Mysida

Mysids can be distinguished from true shrimps (Caridea) and other peracaridans by two key features: the presence of a ventral marsupium formed by oostegites (bristly flaps extending from pereopod bases) in females, and statocysts (circular vesicles functioning as balance organs) on the uropods of the last abdominal segment. The carapace covers the thorax but is attached only to the first three segments. Males possess a fourth pleopod longer than the others with specialized reproductive function.

Primarily marine, occurring in shallow coastal waters, estuaries, and the deep sea. Benthic species burrow in or crawl on mud and sand substrates; pelagic species inhabit mid-water. Some species associate with algae, seagrasses, sea anemones, or hermit crabs. Freshwater and brackish populations occur in lakes, rivers, caves, and groundwater systems, particularly in the Palearctic and Neotropical realms.

Cosmopolitan distribution across world oceans. Freshwater representation is limited to fewer than 10% of species, with approximately 72 freshwater species documented. Notable freshwater concentrations include glacial relict populations in deep lakes, subterranean Tethyan relicts in caves, and extensive radiation of Paramysis (23 species) in the Ponto-Caspian Basin.

Omnivorous, feeding on algae, detritus, and zooplankton. Filter feeding is accomplished using maxillipeds to strain plankton and organic particles from water currents generated by pereopod exopods. Some benthic species groom body surfaces to collect small particles. Scavenging and cannibalism (including predation on newly released young) have been observed.

Direct development without free-swimming larvae. Eggs are fertilized externally and transferred to the female marsupium, where embryos develop directly into miniature adults. Mating occurs at night and lasts minutes; sperm release stimulates egg deposition within approximately one hour. Brood size correlates with body length and environmental conditions. Sexual maturity is reached in 12–20 days under favorable conditions in some species, with new broods produced every 4–7 days.

Most marine species exhibit diel vertical migration: benthic by day, planktonic at night. Locomotion primarily by pleopod swimming. Some species form dense swarms; others are solitary. Several species display commensal relationships with sea anemones or hermit crabs. Mysis relicta and relatives perform diurnal vertical migrations in cold, deep lakes. Mysidium integrum maintains a mutualistic relationship with longfin damselfish, exchanging nutrient provision for predator protection.

Important prey for fish including shad and flounder. Contribute to nutrient cycling through filter feeding and detritus processing. Serve as bioindicators of water quality due to sensitivity to pollution.

Cultured extensively as live food for cephalopods, fish larvae, farmed shrimp, and juvenile seahorses in aquaculture. Americamysis bahia and A. almyra are standard test organisms for pesticide and toxicity bioassays, with heightened sensitivity during moulting periods.

• CarideaTrue shrimps lack the marsupium and statocysts characteristic of Mysida; they also possess free-swimming larvae rather than direct development.
• LophogastridaFormerly grouped with Mysida in Mysidacea but now recognized as distinct; molecular studies do not support their close relationship despite morphological similarities.
• StygiomysidaPreviously included in Mysida but elevated to separate order; includes families Lepidomysidae and Stygiomysidae.

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life