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UTAS Home > IMAS Home > Image Key > Malacostraca > Peracarida >  Mysidacea


A. H. Haworth 1825


Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Superorder Peracarida
Order Mysidacea


  • 5 to 20 mm in length.

Distinguishing characteristics

  • Mysids resemble shrimps.
  • Carapace extends over most or all of the thorax, but is only attached to the first 3 thoracic segments.
  • Rostrum extends anteriorly.
  • Compound eyes are stalked and moveable.
  • 1st antenna is biramous and the 2nd antenna usually has a large scale-like exopodite.
  • 8 pairs of thoracic legs most of which have natatory (swimming) exopodites.
  • 1st and sometimes the 2nd thoracic legs function as maxillipeds or accessory feeding limbs.
  • Mature females have a conspicuous pouch, or marsupium, between the thoracic legs.
  • Eggs are laid directly into the marsupium where they develop to a juvenile stage, resembling an adult, before release. For this reason, mysids are sometimes known as opossum shrimps.
  • Abdomen has 6 segments and bears 5 pairs of pleopods with thin whip-like exopods and endopods.
  • Pleopods may also be reduced in some species and are often sexually modified in males.
  • Uropods on the 6th abdominal segment form a tail fan.
  • Statocysts (balancing organs), clearly visible as circular vesicles, are characteristically located in the uropod endopodite of many species and are a good diagnostic feature of the group.
  • Can vary in colour from pale, almost transparent, through to bright orange or brown


  • Few mysid species are truly planktonic in the epipelagic zone of coastal and offshore waters.
  • Mostly found in the very shallow coastal waters where they live just above or near the bottom, often around a substrate such as a reef, rocky outcrop or kelp bed.
  • Can occur in high abundance in small dense schools or swarms and are an important food source for many inshore fish, e.g. seahorses.
  • In reality they are unlikely to be caught in plankton nets, unless sampling occurs very close inshore at night. However, they are included in this text because they are similar in appearance to euphausiids (see Euphausiacea). In fact, at one time the two orders were erroneously grouped together as Schizopoda.

Additional notes

  • Please note some specimens photographed for this fact sheets were not collected in plankton nets.