aka: Opossum Shrimps
Mysida is an order of small, shrimp-like crustaceans in the Class Malacostraca and Superorder Peracarida, with two families, Mysidae (nine subfamilies) and Petalopthalmidae (two subfamilies); 178 genera; and 1,132 species. Most are marine species, but there are about 72 freshwater species, being predominantly found in the Palearctic and Neotropical biogeographical realms. The first report of a species of mysid shrimp documented in Arkansas was in 2012.
Species within the order are found throughout the world across a broad range of habitats, such as subterranean, freshwater, and brackish. They can also be found in shallow coastal waters and in surface to deep-sea habitats. In marine waters, they can be benthic (living on the seabed) or pelagic (living in mid-water), but they are also important in some freshwater and brackish ecosystems. However, most are found close to shore, crawling on or burrowing into the mud or sand. Most marine species are benthic by day but leave the seabed at night to become planktonic. Many benthic species make daily migrations into higher parts of the water column. For example, Mysis relicta and its close relatives inhabit cold, deep lakes and have a diurnal cycle of vertical migrations. Some mysids live among algae and seagrasses; some are solitary, while many form dense swarms. In general, they are free-living, but a few species, mostly in the subfamily Heteromysinae, are commensal and are associated with sea anemones and hermit crabs. Several taxa have also been described from various freshwater habitats and caves.
The fossil history of this group is extensive, although the soft-bodied Mysida are generally poorly predisposed for fossilization. However, most species of the family Mysidae possess mineralized, endogenous statoliths as main components in their statocysts. A large number of mysidacean fossils are preserved as molds in Carboniferous to Permian sediments. The earliest fossil record from an extant order of mysidaceans is of two species from the Carboniferous period in North America and include the lophogastridans Peachocaris strongi and P. acanthouraea. The earliest fossils credited to the Mysida period are the species Elder unguiculata and Francocaris grimmi from the Jurassic period in Bavaria, Germany.
The first mysidacean species were described in the eighteenth century; however, over 200 years later, there is still debate over the taxonomic organization of the group, as well as where the Mysidacea fit within the Crustacea. Customarily, Mysida were integrated with another, the Lophogastrida, into a broader order Mysidacea, an externally similar group of pelagic crustaceans, but that classification has been found to be invalid. While the previous aggregate had good morphological support, molecular studies did not verify the monophyly of this group. Previously, Mysida included the families Lepidomysidae and Stygiomysidae, but with the inclusion of DNA sequence data of ribosomal genes, these have now been placed in a separate order removed from the Mysida, the Stygiomysida. Nevertheless, although there have been additions and revisions, the taxonomy of the higher Mysida has remained relatively stable.
Although, in many respects, mysids appear similar to some shrimps, the main characteristic separating them from the superorder Eucarida (decapods, krill, Amphionides, and Angustidontida) is their lack of free-swimming larvae. The mysid’s head possesses a pair of large, stalked eyes and two pairs of antennae. The head and first segment (or sometimes the first three segments) of the thorax are fused to form the cephalothorax. The thorax is composed of eight segments, each having branching limbs, entirely concealed beneath a protective carapace, and the abdomen has six segments and usually further smaller limbs. The first two thoracic segments bear maxillipeds, which are used to filter plankton and organic particulate from the water. The other six pairs of thoracic appendages are branching (biramous) limbs known as pereopods, and are used for swimming, as well as for wafting water toward the maxillipeds for feeding. Beneath the thorax, unlike the Caridea (true shrimps), females have a marsupium (brood pouch). This structure is enclosed by the large bristly flaps (flexible oostegites) that extend from the basal segments of the pereopods and form the floor of a chamber roofed by the animal’s sternum. This chamber is where the eggs are brooded, development being direct in most cases. Although these may be absent or vestigial in females, the abdomen has six segments, the first five of which bear pleopods. In males, the fourth pleopod is longer than the others and has a specialized reproductive function.
Most species are from 5 to 25 mm (0.2–1.0 in.) in length and are variable in color, ranging from pale and transparent to bright orange or brown. Within the superorder Peracarida, they differ from other species by featuring statocysts on their uropods (located on the last abdominal segment). These aid the organism in orienting itself in the water and are clearly observed as circular vesicles. Together with the pouch, the statocysts are often used as taxonomic features that help distinguish mysids from other shrimp-like organisms.
Many mysids (pelagic and most other species) are omnivorous pelagic or epi- to hyperbenthic, omnivorous filter feeders that feed on algae, detritus, and zooplankton. As filter feeders, they generate a feeding current with the exopods of their pereopods. This wafts food particles into a ventral food groove along which they are passed before being filtered on the second maxillae by setae (bristles). Larger planktonic prey can also be caught in a trap composed of the endopods of the thoracic appendages. Scavenging and cannibalism are also common, with the adults sometimes cannibalizing their own young once they emerge from the marsupium. In turn, mysids also have many predators and form an important part of the diet of such fish as shad and flounder.
In the laboratory, some mysids are cultured for experimental purposes and are used as a model food source for other cultured marine organisms. Due to their small size and low cost, they are often fed to fish larvae, cephalopods, and commercially farmed shrimp. Their high protein and fat content also makes them a good alternative to live enriched brine shrimp (Artemia) when feeding juvenile fishes (especially those that are difficult to maintain, such as young seahorses) and other small fauna. In these culture systems, brine shrimp juveniles incubated for twenty-four hours are the most common food for mysids and are sometimes enriched with highly unsaturated fatty acids to increase their nutritional value. In addition, mysids are sensitive to water pollution, so they are sometimes used as bio-indicators to monitor water quality.
In terms of reproduction, individual mysids are separate sexes, either male or female, and fertilization is external. The gonads are located in the thorax and are tubular in shape. Males possess two gonopores on the eighth thoracic segment and a pair of long penises. The female gonopores are on the sixth thoracic segment and, to form a brood pouch, the oostegites are attached to the first to seventh pereopods. Copulation usually takes place at night and lasts only a few minutes. During the process, the male inserts his penises into the marsupium and releases sperm. This stimulates the female, and, within an hour, the eggs are usually released into the marsupium. Here, they are fertilized and retained, with direct development of the embryos in the brood pouch and with the young hatching from the eggs as miniature adults. The size of a mysid brood generally correlates with body length of the female and other factors such as density and food availability. The age at which mysids reach sexual maturity depends on water temperature and food availability. This character is normally reached at 12 to 20 days for the species Americamysis (=Mysidopsis) bahia, native to estuarine waters of Florida and Texas. The young are released soon afterward, and although their numbers are usually low, the short reproductive cycle of mysid adults means a new brood can be produced every four to seven days. Despite this low fecundity, these species have a short reproductive cycle, which means they can quickly reproduce in vast numbers.
In Arkansas, the first report of a species of mysid shrimp was documented from the state in 2012. Thirty-two Mysis diluviana were found in the Cache River east of Biscoe at the Monroe–Prairie county line. These M. diluviana obviously represented an introduction, and the authors of the report suggested two possibilities for their occurrence: (1) they escaped from barge traffic in the area that may have released ballast water with M. diluviana individuals in it, or (2) one of the large aquaculture facilities that culture baitfish (Golden Shiner, Notemigonus crysoleucas) may have introduced the opossum shrimps into the White River near its confluence with the Cache River.
Mysid shrimps have been reported to be hosts of various parasites, including a cestode, Cyathocephalus truncatus; nematodes; and an echinorhynchid acanthocephalan cystacanth, Echinorhynchus leidyi.
For additional information:
Audzijonytė, Asta, and Risto Väinölä. “Diversity and Distributions of Circumpolar Fresh- and Brackish-Water Mysis (Crustacea: Mysida): Descriptions of M. relicta Lovén, 1862, M. salemaai n. sp., M. segerstralei n. sp. and M. diluviana n. sp., Based on Molecular and Morphological Characters.” Hydrobiologia 544 (2005): 89–141.
Domingues, Pedro M., Philip E. Turk, Jose P. Andrade, and Philip G. Lee. “Culture of the Mysid, Mysidopsis almyra (Bowman), (Crustacea: Mysidacea) in a Static Water System: Effects of Density and Temperature on Production, Survival and Growth.” Aquaculture Research 30 (1999): 135–143.
———. “Pilot-Scale Production of Mysid Shrimp in a Static Water System.” Aquaculture International 6 (1998): 387–402.
Lussier, Anne, Suzanne M. Kuhn, Melissa J. Chammas, and John Sewall. “Techniques for the Laboratory Culture of Mysidopsis Species (Crustacea: Mysidacea).” Environmental Toxicology and Chemistry 7 (1988): 969–977.
Meland, Kenneth. “Species Diversity and Phylogeny of the Deep-Sea Genus Pseudomma (Crustacea: Mysida).” Zootaxa 649 (2004): 1–30.
Nimmo, D. R., and T. L. Hamaker. “Mysids in Toxicity Testing—A Review.” Hydrobiologia 93 (1982): 171–178.
Porter, Megan L., Kenneth Meland, and Wayne Price. “Global Diversity of Mysids (Crustacea-Mysida) in Freshwater.” Freshwater Animal Diversity Assessment Developments in Hydrobiology 198 (2008): 213–218.
Prychitko, S. B., and R. W. Nero. “Occurrence of the Acanthocephalan Echinorhynchus leidyi (Van Cleave, 1924) in Mysis relicta.” Canadian Journal of Zoology 61 (1983): 460–462.
Schanke, Kevin, Henry W. Robison, Nathan J. Wentz, and Chris T. McAllister. “First Record of the Opossum Shrimp, Mysis diluviana (Crustacea: Mysida) from Arkansas.” Journal of the Arkansas Academy of Science 70 (2016): 289‒292. Online at http://scholarworks.uark.edu/jaas/vol70/iss1/51 (accessed July 6, 2020).
Smith, J. D., and M. W. Lankester. “Development of Swim Bladder Nematodes (Cystidicola spp.) in Their Intermediate Hosts.” Canadian Journal of Zoology 57 (1979): 1736–1744.
Sparrow, R. A. H., P. A. Larkin, and R. A. Rutherglen. “Successful Introduction of Mysis relicta Lovén into Kootenay Lake, British Columbia.” Journal of the Fisheries Research Board of Canada 21 (1964): 1325–1327.
Spencer, C. N., D. S. Potter, R. T. Bukantis, and Jack A. Stanford. “Impact of Predation by Mysis relicta on Zooplankton in Flathead Lake, Montana, USA.” Journal of Plankton Research 21 (1999): 51–64.
Sudo, H. “Effect of Temperature on Growth, Sexual Maturity and Reproduction of Acanthomysis robusta (Crustacea: Mysidacea) Reared in the Laboratory.” Marine Biology 143 (2003): 1095–1107.
Väinölä, R. B., J. Riddoch, R. D. Ward, and R. I. Jones. “Genetic Zoogeography of the Mysis relicta Species Group (Crustacea: Mysidacea) in Northern Europe and North America.” Canadian Journal of Fisheries and Aquatic Sciences 51 (1994):1490–1505.
Wolff, R. J. “Mysis relicta as Intermediate Host of an Acanthocephalan Parasite.” Transactions of the Illinois Academy of Science 77 (1984): 1–2.
Chris T. McAllister
Eastern Oklahoma State College
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