

Order: Anostraca

Number of families: 8

Thumbnail description
Lower crustaceans with elongated bodies and paired eyes on stalks; the body lacks a carapace (hard or bony shell)

Evolution and systematics

On the basis of evidence from the fossil order Lipostraca, and the Upper Cambrian species Rehbachiella kinnekullensis, the anostracan line apparently split off at a very early stage from the rest of the Branchiopoda, about 500 million years ago. Fairy shrimps are widely considered the most primitive living crustaceans. Currently, scientists count eight families in two suborders within the Anostraca. This number is lower than the 11 families listed in older reference works. The change in taxonomy is the result of molecular investigations, which led researchers to group the families Artemiidae and Parartemiidae together to form the suborder Artemiina; and combine the families Chirocephalidae, Branchinectidae, Thamnocephalidae, Branchipodidae, Tanymastigidae, and Streptocephalidae to form the suborder Anostracina. DNA analysis also showed that the (sub)arctic genera Polyartemia and Polyartemiella, which have 17 and 19 pairs of limbs respectively as opposed to the usual 11, do not necessarily represent the most primitive anostracans. Rather, these genera may have resulted from mutations in homeobox genes, which are DNA sequences whose function is to divide an embryo into bands of tissue that will develop into specific organs. In females of the genus Parartemia, the eleventh pair of limbs may be reduced in size or completely missing.

The ancestral form Rehbachiella is known from horizontally banded limestone nodules found in the Orsten formation on the Baltic shore of Sweden. It has a rather large (more than 11) number of limbs, but no or few free abdominal segments. Unlike modern Anostraca, Rehbachiella had paired but sessile (located within the head) eyes. Even in extant fairy shrimps, however, rare cyclopic (one-eyed) mutants occasionally occur. The primitive ancestral anostracan may thus have had two sessile eyes that merged into a single median eye, which separated again at a later stage of evolution into two eyes on stalks.

Physical characteristics

Fairy shrimps are medium-sized branchiopods, usually 0.39–1.18 in (1–3 cm) long; but a few raptorial species, such as Branchinecta gigas may grow as long as 3.9 in (10 cm). The name of the order comes from two Greek words meaning "without" and "piece of hard tile." The fairy shrimp's thoracic limbs are flattened and leaflike, without true joints; the body lacks a carapace (hard or bony shell). Typical anostracans have 11 pairs of limbs, but some atypical species may have as many as 10, 17, or 19 pairs. One peculiar feature of all anostracan species is that they swim upside down. Some are largely translucent and hard to spot in the water; others, however, may develop bands or zones of bright color. The ovisac of females is often deep orange, red, or blue, and the rays in the branches of the tail may also have a distinctive color. The entire animal may develop a bright red or orange color.

The sexes are separate, except in some strains of Artemia, which may be parthenogenetic. Males have modified second antennae that serve to grasp females; the antennae may also demonstrate structural complexity. Females carry a ventral egg sac that may be either short and broad or long and thin, depending on the genus and species.

The eggs or cysts of anostracans are noteworthy because they are surrounded by a thick wall that allows them to resist drought and high temperatures. They develop into a gastrula containing about 4,000 cells, and then stop developing in order to survive adverse conditions. This stage of latency may continue for long periods of time, possibly more than a century in some strains of Artemia. On the other hand, Artemia is the only known genus in which viviparity may occur. In some freshwater streptocephalids, as many as a third of the cysts produced may hatch shortly after being shed and bypass the resting stage.

Distribution

There are no extant marine fairy shrimps, but some species may occur in mountain lakes with almost pure water, while others—mainly Artemia—occur in saturated brine. In the Artemiina, the distribution of Artemia and Parartemia species used to be complementary. Artemia occurred in bodies of salt water on all continents except Australia, and Parartemia only in Australia. In the twentieth century, however, several species of Artemia were successfully introduced in various parts of Australia.

Most families of anostracans are found on three or four continents, but their ranges are often restricted to parts of a continent at the subfamily or genus level. For example, in the Thamnocephalidae, the genus Thamnocephalus is restricted to North America and northern South America, while Dendrocephalus is found exclusively in South America. A peculiar type of disjunct (widely separated) distribution is seen in the genus Branchipodopsis, which is represented in southern Africa by more than 15 species. Elsewhere in Africa, however, it has been reliably recorded only on the horn of Africa and the island of Socotra. The genus also occurs in such widely separated locations as Oman, the Caspian basin, and the Mongolian plateau. At the species (and sometimes genus) level, ranges may be extremely small, often restricted to the type locality. Such is the case with several species of Californian Branchinecta. Other species with extremely small ranges are found further south in Baja California. Another example is the genus Dexteria, limited to the area around Gainesville, Florida, and probably extinct by now as a result of the city's development.

Habitat

The main habitat of fairy shrimps is rain pools. The pools may be filled by periodic and predictable rains, or only erratically at long intervals. A number of anostracans, however, have adapted to two different types of water bodies: high mountain lakes and arctic ponds on the one hand, and saline lakes on the other. The life cycle of fairy shrimps in Arctic or Antarctic ponds is not regulated by alternation between wetting and drying, but by alternation between freezing and thawing. In saline lakes and ponds, such species as Artemia spp., Parartemia spp., and selected Branchinecta may occur from the first inundation, when the salt concentration is low, up to the point of saturation. All these habitats have one common feature: they are free of fishes and other vertebrate predators.

Behavior

Fairy shrimps form swarms that can be quite conspicuous if the animals are brightly colored. On the other hand, many species prefer to live in argillotrophic lakes or pools. The term "argillotrophic" means that the body of water in question produces low levels of phytoplankton because the water is clouded by high levels of suspended clay particles. The animals themselves are responsible for the turbidity, by stirring up sediment from the lake bottom. The turbidity serves a double purpose: it resuspends particles of sediment that are potentially nutritive, and it creates an environment that protects the shrimp from insect and bird predators that hunt by sight.

Beside swimming upside down, fairy shrimp, like zooplankton, migrate vertically over a 24-hour period if they live in sufficiently deep pools; they usually come to the surface at night. Females tend to live below the males in the water column.

Feeding ecology and diet

The vast majority of fairy shrimps are filter feeders. They use specialized endites on their legs to collect food in the ventral food groove. The food is then pushed towards the mouth. Filter feeding allows them to collect particles as small as bacteria and as large as algal cells. Fairy shrimps can consume even rotifers, nauplii (crustacean larvae), and nauplii of their own species.

A few species, like Branchinecta ferox and B. gigas are true raptorial predators, which means that they are adapted to seize prey. They pursue, catch and eat prey the size of large cladocerans and copepods, and other smaller-sized fairy shrimps that share their habitat.

Reproductive biology

Except for some members of the genus Artemia, fairy shrimp are bisexual and oviparous, usually with marked structural differences between the sexes. In males, the second antenna is modified into a remarkably complex clasping organ that is used to hold the female during copulation. In addition to the clasping organ, male anostracans have two penes. Copulation may take place so rapidly as to be hardly visible to the unaided eye, as in most streptocephalids; or last for many hours as the couple swims around in tandem formation, as in Artemia.

Following copulation and internal fertilization, the eggs are deposited in an external brood pouch of variable shape. Each batch, or clutch, may contain several hundred eggs, and a female may produce up to forty clutches in her lifetime; thus total fertility may reach 4,000 eggs per female. The eggs are usually shed freely into the water. They may either sink to the bottom, as is usually the case in freshwater species, or float on the surface, to be deposited eventually along the lake shore.

Conservation status

Anostracan species with wide geographic ranges are usually under little or no threat. In such densely inhabited areas as California's central valley, however, where there is intense competition between urban and agricultural development on the one hand and conservation efforts on the other, many vernal pools have either been drained already or are threatened by obliteration. To a lesser extent, the same is true of endemic species in Baja California and other arid regions of Mexico. Such Florida endemics as Dexteria floridana may already be extinct. The 2002 IUCN Red List includes 28 anostracan species: six are categorized as Critically Endangered; nine as Endangered; 10 as Vulnerable; one as Lower Risk/Conservation Dependent; one as Lower Risk/Near Threatened; and one as Data Deficient. In the United States, five fairy shrimp species are listed as endangered; in California three of these species have been provided with habitat in an attempt to protect them.

Significance to humans

The genus Artemia is of considerable economic importance. The cysts of this species are harvested, cleaned, dried, packed and sold as fish food in the aquarium business. The cysts are also used in industrial aquaculture to feed fish larvae. The Libyan Fezzan desert contains several spring-fed dune lakes that have turned saline with time. Small communities living around these lakes use Artemia as their main source of animal protein. The women collect and dry the shrimp. These communities are called dawada (worm eaters) by the surrounding Arab tribes.

A species of Streptocephalus and a species of Branchinella are found in the hills of northeastern Thailand. These anostracans are fished by local tribespeople and used in a variety of local dishes.

Species accounts

Brine shrimp
Sudanese fairy shrimp

Resources

Books:

Dumont, H. J., and S. V. Negrea. Introduction to the Class Branchiopoda. Leiden, The Netherlands: Backhuys Publishers, 2002.

Erikson, C., and D. Belk. Fairy Shrimp of California's Puddles, Pools and Playas. Eureka, CA: Mad River Press, 1999.

Hamer, M. "Anostraca." In Guides to the Freshwater Invertebrates of Southern Africa, Vol. 1, Crustacea, edited by J. A. Day, B. A. Stewart, I. J. De Moor, and A. E. Louw. Pretoria, South Africa: Water Research Commission, 1999.

Persoone, G., et al. The Brine Shrimp Artemia, 3 vols. Wetteren, Belgium: Universa Press, 1980.

Periodicals:

Belk, D., and J. Brtek. "Checklist of the Anostraca." Hydrobiologia 298 (1995): 315–353.

Brendonck, L. "Redescription of the Fairy Shrimp Streptocephalus proboscideus (Frauenfeld, 1873)." Bulletin de l'Institut Royal des Sciences Naturelles de Belgique 59 (1990):4957.

[Article by: Henri Jean Dumont, PhD, ScD]

An order of branchiopod crustaceans, known as fairy shrimps and, in some cases, brine shrimps. These organisms range up to about 4 in. (100 mm) in length, but usually are much smaller. The trunk consists of 19 to 27 segments, of which usually the first 11, sometimes the first 17 or 19, bear limbs, plus a telson bearing flattened furcal rami (see illustration). The trunk limbs are foliaceous, and all are of the same basic type but differ a little among themselves; each limb is differentiated into a series of endites and is usually filtratory. By beating in metachronal rhythm they propel the animal forward and draw food particles toward it. Anostracans usually swim with their ventral surface uppermost.

Branchinecta paludosa, male, small specimen, lateral aspect.

Most anostracans subsist on minute particles that they sieve from the water with their trunk limbs and pass forward to the mouthparts. Reproduction is usually bisexual, but some brine shrimps are parthenogenetic. Fairy shrimps frequent fresh water, usually temporary pools, in all regions of the world, but are also found in predator-free waters in Arctic and Antarctic regions. See also Branchiopoda.

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IN BRIEF: n. - Small aquatic crustaceans lacking a carapace: fairy shrimps.