MYSIDS: MysidaNO COMMON NAME (Mysis relicta): SPECIES ACCOUNTS
NO COMMON NAME (Stygiomysis cokei): SPECIES ACCOUNTS
The bodies of mysids are usually glassy and transparent, although deep-sea species are often red. Many species have dark, star-shaped patterns made up of clusters of special cells. These clusters of cells give some mysids the ability to change colors to match their background. They can turn dark against a black background or a dark olive green if they are living among green algae (AL-jee).
Most mysids resemble wormlike shrimp and measure between 0.39 to 1.18 inches (10 to 30 millimeters) long. Both pairs of antennae are branched, or biramous (BY-ray-mus). Males have bristles on the bases of the second pair of antennae. The compound eyes are black and mounted on flexible stalks. Each compound eye has multiple lenses. The head and segmented thorax are tightly joined together in a single body region called the cephalothorax (SEH-feh-lo-THOR-acks). A shieldlike carapace covers the head and most of the cephalothorax and is tightly attached with, or fused to, the first 3 or 4 thoracic (thuh-RAE-sik) segments.
The thorax has eight pairs of biramous thoracic limbs, or pereopods (PAIR-ee-oh-pawds). The first and sometimes second pairs of pereopods have pincherlike claws and are used for feeding. The bases of some pereopods in females have plates that form a brooding chamber called the marsupium (mar-SUE-pee-uhm).
The abdomen has six segments. All segments are similar in appearance except for the last, which is twice as long as the others. Each of the first five abdominal segments has a pair of biramous appendages, or pleopods (PLEE-oh-pawds), underneath. The pleopods are usually smaller in the female and sometimes in the male. Males have specialized pleopods that are used for mating. At the tip of the abdomen is a flaplike tail, or telson. On either side of the telson is a biramous uropod (YUR-oh-pawd). Together the telson and uropods form a fanlike tail.
Mysids live on all continents and in all oceans.
Mysids live in a wide variety of aquatic environments. Most species are found in the open sea or along the coast, usually on or near the bottom. Some species burrow in the sand or mud, while others are found in open waters. Species living in open waters are called pelagic (peh-LAJ-ihk) species. Deep-sea mysids are found at depths of 18,700 to 23, 622 feet (5,700 to 7, 200 meters). Other species live in estuaries and other brackish aquatic habitats. A few species live in underground springs or in coastal sea caves.
Most mysids are filter feeders. They strain tiny bits of plant and animal materials from the water as they swim over the bottom. These species also use their pincherlike claws to capture small animals when they are available. Prey items include small crustaceans and mollusks.
BEHAVIOR AND REPRODUCTION
Mysids spend much of their time swimming. They can swim up, down, forward, and backward with equal agility. When threatened they quickly jerk backward by flexing the abdomen and fanlike tail forward against the thorax. Females have very small pleopods and use their pereopods for swimming. Burrowing species rise up into the water at night to feed and sink down to the safety of the bottom during the day to avoid predators. Some species form swarms that may reach several miles in length and three or more feet in diameter.
Both males and females are required for reproduction. Females produce chemicals, or pheromones (FEH-re-moans), to attract males. Males and females sometimes align their bodies, belly-to-belly and head-to-tail, to mate. The male's sperm is either injected or washed into the female's marsupium. Within 30 minutes eggs are released into the marsupium and fertilized.
After hatching the young remain and develop in the marsupium for several weeks or months, depending on species and water temperature. The young eventually leave the marsupium with a full set of appendages and reach adulthood in about a month at water temperatures of 68°F (20°C).
MYSIDS AND PEOPLE
Some mysids are harvested and processed for use as fish food or bait. In Asia, mysids are commonly sold as food for humans. They are also used as research animals because they are easy to collect, handle, and keep in the laboratory.
THE POUCH TRIP
Mysids are sometimes called "opossum shrimps" because the females of some species have leg bases with well developed plates. These plates form an enclosed chamber, or brood pouch, under the thorax. This pouch, called the marsupium, protects the eggs and young and is similar in function to the marsupia of opossums, kangaroos, and other marsupial mammals. The word marsupium is the Latin word for pouch.
The World Conservation Union (IUCN) considers two species of mysids, both of which live in sea caves, to be Critically Endangered: Bermudamysis speluncola and Platyops sterreri. This means these mysids are facing an extremely high risk of extinction in the wild. Mysids are threatened by pollution of coastal waters, dredging of canals, groundwater drainage, and continual use of pesticides. Underground waters are threatened by tourism, agriculture, and urban development.
Physical characteristics: Adults measure 0.59 to 0.98 inches (15 to 25 millimeters) long. The telson is wide and split at the tip.
Geographic range: In North America Mysis relicta is found in the Great Lakes of North America; Green Lake, Trout Lake, and Lake Geneva in Wisconsin; the Finger Lakes of New York; and a few Canadian shield lakes. It has been introduced into other lakes in the western United States, Alabama, and Maine. This species is also found in Europe, Scandinavia, and Russia.
Habitat: This species prefers to live in cold, freshwater lakes at temperatures of 39° to 57°F (4° to 14°C).
Diet: Mysis relicta is a filter feeder, a scavenger, and a predator.
Behavior and reproduction: This species moves up into open water at night and remains near the bottom during the day. They usually live in the dark; their eyes are easily damaged by strong light.
Brood size is determined by the size of the female. Females measuring 0.51 to 0.59 inches (13 to 15 millimeters) carry 10 to 20 eggs, while females 0.66 to 0.82 inches (17 to 21 millimeters) carry as many as 25 to 40 eggs. The hatchlings remain in the marsupium for one to three months. They live a total of one or two years. Females reproduce once to twice in their lifetime.
Mysis relicta and people: Mysis relicta are prey for many species of fish valued as food for humans. These include lake trout, brown trout, rainbow trout, kokanee salmon, burbot, smelts, alewives, and others.
Conservation status: Mysis relicta is not considered endangered or threatened. ∎
Physical characteristics: Adults measure 0.35 to 0.86 inches (9.0 to 22.0 millimeters). The carapace is about one-fifth of the entire length of the body. The first pair of antennae (antennules) are about one-half the body length. The marsupium is made up of four plates. The telson has fifteen spines arranged in five groups of three and is about one-sixth the length of the body.
Geographic range: Stygiomysis cokei is found in coastal inland caves near Quintana Roo, Yucatan Peninsula, Mexico; in Mayan Blue Cave; Carwash Cave; and Naharon Cave.
Habitat: These cave dwellers live in the freshwater layer at depths of 32.8 to 65.6 feet (10 to 20 meters). They are also found in the upper layers with slightly brackish waters. They prefer habitats with relatively constant temperatures ranging between 76.1° to 77.9°F (24.5° to 25.5°C).
Diet: Stygiomysis cokei is primarily a filter feeder.
Behavior and reproduction: Little is known about how this species behaves under natural conditions. When disturbed, they will swim with awkward movements. Nothing is known about their reproductive behavior.
Stygiomysis cokei and people: This species is not known to impact humans or their activities directly.
Conservation status: This species is not considered endangered or threatened. ∎
FOR MORE INFORMATION
Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.
Mauchline, J. The Biology of Mysids and Euphausiids. Part I. The Biology of Mysids. London: Academic Press, 1980.
Grossnickle, N. E. "Feeding Habits of Mysis relicta—An Overview." Hydrobiology 93 (1982): 101-107.
"Biology of Opposum [sic] Shrimps." Mysidacea Gallery.http://www.museum.vic.gov.au/crust/mysibiol.html (accessed on February 21, 2005).
Mysis relicta. http://nas.er.usgs.gov/queries/FactSheet.asp?SpeciesID=1142 (accessed on February 22, 2005).