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Mysidacea are a small group of shrimp-like creatures, often used as live food for both freshwater and saltwater fish. They are often traded under the common name of Mysis Shrimp, Mysid Shrimp or Opossom Shrimp.
Note that despite their name, and their superficial resemblance to shrimp, they are only quite distantly related to the true shrimps, which are classified in the order Decapoda.
These are often sold either as live or frozen.