Anostraca
Anostraca is one 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 are higher than the saline levels 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 fish food in many regions. There are 300 species spread across 8 families.
Taxonomy
| Kingdom | Phylum | Subphylum | Class | Subclass | Order | |
|---|---|---|---|---|---|---|
| Classification | Animalia | Arthropoda | Crustacea | Branchiopoda | Sarsostraca | Anostraca |
Description
A fairy shrimp's body is typically elongated and divided into three distinct parts: head, abdomen, and thorax. The whole animal can measure anywhere from .25 - 1 inch long (6-25 millimeters), however some species may not 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 other arthropods does 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 males and females of the species; 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
Most members of this order have a thorax with 13 segments with the exception of Polyartemiella and Polyartemia which possess 19 and 21 segments 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 fairy shrimp reproduce sexually, however a few reproduce by parthenogenesis. The abdomen is comprised of 6 segments without appendages, and a telson, which bears two flattened cercopods.
Ecology
Fairy shrimp inhabit inland waters including: vernal ponds, salt lakes, and lakes at high altitudes or latitudes. Due to their relatively large size and slow means of locomotion, Anostracans are easy prey for predatory fish and waterfowl [2]. This susceptibility to predation is the reason why they are restricted to areas with a lower quantity of predators.
Anostracans swim gracefully by moving their phyllopodia which are connected to their body segments. When swimming, fairy shrimp will have their ventral side facing upward, which is why it is said that they are "swimming upside-down" [3]. While swimming they filter food indiscriminately from the water, and also scrape algae and other organic materials from solid surfaces. To do this, they turn to have their ventral side against the food surface [5].
Fairy shrimp have the ability to enter diapause, which is a state of biological dormancy where growth and metabolism are arrested, as an egg (or cyst) [4]. This is an especially important biological trait because it assists in both species' dispersal and 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 even the vacuum of space. Diapause is also the best way for the fairy shrimp to colonize new habitats—facilitated by a variety of conditions including wind, predators, and currents as the 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 are collected from lakeshores and then stored and transported dry, after transportation they hatch readily when submerged in salt water. This is a multimillion-dollar industry centered in 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