Facts: Opossum Shrimp (Mysids)

The order Mycida consists of over 1,000 species of small, shrimp-like crustaceans, commonly known as mycids or opossum shrimp, that occupy diverse aquatic environments globally. They earn the common name opossum shrimp because the mature females possess a ventral brood pouch called a marsupium, where the developing embryos are carried. These crustaceans are only distantly related to true shrimp. Opossum shrimp are found in a broad array of habitats, including subterranean, freshwater, brackish, coastal, and deep-sea environments, although the majority of species inhabit marine ecosystems. Opossum shrimp are among the most abundant animals in marine zooplankton. Some species have a large range, while others are restricted to very specific areas. A typical opossum shrimp has a translucent body, one pair of stocked eyes, two pairs of antennae, and eight pairs of limbs. The foremost two pairs of limbs function like arms and are used for filtering food from the water. The six other pairs, known as poreopods, are used for swimming and for wafting water and suspended matter toward the filter-feeding limbs. Most opossum shrimp possess a specialized balance organ called the statocyst, located on their tail fan. Opossum shrimp are tiny, with the majority of species ranging from a tenth of an inch to an inch long. Most are found hovering above various bottom habitats during the daytime, though some live in or on the bottom substrate. Many species migrate daily during something called diovertical migration, where they spend the days in deeper, darker water and migrate closer to the surface at night. They frequently form dense aggregations. These swarms can be extensive, sometimes reported as several miles long and over three feet in diameter. They typically only have a one to two year life cycle. Their breeding season varies from once a year in the Arctic to continuous breeding in warmer latitudes. Larval development of opossum shrimp occurs entirely in the brood pouch of a female. Mating is thought to usually take place at night, following the molting of the female when a male deposits sperm into the female's marsupium, or brood pouch. The female then moves her eggs into her brood pouch, where they become fertilized. The eggs hatch into larvae, and the larvae develop into juveniles. The brood pouch will expand and darken, then the free-swimming juveniles are released into the water. The incubation period is highly variable, depending on the species and water temperature, and can range from a few weeks to several months. The majority of opossum shrimp are filter feeding omnivores, feeding on phytoplankton, detritus, and zooplankton. Scavenging, active predation, and cannibalism are also common, with the adults preying on their young once they emerge from the brood pouch. Opossum shrimp are an important food source for many organisms, such as large invertebrates, fish, birds, and seals. Their short reproductive cycle and high adaptability to water conditions make opossum shrimp ideal organisms to culture in laboratories. They are used as feed for aquaculture organisms, like lake trout. Opossum shrimp have progressively invaded new areas by new connections of water systems and through ballast water in ships. They've also been intentionally introduced into some water systems, thinking they would benefit fish like trout, but they have instead become highly abundant and negatively impacted native organisms by excessively eating the zooplankton that young organisms rely on to grow into adults. For more marine facts, click the SUBSCRIBE button!!