Anostraca is one of the four orders of crustaceans in the class Branchiopoda; its members are referred to as fairy shrimp. They live in vernal pools and hypersaline lakes across the world, and they have even been found in deserts, ice-covered mountain lakes and Antarctic ice. They are usually 6–25 mm (0.24–0.98 in) long (exceptionally up to 170 mm or 6.7 in). Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace. They swim "upside-down" and feed by filtering organic particles from the water or by scraping algae from surfaces. They are an important food for many birds and fish, and some are cultured and harvested for use as fish food. There are 300 species spread across 8 families.
The body of a fairy shrimp is elongated and divided into segments.[3] The whole animal is typically 6–25 millimetres (0.24–0.98 in) long, but one species, Branchinecta gigas does not reach sexual maturity until it reaches 50 mm (2.0 in) long, and can grow to 170 mm (6.7 in) long.[3] The exoskeleton is thin and flexible,[3] and lacks any sign of a carapace.[4] The body can be divided into three distinct parts (tagmata) – head, thorax and abdomen.[4]
The head is morphologically distinct from the thorax. It bears two compound eyes on prominent stalks, and two pairs of antennae.[5] The first pair of antennae are small, usually unsegmented, and uniramous. The second pair are long and cylindrical in females, but in males they are enlarged and specialised for holding the female during mating.[5] In some groups, males have an additional frontal appendage.[5]
The thorax of most anostracans has 13 segments (19 in Polyartemiella and 21 in Polyartemia).[6] All but the last two are very similar, with a pair of biramous phyllopods (flattened, leaf-like appendages).[4] The last two segments are fused together,[3] and their appendages are specialised for reproduction.[6] Most anostracans have separate sexes (gonochorism), but a few reproduce by parthenogenesis.[7] The abdomen comprises 6 segments without appendages, and a telson,[6] which bears two flattened caudal rami or "cercopods".[3]
The head contains two digestive glands and the small lobate stomach into which they empty. This is connected to a long intestine, which terminates in a short rectum, with the anus located on the telson.[5] The haemocoel of anostracans is pumped by a long, tubular heart, which runs through most of the animal's length.[5] A series of slits allow haemocoel into the heart, which is then pumped out of the anterior opening by peristalsis.[5] The nervous system consists of two nerve cords which run the length of the body, with two ganglia and two transverse commissures in most of the body segments.[5]
Gas exchange is thought to take place through the entire body surface, but especially that of the phyllopodia and their associated gills, which may also be responsible for osmotic regulation.[5] Two coiled glands at the bases of the maxillae are used to excrete nitrogenous waste, typically in the form of urea.[5] Most of the animal's nitrogenous waste is, however, in the form of ammonia, which probably diffuses into the environment through the phyllopodia and gills.[5]