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" or "brine shrimp". They swim upside-down and are typically found in vernal ponds and hypersaline lakes (landlocked lakes that contain high levels of sodium chloride and other salts) 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 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 causes their range to be 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 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, hyper salinity, complete desiccation, exposure to UV radiation, and 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].
Feeding
Different anostraca species eat different substances which varies from algae, plankton and organic particles from the water. Most anostracans feed on algae by scraping hard algae covered surfaces with their legs which then makes the algae more accessible to feed on. They also use their legs to filter feed organic particles from the water.
Environmental Impact
Members of this order, most notably brine shrimp, are used as food for fish and other organisms in aquaria and aquaculture. In natural environments they are also eaten by birds, water boatmen, fishes and other crustaceans. 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].
Diversity
Of the four orders of Branchiopoda, Anostraca has the widest diversity range. There are approximately 313 species in it, arranged into 26 genera and eight families:
- Artemiidae – 1 genus, 8 species
- Branchinectidae – 1 genus, 45 species
- Branchipodidae – 5 genera, 35 species
- Chirocephalidae – 9 genera, 81 species
- Parartemiidae – 1 genus, 18 species
- Streptocephalidae – 1 genus, 56 species
- Tanymastigidae – 2 genera, 8 species
- Thamnocephalidae – 6 genera, 62 species
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
[8] The Editors of Encyclopaedia Britannica. “Brine Shrimp | Crustacean.” Encyclopædia Britannica, 2019, www.britannica.com/animal/brine-shrimp.
[9] Luc Brendonck; D. Christopher Rogers; Jorgen Olesen; Stephen Weeks; Walter R. Hoch (2008). "Global diversity of large branchiopods (Crustacea: Branchiopoda) in freshwater". In Estelle V. Balian; Christian Lévêque; Hendrik Segers; Koen Martens (eds.). Freshwater Animal Diversity Assessment. Developments in Hydrobiology 198. pp. 167–176. doi:10.1007/s10750-007-9119-9. ISBN 978-1-4020-8258-0. S2CID 46608816. Reprinted from Hydrobiologia, Volume 595