Superficially, Mysis relicta resembles a crayfish. Differences include a thin carapace not completely covering the thorax, with the last two segments exposed dorsally, and to a lesser degree laterally. Appendages are long, thin, setose, biramous, and many segmented. The first thoracic segment is fused with the head and bears a pair of maxillipeds. The remaining seven segments are distinct with the secondary maxillipeds on the second thoracic segment and a pair of swimming legs on each of the remaining five segments.
Eyes are extremely large, compound, and stalked.
The telson has a wide, bifurcated tip. The first nonindigenous introduction was proposed by Clements et al. Subsequent stockings were preformed in many montane lakes of the western U.S. and Canada. Mysis relicta is an opportunistic feeder with both predatorial and filter feeding habits. Dramatic changes and species extinctions of native zooplankton communities have been attributed to this opportunistic lifestyle.