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 anostra-cans 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

Brine shrimps (Artemia franciscana) are a very important part of the ecosystem in saltwater and brackish habitats. (Photo by E. R.Degginger/Photo Researchers, Inc. Reproduced by permission.)

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.

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