The Order Anostraca (Phylum Arthropoda, Subphylum Crustacea, Class Branchiopoda) includes the fairy or brine shrimps. Worldwide, there are 300 species within twenty-six genera placed in eight families: Artemiidae (one genus, nine species), Branchinectidae (one genus, forty-five species), Branchipodidae (five genera, thirty-five species), Chirocephalidae (nine genera, eighty-one species), Parartemiidae (one genus, thirteen species), Streptocephalidae (one genus, fifty-six species), Tanymastigidae (two genera, eight species), and Thamnocephalidae (six genera, sixty-two species). In Arkansas, seven anostracan species are known: Eubranchipus neglectus, E. serratus, E. moorei, Branchinecta packardi, Thamnocephalus platyurus, Streptocepalus sealii, and S. texanus.
Fairy shrimps are very primitive organisms believed to have diverged during the Ordovician period from the main line of the Branchiopoda. Their fossil record dates back to the Devonian, although some studies report fossils resembling fairy shrimp in the Upper Cambrian. Anostracans are the most diverse of the four orders of the Class Branchiopoda and differ from all other branchiopods by lacking a carapace. The monophyly of this order has been well documented, and most scientists believe Anostraca was the first group to branch off from the Branchiopoda.
Fairy shrimp are exemplary residents of vernal (temporary) ponds and pools, snowmelt pools, hypersaline lakes, desert pools, ice-covered mountain lakes, and even environs of Antarctica. They are most commonly found during spring and early summer months. These crustaceans are generally found in bodies of water that are too small, too temporary, or too saline for fishes.
Anostracans are distributed worldwide on all seven continents. Interestingly, the radiation hypothesis, which favors rapid spread and colonization during the Gondwana fragmentation, closely resembles the current distribution of the order. Most of the extant genera have restricted geographical distributions, with only three genera (Artemia, Branchinella, and Branchinecta) being widespread across the remnants of the former supercontinent Pangaea. The remainder inhabit the former geographical area known as Laurasia.
Fairy shrimps typically tend to be 6.0 to 25 mm (0.24–0.98 in.) long (exceptionally large specimens are up to 170 mm or 6.7 in., and females are slightly larger than males) with a head, thorax, and abdomen. The head region is distinct from the thorax and possesses a pair of compound eyes placed on prominent stalks, a single median eye, and two pairs of antennae. The first antenna is small, usually unsegmented, and uniramous. In females, the second antenna is long and cylindrical, while the male has an enlarged second antenna specialized for holding the female during copulation. An additional frontal appendage is present in some species. The thorax of a fairy shrimp is generally divided into thirteen segments, each of which are similar, except the last two, which are fused together and used in reproduction. All of the first eleven thoracic segments have a pair of biramous phyllopods, which are flattened, leaf-like appendages that function as swimming legs. The abdomen has six segments without appendages, and there is a telson that bears two flattened caudal rami. The exoskeleton is thin and flexible, and gas exchange is assumed to occur over the entire body surface, but especially through the phyllopodia and their associated gills, which are responsible for osmotic regulation. Although most fairy shrimp species have male and female members, a few reproduce by parthenogenesis.
Fairy shrimp swim “upside-down,” or ventral side up, and feed by filtering organic particles from the water or by scraping algae from surfaces such as rocks and sunken wood. At least two species (Branchinecta gigas and B. raptor) are specialized predators on other fairy shrimps. In turn, they are an important food for many fishes and birds and are cultured and harvested for use as fish food. In addition, there is a multimillion-dollar business based on brine shrimp eggs in the Great Salt Lake area of Utah and the San Francisco area of California, as well as an area from the saline soda Mono Lake in the dry Great Basin, where adults are collected and shipped frozen to dealers. Tropical fish hobbyists have used brine shrimp eggs and frozen adults as a wonder food for their fishes for decades. The key to the brine shrimp egg business is the drought-resistant eggs that are collected from the shores of lakes, carefully stored, and later transported dry. The brine shrimp readily hatch when submerged in saltwater.
The distribution of these fairy shrimps is often a mystery. Birds have been implicated in their movement between areas. In addition, anostracan eggs have been suggested to be passed through the digestive tracts of predatory diving beetles of the family Dytiscidae and, thus, transported to various isolated ponds and lakes.
The following seven anostracan species have been documented from Arkansas:
- Eubranchipus neglectus occurs in Alabama, Kentucky, and northern Ohio, and has been documented from Craighead, Greene, and Mississippi Arkansas populations lie along the southern border of their reported range.
- Eubranchipus moorei is reported to occur within the Gulf Coastal Plain from St. Tammany Parish, Louisiana, to Baker County, Georgia. In Arkansas, it has been documented from Jackson and Lawrence The Arkansas records extend the range of this species approximately 600 km (966 mi.) to the north, and into the Mississippi Embayment. All known Arkansas sites to date lie within a small area of northern Jackson and southern Lawrence counties in shallow depressions, usually in bottomland hardwood forests, but all in low rolling sandy hills parallel to the Black River valley. Three sites are near the Black River, while the remaining two sites are within 1 km (1.6 mi.) of Village Creek, a major tributary of the Black River. Waterfowl may be the vehicle for the distribution of E. moorei to the north as has been reported for some anostracan species in eastern Asia. Ducks have been observed feeding at two of the sites adjacent to the Black River.
- Eubranchipus serratus is known from British Columbia, Canada, and in the United States it ranges from the east to the west coasts south to Arizona, Oklahoma, and Virginia. Within Arkansas, E. serratus is known from two counties: Johnson and Mississippi.
- Branchinecta packardi is known from Alberta and Saskatchewan, Canada, and Arizona, Colorado, Kansas, Montana, North Dakota, New Mexico, Nebraska, Oklahoma, Texas, Utah, Wyoming, and several states of Mexico. In Arkansas, it occurs in Mississippi, Jackson, and Stone These Arkansas records lie considerably to the east of the Great Plains, where it is common. Both the Ozark Plateaus and Mississippi Embayment sites are adjacent to rivers of moderate size, and waterfowl may be the distributors there.
- Thamnocephalus platyurus has been reported from Arizona, California, Colorado, Kansas, Missouri, New Mexico, Nebraska, Nevada, Oklahoma, Texas, Utah, Wyoming, and Mexico in addition to Arkansas. Within Arkansas, it is known from the Ozark Plateau in Johnson County.
- Streptocepalus sealii is widely distributed from southern Canada through the United States into southern Mexico. In Arkansas, this species has been found in Poinsett County. The Arkansas site extends the known distribution east of the Great Plains, where it is common, into the Mid-South. The site, in the Ozark Plateaus, lies within the Arkansas River Valley, and again waterfowl may be the agents of dispersal as they move down this river out of the Great Plains.
- Streptocephalus texanus its distribution of extends through Arizona, California, Colorado, Kansas, Missouri, Montana, New Mexico, Nebraska, Oklahoma, Texas, Utah, Wyoming, and into Mexico. In Arkansas, this anostracan has been collected from near Hartman (Johnson County).
Anostracan diversity depends in general on two factors: (1) chemical heterogeneity among (mostly temporary) habitats and thermal variation resulting both from ponds filling at different seasons and, (2) altitudinal and latitudinal effects. Based on limited data for Arkansas species, the influence of seasonal variation appears to be important. Collections of Eubranchipus species (E. neglectus, E. serratus, E. moorei) have been during January, February, and March, when mean monthly air temperatures in northeastern Arkansas are 4.2° C (39.6° F), 5.8° C (42.4° F), and 10.3° C (50.5° F), respectively. Branchinecta packardi was collected in February and May (mean monthly temperatures of 5.8° C [42.4° F] and 21.2° C [70.2° F], respectively). Thamnocephalus was collected in May (mean monthly temperature of 21.2° C [70.2° F]), and Streptocephalus texanus was taken in late May, while Streptocephalus sealii was taken during late June to mid-July (mean monthly temperatures of 25.7° C [78.3° F] and 27.3° C [81.1° F]).
For additional information:
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Henry W. Robison
Chris T. McAllister
Eastern Oklahoma State College
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