Anostraca

Anostraca is on of the four orders of crustaceans that compose the class Branchiopoda, the members of this order are more commonly referred to as "Fairy Shrimp". They swim upside-down and feed by filtering organic particles from the water or by scraping algae from surfaces. They are typically found in vernal ponds and hypersaline lakes (landlocked lakes that contain high levels of sodium chloride and other salts, these saline levels pass that of ocean water) as well as deserts and colder regions such as ice covered mountains. They are an important food source for many birds and fish, and some are cultured and harvested for use as fish food in many regions. There are 300 species spread across 8 families.

Taxonomy

Fairy Shrimp Taxonomy

	Kingdom	Phylum	Subphylum	Class	Subclass	Order
Classification	Animalia	Arthropoda	Crustacea	Branchiopoda	Sarsostraca	Anostraca

Description

A Fairy Shrimps' body is typically elongated and divided into multiple sections, these sections are typically divided into three distinct parts: head, abdomen, and thorax. The whole animal can typically measure anywhere from .25 - 1 inch long (6-25 millimeters), however some species may not even reach sexual maturity until they are 2 inches (50mm) long with the ability to grow up to almost 7 inches (170 mm)[1]. Fairy shrimp possess a thin but flexible exoskeleton that unlike that of other arthropods do not have a carapace (an upper section of exoskeleton found in many groups of animals) [1].

Head

The head of Fairy Shrimp are distinct from the thorax in that the head possesses two compound eyes and then two separate pairs of antennae. The shape of the second pair of antennae differ between male and females of the species in that the males second pair of antennae are enlarged and specialized (instead of long and cylindrical) in order to be able to better hold females during mating [7].

Thorax and Abdomen

When it comes to the thorax most members of this order have 13 segments with the exception of Polyartemiella and Polyartemia which possess 19 and 21 respectively. All segments but the last two are similar in that they have a pair of flattened leaf-like appendages [1]. The last two segments of the body are fused together and the appendages are then specialized for sexual reproduction. Most anostracans have separate sexes, however a few reproduce by parthenogenesis. The abdomen is comprised of 6 segments without appendages, and a telson, which bears two flattened cercopods.

Lifestyle

Fairy Shrimp inhabit inland waters that can range from vernal ponds, salt lakes, and lakes at high altitudes or latitudes. Due to their relatively large size and slow means of locomotion Anostracans are then prime candidates to be an easy food supply for predatory fish and waterfowl [2]. This susceptible nature to predation is the reason why they are restricted to areas with a lower quantity of predators.

Anostracans swim gracefully by movements of their phyllopodia connected to their body segments, when swimming anostracans will have their ventral side being the upmost part of their body, it is for this reason that they are typically described as "swimming upside-down" [3]. During this upside down swimming process they filter food indiscriminately from the water as they swim, however they also scrape algae and other organic materials from solid surfaces, for which they turn to have their ventral side against the food surface [5].

A final very important aspect of the Fairy Shrimp lifestyle is their ability to be able to enter diapause. Diapause is the state of biological dormancy where growth and metabolism are arrested, as an egg (or cyst) [4]. This is an especially important biological trait in that it assists in both species' dispersal and in overcoming adverse environmental conditions. Once a Fairy Shrimp becomes dormant, these cysts can withstand conditions as harsh and diverse as droughts, frosts, hypersalinity, complete desiccation, exposure to UV radiation and in the most extreme scenario; the vacuum of space. Diapause is also the best way for the fairy shrimps to colonize new habitats—facilitated by a variety of conditions including wind, predators, and currents as the soft-bodied adults are unable to leave the freshwater system [4].

Environmental Impact

Members of this order, most notably Brine Shrimp, are used as food for fish and other organisms in aquaria and aquaculture. Their drought-resistant eggs (due to their ability to preform diapause) are collected from lakeshores and are stored and transported dry. They hatch readily when submerged in salt water. This is a multimillion-dollar industry, centered on the Great Salt Lake in Utah and San Francisco Bay in California; adults are collected from Mono Lake and transported frozen [6].

References

[1] Peter H. H. Weekers; Gopal Murugan; Jacques R. Vanfleteren; Denton Belk; Henri J. Dumont (2002). "Phylogenetic analysis of anostracans (Branchiopoda: Anostraca) inferred from nuclear 18S ribosomal DNA (18S rDNA) sequences" Pg. 535–544. doi:10.1016/S1055-7903(02)00289-0. PMID 12450757.

[2] Vernal Pool Fairy Shrimp. National Wildlife Federation. (n.d.). Retrieved May 10, 2022, from https://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Vernal-Pool-Fairy-Shrimp

[3] “Fairy Shrimp.” 2022. Fairy Shrimp | Merced Vernal Pools & Grassland Reserve. Accessed May 10. https://vernalpools.ucmerced.edu/ecosystem/reserve-fairy-shrimp.

[4] Fryer, Geoffrey (1996-03-01). "Diapause, a potent force in the evolution of freshwater crustaceans". Hydrobiologia. 320 (1–3): 1–14. doi:10.1007/bf00016800

[5] Denton Belk (2007). "Branchiopoda". In Sol Felty Light; James T. Carlton (eds.). The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon (4th ed.). University of California Press. pp. 414–417. ISBN 978-0-520-23939-5.

[6] J. M. Melack (2009). "Saline and soda lakes". In Sven Erik Jørgensen (ed.). Ecosystem Ecology. Academic Press. pp. 380–384. ISBN 978-0-444-53466-8.

[7] William David Williams (1980). "Arachnids and Crustaceans". Australian Freshwater Life: the Invertebrates of Australian Inland Waters (2nd ed.). Palgrave Macmillan Australia. pp. 118–184. ISBN 978-0-333-29894-7