any member of the crustacean order Mysidacea. Most of the 450 known species live in the sea; a few live in brackish water; and fewer still live in fresh water. Most are 1 to 3 cm (about 0.4 to 1.2 inches) long. The name opossum shrimp derives from the females' brood pouch, in which young larvae spend several weeks.
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